Cleaned up frontend-level kernel restart logic.

IPython/frontend/qt/console/console_widget.py

@@ -1,1604 +1,1611
 """ An abstract base class for console-type widgets.
 """
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 # Standard library imports
 from os.path import commonprefix
 import re
 import sys
 from textwrap import dedent
 
 # System library imports
 from PyQt4 import QtCore, QtGui
 
 # Local imports
 from IPython.config.configurable import Configurable
 from IPython.frontend.qt.util import MetaQObjectHasTraits, get_font
 from IPython.utils.traitlets import Bool, Enum, Int
 from ansi_code_processor import QtAnsiCodeProcessor
 from completion_widget import CompletionWidget
 
 #-----------------------------------------------------------------------------
 # Classes
 #-----------------------------------------------------------------------------
 
 class ConsoleWidget(Configurable, QtGui.QWidget):
     """ An abstract base class for console-type widgets. This class has 
         functionality for:
 
             * Maintaining a prompt and editing region
             * Providing the traditional Unix-style console keyboard shortcuts 
             * Performing tab completion
             * Paging text
             * Handling ANSI escape codes
 
         ConsoleWidget also provides a number of utility methods that will be
         convenient to implementors of a console-style widget.
     """
     __metaclass__ = MetaQObjectHasTraits
 
     #------ Configuration ------------------------------------------------------
 
     # Whether to process ANSI escape codes.
     ansi_codes = Bool(True, config=True)
 
     # The maximum number of lines of text before truncation. Specifying a
     # non-positive number disables text truncation (not recommended).
     buffer_size = Int(500, config=True)
 
     # Whether to use a list widget or plain text output for tab completion.
     gui_completion = Bool(False, config=True)
 
     # The type of underlying text widget to use. Valid values are 'plain', which
     # specifies a QPlainTextEdit, and 'rich', which specifies a QTextEdit.
     # NOTE: this value can only be specified during initialization.
     kind = Enum(['plain', 'rich'], default_value='plain', config=True)
 
     # The type of paging to use. Valid values are:
     #     'inside' : The widget pages like a traditional terminal.
     #     'hsplit' : When paging is requested, the widget is split
     #                horizontally. The top pane contains the console, and the
     #                bottom pane contains the paged text.
     #     'vsplit' : Similar to 'hsplit', except that a vertical splitter used.
     #     'custom' : No action is taken by the widget beyond emitting a
     #                'custom_page_requested(str)' signal.
     #     'none'   : The text is written directly to the console.
     # NOTE: this value can only be specified during initialization.
     paging = Enum(['inside', 'hsplit', 'vsplit', 'custom', 'none'], 
                   default_value='inside', config=True)
 
     # Whether to override ShortcutEvents for the keybindings defined by this
     # widget (Ctrl+n, Ctrl+a, etc). Enable this if you want this widget to take
     # priority (when it has focus) over, e.g., window-level menu shortcuts.
     override_shortcuts = Bool(False)
 
     #------ Signals ------------------------------------------------------------
 
     # Signals that indicate ConsoleWidget state.
     copy_available = QtCore.pyqtSignal(bool)
     redo_available = QtCore.pyqtSignal(bool)
     undo_available = QtCore.pyqtSignal(bool)
 
     # Signal emitted when paging is needed and the paging style has been
     # specified as 'custom'.
     custom_page_requested = QtCore.pyqtSignal(object)
 
     # Signal emitted when the font is changed.
     font_changed = QtCore.pyqtSignal(QtGui.QFont)
 
     #------ Protected class variables ------------------------------------------
 
     _ctrl_down_remap = { QtCore.Qt.Key_B : QtCore.Qt.Key_Left,
                          QtCore.Qt.Key_F : QtCore.Qt.Key_Right,
                          QtCore.Qt.Key_A : QtCore.Qt.Key_Home,
                          QtCore.Qt.Key_E : QtCore.Qt.Key_End,
                          QtCore.Qt.Key_P : QtCore.Qt.Key_Up,
                          QtCore.Qt.Key_N : QtCore.Qt.Key_Down,
                          QtCore.Qt.Key_D : QtCore.Qt.Key_Delete, }
 
     _shortcuts = set(_ctrl_down_remap.keys() +
                      [ QtCore.Qt.Key_C, QtCore.Qt.Key_G, QtCore.Qt.Key_O,
                        QtCore.Qt.Key_V ])
 
     #---------------------------------------------------------------------------
     # 'QObject' interface
     #---------------------------------------------------------------------------
 
     def __init__(self, parent=None, **kw):
         """ Create a ConsoleWidget.
 
         Parameters:
         -----------
         parent : QWidget, optional [default None]
             The parent for this widget.
         """
         QtGui.QWidget.__init__(self, parent)
         Configurable.__init__(self, **kw)
 
         # Create the layout and underlying text widget.
         layout = QtGui.QStackedLayout(self)
         layout.setContentsMargins(0, 0, 0, 0)
         self._control = self._create_control()
         self._page_control = None
         self._splitter = None
         if self.paging in ('hsplit', 'vsplit'):
             self._splitter = QtGui.QSplitter()
             if self.paging == 'hsplit':
                 self._splitter.setOrientation(QtCore.Qt.Horizontal)
             else:
                 self._splitter.setOrientation(QtCore.Qt.Vertical)
             self._splitter.addWidget(self._control)
             layout.addWidget(self._splitter)
         else:
             layout.addWidget(self._control)
 
         # Create the paging widget, if necessary.
         if self.paging in ('inside', 'hsplit', 'vsplit'):
             self._page_control = self._create_page_control()
             if self._splitter:
                 self._page_control.hide()
                 self._splitter.addWidget(self._page_control)
             else:
                 layout.addWidget(self._page_control)
 
         # Initialize protected variables. Some variables contain useful state
         # information for subclasses; they should be considered read-only.
         self._ansi_processor = QtAnsiCodeProcessor()
         self._completion_widget = CompletionWidget(self._control)
         self._continuation_prompt = '> '
         self._continuation_prompt_html = None
         self._executing = False
         self._prompt = ''
         self._prompt_html = None
         self._prompt_pos = 0
         self._prompt_sep = ''
         self._reading = False
         self._reading_callback = None
         self._tab_width = 8
         self._text_completing_pos = 0
 
         # Set a monospaced font.
         self.reset_font()
 
     def eventFilter(self, obj, event):
         """ Reimplemented to ensure a console-like behavior in the underlying
             text widgets.
         """
         etype = event.type()
         if etype == QtCore.QEvent.KeyPress:
 
             # Re-map keys for all filtered widgets.
             key = event.key()
             if self._control_key_down(event.modifiers()) and \
                     key in self._ctrl_down_remap:
                 new_event = QtGui.QKeyEvent(QtCore.QEvent.KeyPress, 
                                             self._ctrl_down_remap[key],
                                             QtCore.Qt.NoModifier)
                 QtGui.qApp.sendEvent(obj, new_event)
                 return True
 
             elif obj == self._control:
                 return self._event_filter_console_keypress(event)
 
             elif obj == self._page_control:
                 return self._event_filter_page_keypress(event)
 
         # Make middle-click paste safe.
         elif etype == QtCore.QEvent.MouseButtonRelease and \
                 event.button() == QtCore.Qt.MidButton and \
                 obj == self._control.viewport():
             cursor = self._control.cursorForPosition(event.pos())
             self._control.setTextCursor(cursor)
             self.paste(QtGui.QClipboard.Selection)
             return True
 
         # Manually adjust the scrollbars *after* a resize event is dispatched.
         elif etype == QtCore.QEvent.Resize:
             QtCore.QTimer.singleShot(0, self._adjust_scrollbars)
 
         # Override shortcuts for all filtered widgets.
         elif etype == QtCore.QEvent.ShortcutOverride and \
                 self.override_shortcuts and \
                 self._control_key_down(event.modifiers()) and \
                 event.key() in self._shortcuts:
             event.accept()
             return False
 
         # Prevent text from being moved by drag and drop.
         elif etype in (QtCore.QEvent.DragEnter, QtCore.QEvent.DragLeave, 
                        QtCore.QEvent.DragMove, QtCore.QEvent.Drop):
             return True
 
         return super(ConsoleWidget, self).eventFilter(obj, event)
 
     #---------------------------------------------------------------------------
     # 'QWidget' interface
     #---------------------------------------------------------------------------
 
     def sizeHint(self):
         """ Reimplemented to suggest a size that is 80 characters wide and
             25 lines high.
         """
         font_metrics = QtGui.QFontMetrics(self.font)
         margin = (self._control.frameWidth() +
                   self._control.document().documentMargin()) * 2
         style = self.style()
         splitwidth = style.pixelMetric(QtGui.QStyle.PM_SplitterWidth)
 
         # Note 1: Despite my best efforts to take the various margins into
         # account, the width is still coming out a bit too small, so we include
         # a fudge factor of one character here.
         # Note 2: QFontMetrics.maxWidth is not used here or anywhere else due
         # to a Qt bug on certain Mac OS systems where it returns 0.
         width = font_metrics.width(' ') * 81 + margin
         width += style.pixelMetric(QtGui.QStyle.PM_ScrollBarExtent)
         if self.paging == 'hsplit':
             width = width * 2 + splitwidth
 
         height = font_metrics.height() * 25 + margin
         if self.paging == 'vsplit':
             height = height * 2 + splitwidth
 
         return QtCore.QSize(width, height)
 
     #---------------------------------------------------------------------------
     # 'ConsoleWidget' public interface
     #---------------------------------------------------------------------------
 
     def can_copy(self):
         """ Returns whether text can be copied to the clipboard.
         """
         return self._control.textCursor().hasSelection()
 
     def can_cut(self):
         """ Returns whether text can be cut to the clipboard.
         """
         cursor = self._control.textCursor()
         return (cursor.hasSelection() and 
                 self._in_buffer(cursor.anchor()) and 
                 self._in_buffer(cursor.position()))
         
     def can_paste(self):
         """ Returns whether text can be pasted from the clipboard.
         """
         if self._control.textInteractionFlags() & QtCore.Qt.TextEditable:
             return not QtGui.QApplication.clipboard().text().isEmpty()
         return False
 
     def clear(self, keep_input=True):
-        """ Clear the console, then write a new prompt. If 'keep_input' is set,
-            restores the old input buffer when the new prompt is written.
+        """ Clear the console. 
+
+        Parameters:
+        -----------
+        keep_input : bool, optional (default True)
+            If set, restores the old input buffer if a new prompt is written.
         """
-        if keep_input:
-            input_buffer = self.input_buffer
-        self._control.clear()
-        self._show_prompt()
-        if keep_input:
-            self.input_buffer = input_buffer
+        if self._executing:
+            self._control.clear()
+        else:
+            if keep_input:
+                input_buffer = self.input_buffer
+            self._control.clear()
+            self._show_prompt()
+            if keep_input:
+                self.input_buffer = input_buffer
 
     def copy(self):
         """ Copy the currently selected text to the clipboard.
         """
         self._control.copy()
 
     def cut(self):
         """ Copy the currently selected text to the clipboard and delete it
             if it's inside the input buffer.
         """
         self.copy()
         if self.can_cut():
             self._control.textCursor().removeSelectedText()
 
     def execute(self, source=None, hidden=False, interactive=False):
         """ Executes source or the input buffer, possibly prompting for more
         input.
 
         Parameters:
         -----------     
         source : str, optional
 
             The source to execute. If not specified, the input buffer will be
             used. If specified and 'hidden' is False, the input buffer will be
             replaced with the source before execution.
 
         hidden : bool, optional (default False)
 
             If set, no output will be shown and the prompt will not be modified.
             In other words, it will be completely invisible to the user that
             an execution has occurred.
 
         interactive : bool, optional (default False)
 
             Whether the console is to treat the source as having been manually
             entered by the user. The effect of this parameter depends on the
             subclass implementation.
 
         Raises:
         -------
         RuntimeError
             If incomplete input is given and 'hidden' is True. In this case,
             it is not possible to prompt for more input.
 
         Returns:
         --------
         A boolean indicating whether the source was executed.
         """
         # WARNING: The order in which things happen here is very particular, in
         # large part because our syntax highlighting is fragile. If you change
         # something, test carefully!
 
         # Decide what to execute.
         if source is None:
             source = self.input_buffer
             if not hidden:
                 # A newline is appended later, but it should be considered part
                 # of the input buffer.
                 source += '\n'
         elif not hidden:
             self.input_buffer = source
             
         # Execute the source or show a continuation prompt if it is incomplete.
         complete = self._is_complete(source, interactive)
         if hidden:
             if complete:
                 self._execute(source, hidden)
             else:
                 error = 'Incomplete noninteractive input: "%s"'
                 raise RuntimeError(error % source)                
         else:
             if complete:
                 self._append_plain_text('\n')
                 self._executing_input_buffer = self.input_buffer
                 self._executing = True
                 self._prompt_finished()
 
                 # The maximum block count is only in effect during execution.
                 # This ensures that _prompt_pos does not become invalid due to
                 # text truncation.
                 self._control.document().setMaximumBlockCount(self.buffer_size)
 
                 # Setting a positive maximum block count will automatically
                 # disable the undo/redo history, but just to be safe:
                 self._control.setUndoRedoEnabled(False)
 
                 # Perform actual execution.
                 self._execute(source, hidden)
             
             else:
                 # Do this inside an edit block so continuation prompts are
                 # removed seamlessly via undo/redo.
                 cursor = self._get_end_cursor()
                 cursor.beginEditBlock()
                 cursor.insertText('\n')
                 self._insert_continuation_prompt(cursor)
                 cursor.endEditBlock()
 
                 # Do not do this inside the edit block. It works as expected
                 # when using a QPlainTextEdit control, but does not have an 
                 # effect when using a QTextEdit. I believe this is a Qt bug.
                 self._control.moveCursor(QtGui.QTextCursor.End)
 
         return complete
 
     def _get_input_buffer(self):
         """ The text that the user has entered entered at the current prompt.
         """
         # If we're executing, the input buffer may not even exist anymore due to
         # the limit imposed by 'buffer_size'. Therefore, we store it.
         if self._executing:
             return self._executing_input_buffer
 
         cursor = self._get_end_cursor()
         cursor.setPosition(self._prompt_pos, QtGui.QTextCursor.KeepAnchor)
         input_buffer = unicode(cursor.selection().toPlainText())
 
         # Strip out continuation prompts.
         return input_buffer.replace('\n' + self._continuation_prompt, '\n')
 
     def _set_input_buffer(self, string):
         """ Replaces the text in the input buffer with 'string'.
         """
         # For now, it is an error to modify the input buffer during execution.
         if self._executing:
             raise RuntimeError("Cannot change input buffer during execution.")
 
         # Remove old text.
         cursor = self._get_end_cursor()
         cursor.beginEditBlock()
         cursor.setPosition(self._prompt_pos, QtGui.QTextCursor.KeepAnchor)
         cursor.removeSelectedText()
 
         # Insert new text with continuation prompts.
         lines = string.splitlines(True)
         if lines:
             self._append_plain_text(lines[0])
             for i in xrange(1, len(lines)):
                 if self._continuation_prompt_html is None:
                     self._append_plain_text(self._continuation_prompt)
                 else:
                     self._append_html(self._continuation_prompt_html)
                 self._append_plain_text(lines[i])
         cursor.endEditBlock()
         self._control.moveCursor(QtGui.QTextCursor.End)
 
     input_buffer = property(_get_input_buffer, _set_input_buffer)
 
     def _get_font(self):
         """ The base font being used by the ConsoleWidget.
         """
         return self._control.document().defaultFont()
 
     def _set_font(self, font):
         """ Sets the base font for the ConsoleWidget to the specified QFont.
         """
         font_metrics = QtGui.QFontMetrics(font)
         self._control.setTabStopWidth(self.tab_width * font_metrics.width(' '))
 
         self._completion_widget.setFont(font)
         self._control.document().setDefaultFont(font)
         if self._page_control:
             self._page_control.document().setDefaultFont(font)
 
         self.font_changed.emit(font)
 
     font = property(_get_font, _set_font)
 
     def paste(self, mode=QtGui.QClipboard.Clipboard):
         """ Paste the contents of the clipboard into the input region.
 
         Parameters:
         -----------
         mode : QClipboard::Mode, optional [default QClipboard::Clipboard]
 
             Controls which part of the system clipboard is used. This can be
             used to access the selection clipboard in X11 and the Find buffer
             in Mac OS. By default, the regular clipboard is used.
         """
         if self._control.textInteractionFlags() & QtCore.Qt.TextEditable:
             # Remove any trailing newline, which confuses the GUI and forces the
             # user to backspace.
             text = unicode(QtGui.QApplication.clipboard().text(mode)).rstrip()
             self._insert_plain_text_into_buffer(dedent(text))
 
     def print_(self, printer):
         """ Print the contents of the ConsoleWidget to the specified QPrinter.
         """
         self._control.print_(printer)
 
     def prompt_to_top(self):
         """ Moves the prompt to the top of the viewport.
         """
         if not self._executing:
             prompt_cursor = self._get_prompt_cursor()
             if self._get_cursor().blockNumber() < prompt_cursor.blockNumber():
                 self._set_cursor(prompt_cursor)
             self._set_top_cursor(prompt_cursor)
             
     def redo(self):
         """ Redo the last operation. If there is no operation to redo, nothing
             happens.
         """
         self._control.redo()
 
     def reset_font(self):
         """ Sets the font to the default fixed-width font for this platform.
         """
         if sys.platform == 'win32':
             # Consolas ships with Vista/Win7, fallback to Courier if needed
             family, fallback = 'Consolas', 'Courier'
         elif sys.platform == 'darwin':
             # OSX always has Monaco, no need for a fallback
             family, fallback = 'Monaco', None
         else:
             # FIXME: remove Consolas as a default on Linux once our font
             # selections are configurable by the user.
             family, fallback = 'Consolas', 'Monospace'
         font = get_font(family, fallback)
         font.setPointSize(QtGui.qApp.font().pointSize())
         font.setStyleHint(QtGui.QFont.TypeWriter)
         self._set_font(font)
 
     def select_all(self):
         """ Selects all the text in the buffer.
         """
         self._control.selectAll()
 
     def _get_tab_width(self):
         """ The width (in terms of space characters) for tab characters.
         """
         return self._tab_width
 
     def _set_tab_width(self, tab_width):
         """ Sets the width (in terms of space characters) for tab characters.
         """
         font_metrics = QtGui.QFontMetrics(self.font)
         self._control.setTabStopWidth(tab_width * font_metrics.width(' '))
 
         self._tab_width = tab_width
 
     tab_width = property(_get_tab_width, _set_tab_width)
 
     def undo(self):
         """ Undo the last operation. If there is no operation to undo, nothing
             happens.
         """
         self._control.undo()
 
     #---------------------------------------------------------------------------
     # 'ConsoleWidget' abstract interface
     #---------------------------------------------------------------------------
 
     def _is_complete(self, source, interactive):
         """ Returns whether 'source' can be executed. When triggered by an
             Enter/Return key press, 'interactive' is True; otherwise, it is
             False.
         """
         raise NotImplementedError
 
     def _execute(self, source, hidden):
         """ Execute 'source'. If 'hidden', do not show any output.
         """
         raise NotImplementedError
 
     def _prompt_started_hook(self):
         """ Called immediately after a new prompt is displayed.
         """
         pass
 
     def _prompt_finished_hook(self):
         """ Called immediately after a prompt is finished, i.e. when some input
             will be processed and a new prompt displayed.
         """
         pass
 
     def _up_pressed(self):
         """ Called when the up key is pressed. Returns whether to continue
             processing the event.
         """
         return True
 
     def _down_pressed(self):
         """ Called when the down key is pressed. Returns whether to continue
             processing the event.
         """
         return True
 
     def _tab_pressed(self):
         """ Called when the tab key is pressed. Returns whether to continue
             processing the event.
         """
         return False
 
     #--------------------------------------------------------------------------
     # 'ConsoleWidget' protected interface
     #--------------------------------------------------------------------------
 
     def _append_html(self, html):
         """ Appends html at the end of the console buffer.
         """
         cursor = self._get_end_cursor()
         self._insert_html(cursor, html)
 
     def _append_html_fetching_plain_text(self, html):
         """ Appends 'html', then returns the plain text version of it.
         """
         cursor = self._get_end_cursor()
         return self._insert_html_fetching_plain_text(cursor, html)
 
     def _append_plain_text(self, text):
         """ Appends plain text at the end of the console buffer, processing
             ANSI codes if enabled.
         """
         cursor = self._get_end_cursor()
         self._insert_plain_text(cursor, text)
 
     def _append_plain_text_keeping_prompt(self, text):
         """ Writes 'text' after the current prompt, then restores the old prompt
             with its old input buffer.
         """
         input_buffer = self.input_buffer
         self._append_plain_text('\n')
         self._prompt_finished()
 
         self._append_plain_text(text)
         self._show_prompt()
         self.input_buffer = input_buffer
 
     def _cancel_text_completion(self):
         """ If text completion is progress, cancel it.
         """
         if self._text_completing_pos:
             self._clear_temporary_buffer()
             self._text_completing_pos = 0
 
     def _clear_temporary_buffer(self):
         """ Clears the "temporary text" buffer, i.e. all the text following
             the prompt region.
         """
         # Select and remove all text below the input buffer.
         cursor = self._get_prompt_cursor()
         prompt = self._continuation_prompt.lstrip()
         while cursor.movePosition(QtGui.QTextCursor.NextBlock):
             temp_cursor = QtGui.QTextCursor(cursor)
             temp_cursor.select(QtGui.QTextCursor.BlockUnderCursor)
             text = unicode(temp_cursor.selection().toPlainText()).lstrip()
             if not text.startswith(prompt):
                 break
         else:
             # We've reached the end of the input buffer and no text follows.
             return
         cursor.movePosition(QtGui.QTextCursor.Left) # Grab the newline.
         cursor.movePosition(QtGui.QTextCursor.End,
                             QtGui.QTextCursor.KeepAnchor)
         cursor.removeSelectedText()
 
         # After doing this, we have no choice but to clear the undo/redo
         # history. Otherwise, the text is not "temporary" at all, because it
         # can be recalled with undo/redo. Unfortunately, Qt does not expose
         # fine-grained control to the undo/redo system.
         if self._control.isUndoRedoEnabled():
             self._control.setUndoRedoEnabled(False)
             self._control.setUndoRedoEnabled(True)
 
     def _complete_with_items(self, cursor, items):
         """ Performs completion with 'items' at the specified cursor location.
         """
         self._cancel_text_completion()
 
         if len(items) == 1:
             cursor.setPosition(self._control.textCursor().position(), 
                                QtGui.QTextCursor.KeepAnchor)
             cursor.insertText(items[0])
 
         elif len(items) > 1:
             current_pos = self._control.textCursor().position()
             prefix = commonprefix(items)
             if prefix:
                 cursor.setPosition(current_pos, QtGui.QTextCursor.KeepAnchor)
                 cursor.insertText(prefix)
                 current_pos = cursor.position()
 
             if self.gui_completion:
                 cursor.movePosition(QtGui.QTextCursor.Left, n=len(prefix))
                 self._completion_widget.show_items(cursor, items) 
             else:
                 cursor.beginEditBlock()
                 self._append_plain_text('\n')
                 self._page(self._format_as_columns(items))
                 cursor.endEditBlock()
 
                 cursor.setPosition(current_pos)
                 self._control.moveCursor(QtGui.QTextCursor.End)
                 self._control.setTextCursor(cursor)
                 self._text_completing_pos = current_pos
 
     def _context_menu_make(self, pos):
         """ Creates a context menu for the given QPoint (in widget coordinates).
         """
         menu = QtGui.QMenu()
 
         cut_action = menu.addAction('Cut', self.cut)
         cut_action.setEnabled(self.can_cut())
         cut_action.setShortcut(QtGui.QKeySequence.Cut)
 
         copy_action = menu.addAction('Copy', self.copy)
         copy_action.setEnabled(self.can_copy())
         copy_action.setShortcut(QtGui.QKeySequence.Copy)
 
         paste_action = menu.addAction('Paste', self.paste)
         paste_action.setEnabled(self.can_paste())
         paste_action.setShortcut(QtGui.QKeySequence.Paste)
 
         menu.addSeparator()
         menu.addAction('Select All', self.select_all)
         
         return menu
 
     def _control_key_down(self, modifiers, include_command=True):
         """ Given a KeyboardModifiers flags object, return whether the Control
         key is down.
 
         Parameters:
         -----------
         include_command : bool, optional (default True)
             Whether to treat the Command key as a (mutually exclusive) synonym
             for Control when in Mac OS.
         """
         # Note that on Mac OS, ControlModifier corresponds to the Command key
         # while MetaModifier corresponds to the Control key.
         if sys.platform == 'darwin':
             down = include_command and (modifiers & QtCore.Qt.ControlModifier)
             return bool(down) ^ bool(modifiers & QtCore.Qt.MetaModifier)
         else:
             return bool(modifiers & QtCore.Qt.ControlModifier)
 
     def _create_control(self):
         """ Creates and connects the underlying text widget.
         """
         # Create the underlying control.
         if self.kind == 'plain':
             control = QtGui.QPlainTextEdit()
         elif self.kind == 'rich':
             control = QtGui.QTextEdit()
             control.setAcceptRichText(False)
 
         # Install event filters. The filter on the viewport is needed for
         # mouse events and drag events.
         control.installEventFilter(self)
         control.viewport().installEventFilter(self)
 
         # Connect signals.
         control.cursorPositionChanged.connect(self._cursor_position_changed)
         control.customContextMenuRequested.connect(
             self._custom_context_menu_requested)
         control.copyAvailable.connect(self.copy_available)
         control.redoAvailable.connect(self.redo_available)
         control.undoAvailable.connect(self.undo_available)
 
         # Hijack the document size change signal to prevent Qt from adjusting
         # the viewport's scrollbar. We are relying on an implementation detail
         # of Q(Plain)TextEdit here, which is potentially dangerous, but without
         # this functionality we cannot create a nice terminal interface.
         layout = control.document().documentLayout()
         layout.documentSizeChanged.disconnect()
         layout.documentSizeChanged.connect(self._adjust_scrollbars)
 
         # Configure the control.
         control.setContextMenuPolicy(QtCore.Qt.CustomContextMenu)
         control.setReadOnly(True)
         control.setUndoRedoEnabled(False)
         control.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOn)
         return control
 
     def _create_page_control(self):
         """ Creates and connects the underlying paging widget.
         """
         if self.kind == 'plain':
             control = QtGui.QPlainTextEdit()
         elif self.kind == 'rich':
             control = QtGui.QTextEdit()
         control.installEventFilter(self)
         control.setReadOnly(True)
         control.setUndoRedoEnabled(False)
         control.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOn)
         return control
 
     def _event_filter_console_keypress(self, event):
         """ Filter key events for the underlying text widget to create a
             console-like interface.
         """
         intercepted = False
         cursor = self._control.textCursor()
         position = cursor.position()
         key = event.key()
         ctrl_down = self._control_key_down(event.modifiers())
         alt_down = event.modifiers() & QtCore.Qt.AltModifier
         shift_down = event.modifiers() & QtCore.Qt.ShiftModifier
 
         #------ Special sequences ----------------------------------------------
 
         if event.matches(QtGui.QKeySequence.Copy):
             self.copy()
             intercepted = True
 
         elif event.matches(QtGui.QKeySequence.Cut):
             self.cut()
             intercepted = True
 
         elif event.matches(QtGui.QKeySequence.Paste):
             self.paste()
             intercepted = True
 
         #------ Special modifier logic -----------------------------------------
 
         elif key in (QtCore.Qt.Key_Return, QtCore.Qt.Key_Enter):
             intercepted = True
                 
             # Special handling when tab completing in text mode.
             self._cancel_text_completion()
 
             if self._in_buffer(position):
                 if self._reading:
                     self._append_plain_text('\n')
                     self._reading = False
                     if self._reading_callback:
                         self._reading_callback()
 
                 # If the input buffer is a single line or there is only
                 # whitespace after the cursor, execute. Otherwise, split the
                 # line with a continuation prompt.
                 elif not self._executing:
                     cursor.movePosition(QtGui.QTextCursor.End,
                                         QtGui.QTextCursor.KeepAnchor)
                     at_end = cursor.selectedText().trimmed().isEmpty() 
                     single_line = (self._get_end_cursor().blockNumber() ==
                                    self._get_prompt_cursor().blockNumber())
                     if (at_end or shift_down or single_line) and not ctrl_down:
                         self.execute(interactive = not shift_down)
                     else:
                         # Do this inside an edit block for clean undo/redo.
                         cursor.beginEditBlock()
                         cursor.setPosition(position)
                         cursor.insertText('\n')
                         self._insert_continuation_prompt(cursor)
                         cursor.endEditBlock()
 
                         # Ensure that the whole input buffer is visible.
                         # FIXME: This will not be usable if the input buffer is
                         # taller than the console widget.
                         self._control.moveCursor(QtGui.QTextCursor.End)
                         self._control.setTextCursor(cursor)
 
         #------ Control/Cmd modifier -------------------------------------------
 
         elif ctrl_down:
             if key == QtCore.Qt.Key_G:
                 self._keyboard_quit()
                 intercepted = True
 
             elif key == QtCore.Qt.Key_K:
                 if self._in_buffer(position):
                     cursor.movePosition(QtGui.QTextCursor.EndOfLine,
                                         QtGui.QTextCursor.KeepAnchor)
                     if not cursor.hasSelection():
                         # Line deletion (remove continuation prompt)
                         cursor.movePosition(QtGui.QTextCursor.NextBlock,
                                             QtGui.QTextCursor.KeepAnchor)
                         cursor.movePosition(QtGui.QTextCursor.Right,
                                             QtGui.QTextCursor.KeepAnchor,
                                             len(self._continuation_prompt))
                     cursor.removeSelectedText()
                 intercepted = True
 
             elif key == QtCore.Qt.Key_L:
                 self.prompt_to_top()
                 intercepted = True
 
             elif key == QtCore.Qt.Key_O:
                 if self._page_control and self._page_control.isVisible():
                     self._page_control.setFocus()
                 intercept = True
 
             elif key == QtCore.Qt.Key_Y:
                 self.paste()
                 intercepted = True
 
             elif key in (QtCore.Qt.Key_Backspace, QtCore.Qt.Key_Delete):
                 intercepted = True
 
         #------ Alt modifier ---------------------------------------------------
 
         elif alt_down:
             if key == QtCore.Qt.Key_B:
                 self._set_cursor(self._get_word_start_cursor(position))
                 intercepted = True
                 
             elif key == QtCore.Qt.Key_F:
                 self._set_cursor(self._get_word_end_cursor(position))
                 intercepted = True
 
             elif key == QtCore.Qt.Key_Backspace:
                 cursor = self._get_word_start_cursor(position)
                 cursor.setPosition(position, QtGui.QTextCursor.KeepAnchor)
                 cursor.removeSelectedText()
                 intercepted = True
 
             elif key == QtCore.Qt.Key_D:
                 cursor = self._get_word_end_cursor(position)
                 cursor.setPosition(position, QtGui.QTextCursor.KeepAnchor)
                 cursor.removeSelectedText()
                 intercepted = True
 
             elif key == QtCore.Qt.Key_Delete:
                 intercepted = True
 
             elif key == QtCore.Qt.Key_Greater:
                 self._control.moveCursor(QtGui.QTextCursor.End)
                 intercepted = True
                 
             elif key == QtCore.Qt.Key_Less:
                 self._control.setTextCursor(self._get_prompt_cursor())
                 intercepted = True
 
         #------ No modifiers ---------------------------------------------------
 
         else:
             if key == QtCore.Qt.Key_Escape:
                 self._keyboard_quit()
                 intercepted = True
 
             elif key == QtCore.Qt.Key_Up:
                 if self._reading or not self._up_pressed():
                     intercepted = True
                 else:
                     prompt_line = self._get_prompt_cursor().blockNumber()
                     intercepted = cursor.blockNumber() <= prompt_line
 
             elif key == QtCore.Qt.Key_Down:
                 if self._reading or not self._down_pressed():
                     intercepted = True
                 else:
                     end_line = self._get_end_cursor().blockNumber()
                     intercepted = cursor.blockNumber() == end_line
 
             elif key == QtCore.Qt.Key_Tab:
                 if not self._reading:
                     intercepted = not self._tab_pressed()
 
             elif key == QtCore.Qt.Key_Left:
 
                 # Move to the previous line
                 line, col = cursor.blockNumber(), cursor.columnNumber()
                 if line > self._get_prompt_cursor().blockNumber() and \
                         col == len(self._continuation_prompt):
                     self._control.moveCursor(QtGui.QTextCursor.PreviousBlock)
                     self._control.moveCursor(QtGui.QTextCursor.EndOfBlock)
                     intercepted = True
 
                 # Regular left movement
                 else:
                     intercepted = not self._in_buffer(position - 1)
                     
             elif key == QtCore.Qt.Key_Right:
                 original_block_number = cursor.blockNumber()
                 cursor.movePosition(QtGui.QTextCursor.Right)
                 if cursor.blockNumber() != original_block_number:
                     cursor.movePosition(QtGui.QTextCursor.Right,
                                         n=len(self._continuation_prompt))
                 self._set_cursor(cursor)
                 intercepted = True
 
             elif key == QtCore.Qt.Key_Home:
                 start_line = cursor.blockNumber()
                 if start_line == self._get_prompt_cursor().blockNumber():
                     start_pos = self._prompt_pos
                 else:
                     cursor.movePosition(QtGui.QTextCursor.StartOfBlock,
                                         QtGui.QTextCursor.KeepAnchor)
                     start_pos = cursor.position()
                     start_pos += len(self._continuation_prompt)
                     cursor.setPosition(position)
                 if shift_down and self._in_buffer(position):
                     cursor.setPosition(start_pos, QtGui.QTextCursor.KeepAnchor)
                 else:
                     cursor.setPosition(start_pos)
                 self._set_cursor(cursor)
                 intercepted = True
 
             elif key == QtCore.Qt.Key_Backspace:
 
                 # Line deletion (remove continuation prompt)
                 line, col = cursor.blockNumber(), cursor.columnNumber()
                 if not self._reading and \
                         col == len(self._continuation_prompt) and \
                         line > self._get_prompt_cursor().blockNumber():
                     cursor.beginEditBlock()
                     cursor.movePosition(QtGui.QTextCursor.StartOfBlock,
                                         QtGui.QTextCursor.KeepAnchor)
                     cursor.removeSelectedText()
                     cursor.deletePreviousChar()
                     cursor.endEditBlock()
                     intercepted = True
 
                 # Regular backwards deletion
                 else:
                     anchor = cursor.anchor()
                     if anchor == position:
                         intercepted = not self._in_buffer(position - 1)
                     else:
                         intercepted = not self._in_buffer(min(anchor, position))
 
             elif key == QtCore.Qt.Key_Delete:
 
                 # Line deletion (remove continuation prompt)
                 if not self._reading and self._in_buffer(position) and \
                         cursor.atBlockEnd() and not cursor.hasSelection():
                     cursor.movePosition(QtGui.QTextCursor.NextBlock,
                                         QtGui.QTextCursor.KeepAnchor)
                     cursor.movePosition(QtGui.QTextCursor.Right,
                                         QtGui.QTextCursor.KeepAnchor,
                                         len(self._continuation_prompt))
                     cursor.removeSelectedText()
                     intercepted = True
 
                 # Regular forwards deletion:
                 else:
                     anchor = cursor.anchor()
                     intercepted = (not self._in_buffer(anchor) or
                                    not self._in_buffer(position))
 
         # Don't move the cursor if control is down to allow copy-paste using
         # the keyboard in any part of the buffer.
         if not ctrl_down:
             self._keep_cursor_in_buffer()
 
         return intercepted
 
     def _event_filter_page_keypress(self, event):
         """ Filter key events for the paging widget to create console-like 
             interface.
         """
         key = event.key()
         ctrl_down = self._control_key_down(event.modifiers())
         alt_down = event.modifiers() & QtCore.Qt.AltModifier
 
         if ctrl_down:
             if key == QtCore.Qt.Key_O:
                 self._control.setFocus()
                 intercept = True
 
         elif alt_down:
             if key == QtCore.Qt.Key_Greater:
                 self._page_control.moveCursor(QtGui.QTextCursor.End)
                 intercepted = True
                 
             elif key == QtCore.Qt.Key_Less:
                 self._page_control.moveCursor(QtGui.QTextCursor.Start)
                 intercepted = True
 
         elif key in (QtCore.Qt.Key_Q, QtCore.Qt.Key_Escape):
             if self._splitter:
                 self._page_control.hide()
             else:
                 self.layout().setCurrentWidget(self._control)
             return True
 
         elif key in (QtCore.Qt.Key_Enter, QtCore.Qt.Key_Return):
             new_event = QtGui.QKeyEvent(QtCore.QEvent.KeyPress, 
                                         QtCore.Qt.Key_PageDown, 
                                         QtCore.Qt.NoModifier)
             QtGui.qApp.sendEvent(self._page_control, new_event)
             return True
 
         elif key == QtCore.Qt.Key_Backspace:
             new_event = QtGui.QKeyEvent(QtCore.QEvent.KeyPress,
                                         QtCore.Qt.Key_PageUp, 
                                         QtCore.Qt.NoModifier)
             QtGui.qApp.sendEvent(self._page_control, new_event)
             return True
 
         return False
 
     def _format_as_columns(self, items, separator='  '):
         """ Transform a list of strings into a single string with columns.
 
         Parameters
         ----------
         items : sequence of strings
             The strings to process.
 
         separator : str, optional [default is two spaces]
             The string that separates columns.
 
         Returns
         -------
         The formatted string.
         """
         # Note: this code is adapted from columnize 0.3.2.
         # See http://code.google.com/p/pycolumnize/
 
         # Calculate the number of characters available.
         width = self._control.viewport().width()
         char_width = QtGui.QFontMetrics(self.font).width(' ')
         displaywidth = max(10, (width / char_width) - 1)
 
         # Some degenerate cases.
         size = len(items)
         if size == 0: 
             return '\n'
         elif size == 1:
             return '%s\n' % items[0]
 
         # Try every row count from 1 upwards
         array_index = lambda nrows, row, col: nrows*col + row
         for nrows in range(1, size):
             ncols = (size + nrows - 1) // nrows
             colwidths = []
             totwidth = -len(separator)
             for col in range(ncols):
                 # Get max column width for this column
                 colwidth = 0
                 for row in range(nrows):
                     i = array_index(nrows, row, col)
                     if i >= size: break
                     x = items[i]
                     colwidth = max(colwidth, len(x))
                 colwidths.append(colwidth)
                 totwidth += colwidth + len(separator)
                 if totwidth > displaywidth: 
                     break
             if totwidth <= displaywidth: 
                 break
 
         # The smallest number of rows computed and the max widths for each
         # column has been obtained. Now we just have to format each of the rows.
         string = ''
         for row in range(nrows):
             texts = []
             for col in range(ncols):
                 i = row + nrows*col
                 if i >= size:
                     texts.append('')
                 else:
                     texts.append(items[i])
             while texts and not texts[-1]:
                 del texts[-1]
             for col in range(len(texts)):
                 texts[col] = texts[col].ljust(colwidths[col])
             string += '%s\n' % separator.join(texts)
         return string
 
     def _get_block_plain_text(self, block):
         """ Given a QTextBlock, return its unformatted text.
         """
         cursor = QtGui.QTextCursor(block)
         cursor.movePosition(QtGui.QTextCursor.StartOfBlock)
         cursor.movePosition(QtGui.QTextCursor.EndOfBlock, 
                             QtGui.QTextCursor.KeepAnchor)
         return unicode(cursor.selection().toPlainText())
 
     def _get_cursor(self):
         """ Convenience method that returns a cursor for the current position.
         """
         return self._control.textCursor()
                 
     def _get_end_cursor(self):
         """ Convenience method that returns a cursor for the last character.
         """
         cursor = self._control.textCursor()
         cursor.movePosition(QtGui.QTextCursor.End)
         return cursor
 
     def _get_input_buffer_cursor_column(self):
         """ Returns the column of the cursor in the input buffer, excluding the
             contribution by the prompt, or -1 if there is no such column.
         """
         prompt = self._get_input_buffer_cursor_prompt()
         if prompt is None:
             return -1
         else:
             cursor = self._control.textCursor()
             return cursor.columnNumber() - len(prompt)
 
     def _get_input_buffer_cursor_line(self):
         """ Returns the text of the line of the input buffer that contains the
             cursor, or None if there is no such line.
         """
         prompt = self._get_input_buffer_cursor_prompt()
         if prompt is None:
             return None
         else:
             cursor = self._control.textCursor()
             text = self._get_block_plain_text(cursor.block())
             return text[len(prompt):]
 
     def _get_input_buffer_cursor_prompt(self):
         """ Returns the (plain text) prompt for line of the input buffer that
             contains the cursor, or None if there is no such line.
         """
         if self._executing:
             return None
         cursor = self._control.textCursor()
         if cursor.position() >= self._prompt_pos:
             if cursor.blockNumber() == self._get_prompt_cursor().blockNumber():
                 return self._prompt
             else:
                 return self._continuation_prompt
         else:
             return None
 
     def _get_prompt_cursor(self):
         """ Convenience method that returns a cursor for the prompt position.
         """
         cursor = self._control.textCursor()
         cursor.setPosition(self._prompt_pos)
         return cursor
 
     def _get_selection_cursor(self, start, end):
         """ Convenience method that returns a cursor with text selected between
             the positions 'start' and 'end'.
         """
         cursor = self._control.textCursor()
         cursor.setPosition(start)
         cursor.setPosition(end, QtGui.QTextCursor.KeepAnchor)
         return cursor
 
     def _get_word_start_cursor(self, position):
         """ Find the start of the word to the left the given position. If a
             sequence of non-word characters precedes the first word, skip over
             them. (This emulates the behavior of bash, emacs, etc.)
         """
         document = self._control.document()
         position -= 1
         while position >= self._prompt_pos and \
                 not document.characterAt(position).isLetterOrNumber():
             position -= 1
         while position >= self._prompt_pos and \
                 document.characterAt(position).isLetterOrNumber():
             position -= 1
         cursor = self._control.textCursor()
         cursor.setPosition(position + 1)
         return cursor
 
     def _get_word_end_cursor(self, position):
         """ Find the end of the word to the right the given position. If a
             sequence of non-word characters precedes the first word, skip over
             them. (This emulates the behavior of bash, emacs, etc.)
         """
         document = self._control.document()
         end = self._get_end_cursor().position()
         while position < end and \
                 not document.characterAt(position).isLetterOrNumber():
             position += 1
         while position < end and \
                 document.characterAt(position).isLetterOrNumber():
             position += 1
         cursor = self._control.textCursor()
         cursor.setPosition(position)
         return cursor
 
     def _insert_continuation_prompt(self, cursor):
         """ Inserts new continuation prompt using the specified cursor.
         """
         if self._continuation_prompt_html is None:
             self._insert_plain_text(cursor, self._continuation_prompt)
         else:
             self._continuation_prompt = self._insert_html_fetching_plain_text(
                 cursor, self._continuation_prompt_html)
 
     def _insert_html(self, cursor, html):
         """ Inserts HTML using the specified cursor in such a way that future
             formatting is unaffected.
         """
         cursor.beginEditBlock()
         cursor.insertHtml(html)
 
         # After inserting HTML, the text document "remembers" it's in "html
         # mode", which means that subsequent calls adding plain text will result
         # in unwanted formatting, lost tab characters, etc. The following code
         # hacks around this behavior, which I consider to be a bug in Qt, by
         # (crudely) resetting the document's style state.
         cursor.movePosition(QtGui.QTextCursor.Left,
                             QtGui.QTextCursor.KeepAnchor)
         if cursor.selection().toPlainText() == ' ':
             cursor.removeSelectedText()
         else:
             cursor.movePosition(QtGui.QTextCursor.Right)
         cursor.insertText(' ', QtGui.QTextCharFormat())
         cursor.endEditBlock()
 
     def _insert_html_fetching_plain_text(self, cursor, html):
         """ Inserts HTML using the specified cursor, then returns its plain text
             version.
         """
         cursor.beginEditBlock()
         cursor.removeSelectedText()
 
         start = cursor.position()
         self._insert_html(cursor, html)
         end = cursor.position()
         cursor.setPosition(start, QtGui.QTextCursor.KeepAnchor)
         text = unicode(cursor.selection().toPlainText())
 
         cursor.setPosition(end)
         cursor.endEditBlock()
         return text
 
     def _insert_plain_text(self, cursor, text):
         """ Inserts plain text using the specified cursor, processing ANSI codes
             if enabled.
         """
         cursor.beginEditBlock()
         if self.ansi_codes:
             for substring in self._ansi_processor.split_string(text):
                 for act in self._ansi_processor.actions:
 
                     # Unlike real terminal emulators, we don't distinguish
                     # between the screen and the scrollback buffer. A screen
                     # erase request clears everything.
                     if act.action == 'erase' and act.area == 'screen':
                         cursor.select(QtGui.QTextCursor.Document)
                         cursor.removeSelectedText()
 
                     # Simulate a form feed by scrolling just past the last line.
                     elif act.action == 'scroll' and act.unit == 'page':
                         cursor.insertText('\n')
                         cursor.endEditBlock()
                         self._set_top_cursor(cursor)
                         cursor.joinPreviousEditBlock()
                         cursor.deletePreviousChar()
                         
                 format = self._ansi_processor.get_format()
                 cursor.insertText(substring, format)
         else:
             cursor.insertText(text)
         cursor.endEditBlock()
 
     def _insert_plain_text_into_buffer(self, text):
         """ Inserts text into the input buffer at the current cursor position,
             ensuring that continuation prompts are inserted as necessary.
         """
         lines = unicode(text).splitlines(True)
         if lines:
             self._keep_cursor_in_buffer()
             cursor = self._control.textCursor()
             cursor.beginEditBlock()
             cursor.insertText(lines[0])
             for line in lines[1:]:
                 if self._continuation_prompt_html is None:
                     cursor.insertText(self._continuation_prompt)
                 else:
                     self._continuation_prompt = \
                         self._insert_html_fetching_plain_text(
                             cursor, self._continuation_prompt_html)
                 cursor.insertText(line)
             cursor.endEditBlock()
             self._control.setTextCursor(cursor)
 
     def _in_buffer(self, position=None):
         """ Returns whether the current cursor (or, if specified, a position) is
             inside the editing region.
         """
         cursor = self._control.textCursor()
         if position is None:
             position = cursor.position()
         else:
             cursor.setPosition(position)
         line = cursor.blockNumber()
         prompt_line = self._get_prompt_cursor().blockNumber()
         if line == prompt_line:
             return position >= self._prompt_pos
         elif line > prompt_line:
             cursor.movePosition(QtGui.QTextCursor.StartOfBlock)
             prompt_pos = cursor.position() + len(self._continuation_prompt)
             return position >= prompt_pos
         return False
 
     def _keep_cursor_in_buffer(self):
         """ Ensures that the cursor is inside the editing region. Returns
             whether the cursor was moved.
         """
         moved = not self._in_buffer()
         if moved:
             cursor = self._control.textCursor()
             cursor.movePosition(QtGui.QTextCursor.End)
             self._control.setTextCursor(cursor)
         return moved
 
     def _keyboard_quit(self):
         """ Cancels the current editing task ala Ctrl-G in Emacs.
         """
         if self._text_completing_pos:
             self._cancel_text_completion()
         else:
             self.input_buffer = ''
         
     def _page(self, text, html=False):
         """ Displays text using the pager if it exceeds the height of the
         viewport.
 
         Parameters:
         -----------
         html : bool, optional (default False)
             If set, the text will be interpreted as HTML instead of plain text.
         """
         line_height = QtGui.QFontMetrics(self.font).height()
         minlines = self._control.viewport().height() / line_height
         if self.paging != 'none' and \
                 re.match("(?:[^\n]*\n){%i}" % minlines, text):
             if self.paging == 'custom':
                 self.custom_page_requested.emit(text)
             else:
                 self._page_control.clear()
                 cursor = self._page_control.textCursor()
                 if html:
                     self._insert_html(cursor, text)
                 else:
                     self._insert_plain_text(cursor, text)
                 self._page_control.moveCursor(QtGui.QTextCursor.Start)
 
                 self._page_control.viewport().resize(self._control.size())
                 if self._splitter:
                     self._page_control.show()
                     self._page_control.setFocus()
                 else:
                     self.layout().setCurrentWidget(self._page_control)
         elif html:
             self._append_plain_html(text)
         else:
             self._append_plain_text(text)
 
     def _prompt_finished(self):
         """ Called immediately after a prompt is finished, i.e. when some input
             will be processed and a new prompt displayed.
         """
         # Flush all state from the input splitter so the next round of
         # reading input starts with a clean buffer.
         self._input_splitter.reset()
 
         self._control.setReadOnly(True)
         self._prompt_finished_hook()
 
     def _prompt_started(self):
         """ Called immediately after a new prompt is displayed.
         """
         # Temporarily disable the maximum block count to permit undo/redo and 
         # to ensure that the prompt position does not change due to truncation.
         self._control.document().setMaximumBlockCount(0)
         self._control.setUndoRedoEnabled(True)
 
         self._control.setReadOnly(False)
         self._control.moveCursor(QtGui.QTextCursor.End)
         self._executing = False
         self._prompt_started_hook()
 
     def _readline(self, prompt='', callback=None):
         """ Reads one line of input from the user. 
 
         Parameters
         ----------
         prompt : str, optional
             The prompt to print before reading the line.
 
         callback : callable, optional
             A callback to execute with the read line. If not specified, input is
             read *synchronously* and this method does not return until it has
             been read.
 
         Returns
         -------
         If a callback is specified, returns nothing. Otherwise, returns the
         input string with the trailing newline stripped.
         """
         if self._reading:
             raise RuntimeError('Cannot read a line. Widget is already reading.')
 
         if not callback and not self.isVisible():
             # If the user cannot see the widget, this function cannot return.
             raise RuntimeError('Cannot synchronously read a line if the widget '
                                'is not visible!')
 
         self._reading = True
         self._show_prompt(prompt, newline=False)
 
         if callback is None:
             self._reading_callback = None
             while self._reading:
                 QtCore.QCoreApplication.processEvents()
             return self.input_buffer.rstrip('\n')
 
         else:
             self._reading_callback = lambda: \
                 callback(self.input_buffer.rstrip('\n'))
 
     def _set_continuation_prompt(self, prompt, html=False):
         """ Sets the continuation prompt.
 
         Parameters
         ----------
         prompt : str
             The prompt to show when more input is needed.
 
         html : bool, optional (default False)
             If set, the prompt will be inserted as formatted HTML. Otherwise,
             the prompt will be treated as plain text, though ANSI color codes
             will be handled.
         """
         if html:
             self._continuation_prompt_html = prompt
         else:
             self._continuation_prompt = prompt
             self._continuation_prompt_html = None
             
     def _set_cursor(self, cursor):
         """ Convenience method to set the current cursor.
         """
         self._control.setTextCursor(cursor)
 
     def _set_top_cursor(self, cursor):
         """ Scrolls the viewport so that the specified cursor is at the top.
         """
         scrollbar = self._control.verticalScrollBar()
         scrollbar.setValue(scrollbar.maximum())
         original_cursor = self._control.textCursor()
         self._control.setTextCursor(cursor)
         self._control.ensureCursorVisible()
         self._control.setTextCursor(original_cursor)
 
     def _show_prompt(self, prompt=None, html=False, newline=True):
         """ Writes a new prompt at the end of the buffer.
 
         Parameters
         ----------
         prompt : str, optional
             The prompt to show. If not specified, the previous prompt is used.
 
         html : bool, optional (default False)
             Only relevant when a prompt is specified. If set, the prompt will
             be inserted as formatted HTML. Otherwise, the prompt will be treated
             as plain text, though ANSI color codes will be handled.
 
         newline : bool, optional (default True)
             If set, a new line will be written before showing the prompt if 
             there is not already a newline at the end of the buffer.
         """
         # Insert a preliminary newline, if necessary.
         if newline:
             cursor = self._get_end_cursor()
             if cursor.position() > 0:
                 cursor.movePosition(QtGui.QTextCursor.Left, 
                                     QtGui.QTextCursor.KeepAnchor)
                 if unicode(cursor.selection().toPlainText()) != '\n':
                     self._append_plain_text('\n')
 
         # Write the prompt.
         self._append_plain_text(self._prompt_sep)
         if prompt is None:
             if self._prompt_html is None:
                 self._append_plain_text(self._prompt)
             else:
                 self._append_html(self._prompt_html)
         else:
             if html:
                 self._prompt = self._append_html_fetching_plain_text(prompt)
                 self._prompt_html = prompt
             else:
                 self._append_plain_text(prompt)
                 self._prompt = prompt
                 self._prompt_html = None
 
         self._prompt_pos = self._get_end_cursor().position()
         self._prompt_started()
 
     #------ Signal handlers ----------------------------------------------------
 
     def _adjust_scrollbars(self):
         """ Expands the vertical scrollbar beyond the range set by Qt.
         """
         # This code is adapted from _q_adjustScrollbars in qplaintextedit.cpp
         # and qtextedit.cpp.
         document = self._control.document()
         scrollbar = self._control.verticalScrollBar()
         viewport_height = self._control.viewport().height()
         if isinstance(self._control, QtGui.QPlainTextEdit):
             maximum = max(0, document.lineCount() - 1)
             step = viewport_height / self._control.fontMetrics().lineSpacing()
         else:
             # QTextEdit does not do line-based layout and blocks will not in
             # general have the same height. Therefore it does not make sense to
             # attempt to scroll in line height increments.
             maximum = document.size().height()
             step = viewport_height
         diff = maximum - scrollbar.maximum()
         scrollbar.setRange(0, maximum)
         scrollbar.setPageStep(step)
         # Compensate for undesirable scrolling that occurs automatically due to
         # maximumBlockCount() text truncation.
         if diff < 0 and document.blockCount() == document.maximumBlockCount():
             scrollbar.setValue(scrollbar.value() + diff)
 
     def _cursor_position_changed(self):
         """ Clears the temporary buffer based on the cursor position.
         """
         if self._text_completing_pos:
             document = self._control.document()
             if self._text_completing_pos < document.characterCount():
                 cursor = self._control.textCursor()
                 pos = cursor.position()
                 text_cursor = self._control.textCursor()
                 text_cursor.setPosition(self._text_completing_pos)
                 if pos < self._text_completing_pos or \
                         cursor.blockNumber() > text_cursor.blockNumber():
                     self._clear_temporary_buffer()
                     self._text_completing_pos = 0
             else:
                 self._clear_temporary_buffer()
                 self._text_completing_pos = 0
 
     def _custom_context_menu_requested(self, pos):
         """ Shows a context menu at the given QPoint (in widget coordinates).
         """
         menu = self._context_menu_make(pos)
         menu.exec_(self._control.mapToGlobal(pos))

IPython/frontend/qt/console/frontend_widget.py

@@ -1,548 +1,549
 from __future__ import print_function
 
 # Standard library imports
 from collections import namedtuple
 import sys
 
 # System library imports
 from pygments.lexers import PythonLexer
 from PyQt4 import QtCore, QtGui
 
 # Local imports
 from IPython.core.inputsplitter import InputSplitter, transform_classic_prompt
 from IPython.frontend.qt.base_frontend_mixin import BaseFrontendMixin
 from IPython.utils.traitlets import Bool
 from bracket_matcher import BracketMatcher
 from call_tip_widget import CallTipWidget
 from completion_lexer import CompletionLexer
 from history_console_widget import HistoryConsoleWidget
 from pygments_highlighter import PygmentsHighlighter
 
 
 class FrontendHighlighter(PygmentsHighlighter):
     """ A PygmentsHighlighter that can be turned on and off and that ignores
         prompts.
     """
 
     def __init__(self, frontend):
         super(FrontendHighlighter, self).__init__(frontend._control.document())
         self._current_offset = 0
         self._frontend = frontend
         self.highlighting_on = False
 
     def highlightBlock(self, qstring):
         """ Highlight a block of text. Reimplemented to highlight selectively.
         """
         if not self.highlighting_on:
             return
 
         # The input to this function is unicode string that may contain
         # paragraph break characters, non-breaking spaces, etc. Here we acquire
         # the string as plain text so we can compare it.
         current_block = self.currentBlock()
         string = self._frontend._get_block_plain_text(current_block)
 
         # Decide whether to check for the regular or continuation prompt.
         if current_block.contains(self._frontend._prompt_pos):
             prompt = self._frontend._prompt
         else:
             prompt = self._frontend._continuation_prompt
 
         # Don't highlight the part of the string that contains the prompt.
         if string.startswith(prompt):
             self._current_offset = len(prompt)
             qstring.remove(0, len(prompt))
         else:
             self._current_offset = 0
 
         PygmentsHighlighter.highlightBlock(self, qstring)
 
     def rehighlightBlock(self, block):
         """ Reimplemented to temporarily enable highlighting if disabled.
         """
         old = self.highlighting_on
         self.highlighting_on = True
         super(FrontendHighlighter, self).rehighlightBlock(block)
         self.highlighting_on = old
 
     def setFormat(self, start, count, format):
         """ Reimplemented to highlight selectively.
         """
         start += self._current_offset
         PygmentsHighlighter.setFormat(self, start, count, format)
 
 
 class FrontendWidget(HistoryConsoleWidget, BaseFrontendMixin):
     """ A Qt frontend for a generic Python kernel.
     """
 
     # An option and corresponding signal for overriding the default kernel
     # interrupt behavior.
     custom_interrupt = Bool(False)
     custom_interrupt_requested = QtCore.pyqtSignal()
 
     # An option and corresponding signals for overriding the default kernel
     # restart behavior.
     custom_restart = Bool(False)
     custom_restart_kernel_died = QtCore.pyqtSignal(float)
     custom_restart_requested = QtCore.pyqtSignal()
    
     # Emitted when an 'execute_reply' has been received from the kernel and
     # processed by the FrontendWidget.
     executed = QtCore.pyqtSignal(object)
 
     # Emitted when an exit request has been received from the kernel.
     exit_requested = QtCore.pyqtSignal()
     
     # Protected class variables.
     _CallTipRequest = namedtuple('_CallTipRequest', ['id', 'pos'])
     _CompletionRequest = namedtuple('_CompletionRequest', ['id', 'pos'])
     _ExecutionRequest = namedtuple('_ExecutionRequest', ['id', 'kind'])
     _input_splitter_class = InputSplitter
 
     #---------------------------------------------------------------------------
     # 'object' interface
     #---------------------------------------------------------------------------
     
     def __init__(self, *args, **kw):
         super(FrontendWidget, self).__init__(*args, **kw)
 
         # FrontendWidget protected variables.
         self._bracket_matcher = BracketMatcher(self._control)
         self._call_tip_widget = CallTipWidget(self._control)
         self._completion_lexer = CompletionLexer(PythonLexer())
         self._copy_raw_action = QtGui.QAction('Copy (Raw Text)', None)
         self._hidden = False
         self._highlighter = FrontendHighlighter(self)
         self._input_splitter = self._input_splitter_class(input_mode='cell')
         self._kernel_manager = None
-        self._possible_kernel_restart = False
         self._request_info = {}
 
         # Configure the ConsoleWidget.
         self.tab_width = 4
         self._set_continuation_prompt('... ')
 
         # Configure the CallTipWidget.
         self._call_tip_widget.setFont(self.font)
         self.font_changed.connect(self._call_tip_widget.setFont)
 
         # Configure actions.
         action = self._copy_raw_action
         key = QtCore.Qt.CTRL | QtCore.Qt.SHIFT | QtCore.Qt.Key_C
         action.setEnabled(False)
         action.setShortcut(QtGui.QKeySequence(key))
         action.setShortcutContext(QtCore.Qt.WidgetWithChildrenShortcut)
         action.triggered.connect(self.copy_raw)
         self.copy_available.connect(action.setEnabled)
         self.addAction(action)
 
         # Connect signal handlers.
         document = self._control.document()
         document.contentsChange.connect(self._document_contents_change)
 
     #---------------------------------------------------------------------------
     # 'ConsoleWidget' public interface
     #---------------------------------------------------------------------------
 
     def copy(self):
         """ Copy the currently selected text to the clipboard, removing prompts.
         """
         text = unicode(self._control.textCursor().selection().toPlainText())
         if text:
             lines = map(transform_classic_prompt, text.splitlines())
             text = '\n'.join(lines)
             QtGui.QApplication.clipboard().setText(text)
 
     #---------------------------------------------------------------------------
     # 'ConsoleWidget' abstract interface
     #---------------------------------------------------------------------------
 
     def _is_complete(self, source, interactive):
         """ Returns whether 'source' can be completely processed and a new
             prompt created. When triggered by an Enter/Return key press,
             'interactive' is True; otherwise, it is False.
         """
         complete = self._input_splitter.push(source)
         if interactive:
             complete = not self._input_splitter.push_accepts_more()
         return complete
 
     def _execute(self, source, hidden):
         """ Execute 'source'. If 'hidden', do not show any output.
 
         See parent class :meth:`execute` docstring for full details.
         """
         msg_id = self.kernel_manager.xreq_channel.execute(source, hidden)
         self._request_info['execute'] = self._ExecutionRequest(msg_id, 'user')
         self._hidden = hidden
         
     def _prompt_started_hook(self):
         """ Called immediately after a new prompt is displayed.
         """
         if not self._reading:
             self._highlighter.highlighting_on = True
 
     def _prompt_finished_hook(self):
         """ Called immediately after a prompt is finished, i.e. when some input
             will be processed and a new prompt displayed.
         """
         if not self._reading:
             self._highlighter.highlighting_on = False
 
     def _tab_pressed(self):
         """ Called when the tab key is pressed. Returns whether to continue
             processing the event.
         """
         # Perform tab completion if:
         # 1) The cursor is in the input buffer.
         # 2) There is a non-whitespace character before the cursor.
         text = self._get_input_buffer_cursor_line()
         if text is None:
             return False
         complete = bool(text[:self._get_input_buffer_cursor_column()].strip())
         if complete:
             self._complete()
         return not complete
 
     #---------------------------------------------------------------------------
     # 'ConsoleWidget' protected interface
     #---------------------------------------------------------------------------
 
     def _context_menu_make(self, pos):
         """ Reimplemented to add an action for raw copy.
         """
         menu = super(FrontendWidget, self)._context_menu_make(pos)
         for before_action in menu.actions():
             if before_action.shortcut().matches(QtGui.QKeySequence.Paste) == \
                     QtGui.QKeySequence.ExactMatch:
                 menu.insertAction(before_action, self._copy_raw_action)
                 break
         return menu
 
     def _event_filter_console_keypress(self, event):
         """ Reimplemented for execution interruption and smart backspace.
         """
         key = event.key()
         if self._control_key_down(event.modifiers(), include_command=False):
 
             if key == QtCore.Qt.Key_C and self._executing:
                 self.interrupt_kernel()
                 return True
 
             elif key == QtCore.Qt.Key_Period:
                 message = 'Are you sure you want to restart the kernel?'
                 self.restart_kernel(message, now=False)
                 return True
 
         elif not event.modifiers() & QtCore.Qt.AltModifier:
 
             # Smart backspace: remove four characters in one backspace if:
             # 1) everything left of the cursor is whitespace
             # 2) the four characters immediately left of the cursor are spaces
             if key == QtCore.Qt.Key_Backspace:
                 col = self._get_input_buffer_cursor_column()
                 cursor = self._control.textCursor()
                 if col > 3 and not cursor.hasSelection():
                     text = self._get_input_buffer_cursor_line()[:col]
                     if text.endswith('    ') and not text.strip():
                         cursor.movePosition(QtGui.QTextCursor.Left,
                                             QtGui.QTextCursor.KeepAnchor, 4)
                         cursor.removeSelectedText()
                         return True
 
         return super(FrontendWidget, self)._event_filter_console_keypress(event)
 
     def _insert_continuation_prompt(self, cursor):
         """ Reimplemented for auto-indentation.
         """
         super(FrontendWidget, self)._insert_continuation_prompt(cursor)
         cursor.insertText(' ' * self._input_splitter.indent_spaces)
 
     #---------------------------------------------------------------------------
     # 'BaseFrontendMixin' abstract interface
     #---------------------------------------------------------------------------
 
     def _handle_complete_reply(self, rep):
         """ Handle replies for tab completion.
         """
         cursor = self._get_cursor()
         info = self._request_info.get('complete')
         if info and info.id == rep['parent_header']['msg_id'] and \
                 info.pos == cursor.position():
             text = '.'.join(self._get_context())
             cursor.movePosition(QtGui.QTextCursor.Left, n=len(text))
             self._complete_with_items(cursor, rep['content']['matches'])
 
     def _handle_execute_reply(self, msg):
         """ Handles replies for code execution.
         """
         info = self._request_info.get('execute')
         if info and info.id == msg['parent_header']['msg_id'] and \
                 info.kind == 'user' and not self._hidden:
             # Make sure that all output from the SUB channel has been processed
             # before writing a new prompt.
             self.kernel_manager.sub_channel.flush()
 
             # Reset the ANSI style information to prevent bad text in stdout
             # from messing up our colors. We're not a true terminal so we're
             # allowed to do this.
             if self.ansi_codes:
                 self._ansi_processor.reset_sgr()
 
             content = msg['content']
             status = content['status']
             if status == 'ok':
                 self._process_execute_ok(msg)
             elif status == 'error':
                 self._process_execute_error(msg)
             elif status == 'abort':
                 self._process_execute_abort(msg)
 
             self._show_interpreter_prompt_for_reply(msg)
             self.executed.emit(msg)
 
     def _handle_input_request(self, msg):
         """ Handle requests for raw_input.
         """
         if self._hidden:
             raise RuntimeError('Request for raw input during hidden execution.')
 
         # Make sure that all output from the SUB channel has been processed
         # before entering readline mode.
         self.kernel_manager.sub_channel.flush()
 
         def callback(line):
             self.kernel_manager.rep_channel.input(line)
         self._readline(msg['content']['prompt'], callback=callback)
 
     def _handle_kernel_died(self, since_last_heartbeat):
         """ Handle the kernel's death by asking if the user wants to restart.
         """
-        message = 'The kernel heartbeat has been inactive for %.2f ' \
-                  'seconds. Do you want to restart the kernel? You may ' \
-                  'first want to check the network connection.' % \
-                  since_last_heartbeat
         if self.custom_restart:
             self.custom_restart_kernel_died.emit(since_last_heartbeat)
         else:
+            message = 'The kernel heartbeat has been inactive for %.2f ' \
+                'seconds. Do you want to restart the kernel? You may ' \
+                'first want to check the network connection.' % \
+                since_last_heartbeat
             self.restart_kernel(message, now=True)
 
     def _handle_object_info_reply(self, rep):
         """ Handle replies for call tips.
         """
         cursor = self._get_cursor()
         info = self._request_info.get('call_tip')
         if info and info.id == rep['parent_header']['msg_id'] and \
                 info.pos == cursor.position():
             doc = rep['content']['docstring']
             if doc:
                 self._call_tip_widget.show_docstring(doc)
 
     def _handle_pyout(self, msg):
         """ Handle display hook output.
         """
         if not self._hidden and self._is_from_this_session(msg):
             self._append_plain_text(msg['content']['data'] + '\n')
 
     def _handle_stream(self, msg):
         """ Handle stdout, stderr, and stdin.
         """
         if not self._hidden and self._is_from_this_session(msg):
             # Most consoles treat tabs as being 8 space characters. Convert tabs
             # to spaces so that output looks as expected regardless of this
             # widget's tab width.
             text = msg['content']['data'].expandtabs(8)
             
             self._append_plain_text(text)
             self._control.moveCursor(QtGui.QTextCursor.End)
     
     def _started_channels(self):
         """ Called when the KernelManager channels have started listening or 
             when the frontend is assigned an already listening KernelManager.
         """
-        self._control.clear()
-        self._append_plain_text(self._get_banner())
-        self._show_interpreter_prompt()
-
-    def _stopped_channels(self):
-        """ Called when the KernelManager channels have stopped listening or
-            when a listening KernelManager is removed from the frontend.
-        """
-        self._executing = self._reading = False
-        self._highlighter.highlighting_on = False
+        self.reset()
 
     #---------------------------------------------------------------------------
     # 'FrontendWidget' public interface
     #---------------------------------------------------------------------------
 
     def copy_raw(self):
         """ Copy the currently selected text to the clipboard without attempting
             to remove prompts or otherwise alter the text.
         """
         self._control.copy()
 
     def execute_file(self, path, hidden=False):
         """ Attempts to execute file with 'path'. If 'hidden', no output is
             shown.
         """
         self.execute('execfile("%s")' % path, hidden=hidden)
 
     def interrupt_kernel(self):
         """ Attempts to interrupt the running kernel.
         """
         if self.custom_interrupt:
             self.custom_interrupt_requested.emit()
         elif self.kernel_manager.has_kernel:
             self.kernel_manager.interrupt_kernel()
         else:
             self._append_plain_text('Kernel process is either remote or '
                                     'unspecified. Cannot interrupt.\n')
 
+    def reset(self):
+        """ Resets the widget to its initial state. Similar to ``clear``, but
+            also re-writes the banner and aborts execution if necessary.
+        """ 
+        if self._executing:
+            self._executing = False
+            self._request_info['execute'] = None
+        self._reading = False
+        self._highlighter.highlighting_on = False
+
+        self._control.clear()
+        self._append_plain_text(self._get_banner())
+        self._show_interpreter_prompt()
+
     def restart_kernel(self, message, now=False):
         """ Attempts to restart the running kernel.
         """
-        # FIXME: now should be configurable via a checkbox in the
-        # dialog.  Right now at least the heartbeat path sets it to True and
-        # the manual restart to False.  But those should just be the
-        # pre-selected states of a checkbox that the user could override if so
-        # desired.  But I don't know enough Qt to go implementing the checkbox
-        # now.
-
-        # We want to make sure that if this dialog is already happening, that
-        # other signals don't trigger it again. This can happen when the 
-        # kernel_died heartbeat signal is emitted and the user is slow to
-        # respond to the dialog.
-        if not self._possible_kernel_restart:
-            if self.custom_restart:
-                self.custom_restart_requested.emit()
-            elif self.kernel_manager.has_kernel:
-                # Setting this to True will prevent this logic from happening
-                # again until the current pass is completed.
-                self._possible_kernel_restart = True
-                buttons = QtGui.QMessageBox.Yes | QtGui.QMessageBox.No
-                result = QtGui.QMessageBox.question(self, 'Restart kernel?',
-                                                    message, buttons)
-                if result == QtGui.QMessageBox.Yes:
-                    try:
-                        self.kernel_manager.restart_kernel(now=now)
-                    except RuntimeError:
-                        message = 'Kernel started externally. Cannot restart.\n'
-                        self._append_plain_text(message)
-                    else:
-                        self._stopped_channels()
-                        self._append_plain_text('Kernel restarting...\n')
-                        self._show_interpreter_prompt()
-                # This might need to be moved to another location?
-                self._possible_kernel_restart = False
+        # FIXME: now should be configurable via a checkbox in the dialog.  Right
+        # now at least the heartbeat path sets it to True and the manual restart
+        # to False.  But those should just be the pre-selected states of a
+        # checkbox that the user could override if so desired.  But I don't know
+        # enough Qt to go implementing the checkbox now.
+
+        if self.custom_restart:
+            self.custom_restart_requested.emit()
+
+        elif self.kernel_manager.has_kernel:
+            # Pause the heart beat channel to prevent further warnings.
+            self.kernel_manager.hb_channel.pause()
+
+            # Prompt the user to restart the kernel. Un-pause the heartbeat if
+            # they decline. (If they accept, the heartbeat will be un-paused
+            # automatically when the kernel is restarted.)
+            buttons = QtGui.QMessageBox.Yes | QtGui.QMessageBox.No
+            result = QtGui.QMessageBox.question(self, 'Restart kernel?',
+                                                message, buttons)
+            if result == QtGui.QMessageBox.Yes:
+                try:
+                    self.kernel_manager.restart_kernel(now=now)
+                except RuntimeError:
+                    self._append_plain_text('Kernel started externally. '
+                                            'Cannot restart.\n')
+                else:
+                    self.reset()
             else:
-                self._append_plain_text('Kernel process is either remote or '
-                                        'unspecified. Cannot restart.\n')
+                self.kernel_manager.hb_channel.unpause()
+
+        else:
+            self._append_plain_text('Kernel process is either remote or '
+                                    'unspecified. Cannot restart.\n')
 
     #---------------------------------------------------------------------------
     # 'FrontendWidget' protected interface
     #---------------------------------------------------------------------------
 
     def _call_tip(self):
         """ Shows a call tip, if appropriate, at the current cursor location.
         """
         # Decide if it makes sense to show a call tip
         cursor = self._get_cursor()
         cursor.movePosition(QtGui.QTextCursor.Left)
         if cursor.document().characterAt(cursor.position()).toAscii() != '(':
             return False
         context = self._get_context(cursor)
         if not context:
             return False
 
         # Send the metadata request to the kernel
         name = '.'.join(context)
         msg_id = self.kernel_manager.xreq_channel.object_info(name)
         pos = self._get_cursor().position()
         self._request_info['call_tip'] = self._CallTipRequest(msg_id, pos)
         return True
 
     def _complete(self):
         """ Performs completion at the current cursor location.
         """
         context = self._get_context()
         if context:
             # Send the completion request to the kernel
             msg_id = self.kernel_manager.xreq_channel.complete(
                 '.'.join(context),                       # text
                 self._get_input_buffer_cursor_line(),    # line
                 self._get_input_buffer_cursor_column(),  # cursor_pos
                 self.input_buffer)                       # block 
             pos = self._get_cursor().position()
             info = self._CompletionRequest(msg_id, pos)
             self._request_info['complete'] = info
 
     def _get_banner(self):
         """ Gets a banner to display at the beginning of a session.
         """
         banner = 'Python %s on %s\nType "help", "copyright", "credits" or ' \
             '"license" for more information.'
         return banner % (sys.version, sys.platform)
 
     def _get_context(self, cursor=None):
         """ Gets the context for the specified cursor (or the current cursor
             if none is specified).
         """
         if cursor is None:
             cursor = self._get_cursor()
         cursor.movePosition(QtGui.QTextCursor.StartOfBlock, 
                             QtGui.QTextCursor.KeepAnchor)
         text = unicode(cursor.selection().toPlainText())
         return self._completion_lexer.get_context(text)
 
     def _process_execute_abort(self, msg):
         """ Process a reply for an aborted execution request.
         """
         self._append_plain_text("ERROR: execution aborted\n")
 
     def _process_execute_error(self, msg):
         """ Process a reply for an execution request that resulted in an error.
         """
         content = msg['content']
         traceback = ''.join(content['traceback'])
         self._append_plain_text(traceback)
 
     def _process_execute_ok(self, msg):
         """ Process a reply for a successful execution equest.
         """
         payload = msg['content']['payload']
         for item in payload:
             if not self._process_execute_payload(item):
                 warning = 'Warning: received unknown payload of type %s'
                 print(warning % repr(item['source']))
 
     def _process_execute_payload(self, item):
         """ Process a single payload item from the list of payload items in an
             execution reply. Returns whether the payload was handled.
         """
         # The basic FrontendWidget doesn't handle payloads, as they are a
         # mechanism for going beyond the standard Python interpreter model.
         return False
 
     def _show_interpreter_prompt(self):
         """ Shows a prompt for the interpreter.
         """
         self._show_prompt('>>> ')
 
     def _show_interpreter_prompt_for_reply(self, msg):
         """ Shows a prompt for the interpreter given an 'execute_reply' message.
         """
         self._show_interpreter_prompt()
 
     #------ Signal handlers ----------------------------------------------------
 
     def _document_contents_change(self, position, removed, added):
         """ Called whenever the document's content changes. Display a call tip
             if appropriate.
         """
         # Calculate where the cursor should be *after* the change:
         position += added
 
         document = self._control.document()
         if position == self._get_cursor().position():
             self._call_tip()