Indent on new line by looking at the text before the cursor.

that would allow edit like the following to insert the right amount of
space on the newline (`|` is the cursor)

    class Bar:
        def __init__(self):|<enter>
            self.filed=0

Instead of inserting a blank line with 0 space.

Closes  #9283

IPython/terminal/shortcuts.py

@@ -1,203 +1,205 b''
 import signal
 import sys
 
 from prompt_toolkit.enums import DEFAULT_BUFFER, SEARCH_BUFFER
 from prompt_toolkit.filters import (HasFocus, HasSelection, Condition,
     ViInsertMode, EmacsInsertMode, HasCompletions)
 from prompt_toolkit.filters.cli import ViMode, ViNavigationMode
 from prompt_toolkit.keys import Keys
 from prompt_toolkit.key_binding.bindings.completion import display_completions_like_readline
 
 from IPython.utils.decorators import undoc
 
 @Condition
 def cursor_in_leading_ws(cli):
     before = cli.application.buffer.document.current_line_before_cursor
     return (not before) or before.isspace()
 
 def register_ipython_shortcuts(registry, shell):
     """Set up the prompt_toolkit keyboard shortcuts for IPython"""
     insert_mode = ViInsertMode() | EmacsInsertMode()
 
     # Ctrl+J == Enter, seemingly
     registry.add_binding(Keys.ControlJ,
                          filter=(HasFocus(DEFAULT_BUFFER)
                                  & ~HasSelection()
                                  & insert_mode
                         ))(newline_or_execute_outer(shell))
 
     registry.add_binding(Keys.ControlBackslash)(force_exit)
 
     registry.add_binding(Keys.ControlP,
                          filter=(ViInsertMode() & HasFocus(DEFAULT_BUFFER)
                         ))(previous_history_or_previous_completion)
 
     registry.add_binding(Keys.ControlN,
                          filter=(ViInsertMode() & HasFocus(DEFAULT_BUFFER)
                         ))(next_history_or_next_completion)
 
     registry.add_binding(Keys.ControlG,
                          filter=(HasFocus(DEFAULT_BUFFER) & HasCompletions()
                         ))(dismiss_completion)
 
     registry.add_binding(Keys.ControlC, filter=HasFocus(DEFAULT_BUFFER)
                         )(reset_buffer)
 
     registry.add_binding(Keys.ControlC, filter=HasFocus(SEARCH_BUFFER)
                         )(reset_search_buffer)
 
     supports_suspend = Condition(lambda cli: hasattr(signal, 'SIGTSTP'))
     registry.add_binding(Keys.ControlZ, filter=supports_suspend
                         )(suspend_to_bg)
 
     # Ctrl+I == Tab
     registry.add_binding(Keys.ControlI,
                          filter=(HasFocus(DEFAULT_BUFFER)
                                  & ~HasSelection()
                                  & insert_mode
                                  & cursor_in_leading_ws
                         ))(indent_buffer)
 
     registry.add_binding(Keys.ControlO,
                          filter=(HasFocus(DEFAULT_BUFFER)
                                 & EmacsInsertMode()))(newline_with_copy_margin)
 
     registry.add_binding(Keys.F2,
                          filter=HasFocus(DEFAULT_BUFFER)
                         )(open_input_in_editor)
 
     registry.add_binding('v',
                          filter=HasFocus(DEFAULT_BUFFER) & ViNavigationMode()
                         )(open_input_in_editor)
 
     if shell.display_completions == 'readlinelike':
         registry.add_binding(Keys.ControlI,
                              filter=(HasFocus(DEFAULT_BUFFER)
                                      & ~HasSelection()
                                      & insert_mode
                                      & ~cursor_in_leading_ws
                             ))(display_completions_like_readline)
 
     if sys.platform == 'win32':
         registry.add_binding(Keys.ControlV,
                              filter=(
                              HasFocus(
                              DEFAULT_BUFFER) & ~ViMode()
                             ))(win_paste)
 
 
 def newline_or_execute_outer(shell):
     def newline_or_execute(event):
         """When the user presses return, insert a newline or execute the code."""
         b = event.current_buffer
         d = b.document
 
         if b.complete_state:
             cc = b.complete_state.current_completion
             if cc:
                 b.apply_completion(cc)
             else:
                 b.cancel_completion()
             return
 
-        if not (d.on_last_line or d.cursor_position_row >= d.line_count
-            - d.empty_line_count_at_the_end()):
-            b.newline()
-            return
+        before_text = d.text[:d.cursor_position]
+        status, indent = shell.input_splitter.check_complete(before_text + '\n')
 
-        status, indent = shell.input_splitter.check_complete(d.text + '\n')
+        if not (d.on_last_line or
+                d.cursor_position_row >= d.line_count - d.empty_line_count_at_the_end()
+                ):
+            b.insert_text('\n' + (' ' * (indent or 0)))
+            return
 
         if (status != 'incomplete') and b.accept_action.is_returnable:
             b.accept_action.validate_and_handle(event.cli, b)
         else:
             b.insert_text('\n' + (' ' * (indent or 0)))
     return newline_or_execute
 
 
 def previous_history_or_previous_completion(event):
     """
     Control-P in vi edit mode on readline is history next, unlike default prompt toolkit.
 
     If completer is open this still select previous completion.
     """
     event.current_buffer.auto_up()
 
 
 def next_history_or_next_completion(event):
     """
     Control-N in vi edit mode on readline is history previous, unlike default prompt toolkit.
 
     If completer is open this still select next completion.
     """
     event.current_buffer.auto_down()
 
 
 def dismiss_completion(event):
     b = event.current_buffer
     if b.complete_state:
         b.cancel_completion()
 
 
 def reset_buffer(event):
     b = event.current_buffer
     if b.complete_state:
         b.cancel_completion()
     else:
         b.reset()
 
 
 def reset_search_buffer(event):
     if event.current_buffer.document.text:
         event.current_buffer.reset()
     else:
         event.cli.push_focus(DEFAULT_BUFFER)
 
 def suspend_to_bg(event):
     event.cli.suspend_to_background()
 
 def force_exit(event):
     """
     Force exit (with a non-zero return value)
     """
     sys.exit("Quit")
 
 def indent_buffer(event):
     event.current_buffer.insert_text(' ' * 4)
 
 def newline_with_copy_margin(event):
     """
     Preserve margin and cursor position when using
     Control-O to insert a newline in EMACS mode
     """
     b = event.current_buffer
     cursor_start_pos = b.document.cursor_position_col
     b.newline(copy_margin=True)
     b.cursor_up(count=1)
     cursor_end_pos = b.document.cursor_position_col
     if cursor_start_pos != cursor_end_pos:
         pos_diff = cursor_start_pos - cursor_end_pos
         b.cursor_right(count=pos_diff)
 
 def open_input_in_editor(event):
     event.cli.current_buffer.tempfile_suffix = ".py"
     event.cli.current_buffer.open_in_editor(event.cli)
 
 
 if sys.platform == 'win32':
     from IPython.core.error import TryNext
     from IPython.lib.clipboard import (ClipboardEmpty,
                                        win32_clipboard_get,
                                        tkinter_clipboard_get)
 
     @undoc
     def win_paste(event):
         try:
             text = win32_clipboard_get()
         except TryNext:
             try:
                 text = tkinter_clipboard_get()
             except (TryNext, ClipboardEmpty):
                 return
         except ClipboardEmpty:
             return
         event.current_buffer.insert_text(text.replace('\t', ' ' * 4))