Merge pull request #12848 from meeseeksmachine/auto-backport-of-pr-12844-on-7.x

Backport PR  #12844 on branch 7.x (Fix UnboundLocalError in IPDB when running 'context' command without argument)

IPython/core/debugger.py

@@ -1,847 +1,847 b''
 # -*- coding: utf-8 -*-
 """
 Pdb debugger class.
 
 Modified from the standard pdb.Pdb class to avoid including readline, so that
 the command line completion of other programs which include this isn't
 damaged.
 
 In the future, this class will be expanded with improvements over the standard
 pdb.
 
 The code in this file is mainly lifted out of cmd.py in Python 2.2, with minor
 changes. Licensing should therefore be under the standard Python terms.  For
 details on the PSF (Python Software Foundation) standard license, see:
 
 https://docs.python.org/2/license.html
 """
 
 #*****************************************************************************
 #
 #       This file is licensed under the PSF license.
 #
 #       Copyright (C) 2001 Python Software Foundation, www.python.org
 #       Copyright (C) 2005-2006 Fernando Perez. <[e-mail]>
 #
 #
 #*****************************************************************************
 
 import bdb
 import functools
 import inspect
 import linecache
 import sys
 import warnings
 import re
 
 from IPython import get_ipython
 from IPython.utils import PyColorize
 from IPython.utils import coloransi, py3compat
 from IPython.core.excolors import exception_colors
 from IPython.testing.skipdoctest import skip_doctest
 
 
 prompt = 'ipdb> '
 
 #We have to check this directly from sys.argv, config struct not yet available
 from pdb import Pdb as OldPdb
 
 # Allow the set_trace code to operate outside of an ipython instance, even if
 # it does so with some limitations.  The rest of this support is implemented in
 # the Tracer constructor.
 
 def make_arrow(pad):
     """generate the leading arrow in front of traceback or debugger"""
     if pad >= 2:
         return '-'*(pad-2) + '> '
     elif pad == 1:
         return '>'
     return ''
 
 
 def BdbQuit_excepthook(et, ev, tb, excepthook=None):
     """Exception hook which handles `BdbQuit` exceptions.
 
     All other exceptions are processed using the `excepthook`
     parameter.
     """
     warnings.warn("`BdbQuit_excepthook` is deprecated since version 5.1",
                   DeprecationWarning, stacklevel=2)
     if et==bdb.BdbQuit:
         print('Exiting Debugger.')
     elif excepthook is not None:
         excepthook(et, ev, tb)
     else:
         # Backwards compatibility. Raise deprecation warning?
         BdbQuit_excepthook.excepthook_ori(et,ev,tb)
 
 
 def BdbQuit_IPython_excepthook(self,et,ev,tb,tb_offset=None):
     warnings.warn(
         "`BdbQuit_IPython_excepthook` is deprecated since version 5.1",
         DeprecationWarning, stacklevel=2)
     print('Exiting Debugger.')
 
 
 class Tracer(object):
     """
     DEPRECATED
 
     Class for local debugging, similar to pdb.set_trace.
 
     Instances of this class, when called, behave like pdb.set_trace, but
     providing IPython's enhanced capabilities.
 
     This is implemented as a class which must be initialized in your own code
     and not as a standalone function because we need to detect at runtime
     whether IPython is already active or not.  That detection is done in the
     constructor, ensuring that this code plays nicely with a running IPython,
     while functioning acceptably (though with limitations) if outside of it.
     """
 
     @skip_doctest
     def __init__(self, colors=None):
         """
         DEPRECATED
 
         Create a local debugger instance.
 
         Parameters
         ----------
 
         colors : str, optional
             The name of the color scheme to use, it must be one of IPython's
             valid color schemes.  If not given, the function will default to
             the current IPython scheme when running inside IPython, and to
             'NoColor' otherwise.
 
         Examples
         --------
         ::
 
             from IPython.core.debugger import Tracer; debug_here = Tracer()
 
         Later in your code::
 
             debug_here()  # -> will open up the debugger at that point.
 
         Once the debugger activates, you can use all of its regular commands to
         step through code, set breakpoints, etc.  See the pdb documentation
         from the Python standard library for usage details.
         """
         warnings.warn("`Tracer` is deprecated since version 5.1, directly use "
                       "`IPython.core.debugger.Pdb.set_trace()`",
                       DeprecationWarning, stacklevel=2)
 
         ip = get_ipython()
         if ip is None:
             # Outside of ipython, we set our own exception hook manually
             sys.excepthook = functools.partial(BdbQuit_excepthook,
                                                excepthook=sys.excepthook)
             def_colors = 'NoColor'
         else:
             # In ipython, we use its custom exception handler mechanism
             def_colors = ip.colors
             ip.set_custom_exc((bdb.BdbQuit,), BdbQuit_IPython_excepthook)
 
         if colors is None:
             colors = def_colors
 
         # The stdlib debugger internally uses a modified repr from the `repr`
         # module, that limits the length of printed strings to a hardcoded
         # limit of 30 characters.  That much trimming is too aggressive, let's
         # at least raise that limit to 80 chars, which should be enough for
         # most interactive uses.
         try:
             from reprlib import aRepr
             aRepr.maxstring = 80
         except:
             # This is only a user-facing convenience, so any error we encounter
             # here can be warned about but can be otherwise ignored.  These
             # printouts will tell us about problems if this API changes
             import traceback
             traceback.print_exc()
 
         self.debugger = Pdb(colors)
 
     def __call__(self):
         """Starts an interactive debugger at the point where called.
 
         This is similar to the pdb.set_trace() function from the std lib, but
         using IPython's enhanced debugger."""
 
         self.debugger.set_trace(sys._getframe().f_back)
 
 
 RGX_EXTRA_INDENT = re.compile(r'(?<=\n)\s+')
 
 
 def strip_indentation(multiline_string):
     return RGX_EXTRA_INDENT.sub('', multiline_string)
 
 
 def decorate_fn_with_doc(new_fn, old_fn, additional_text=""):
     """Make new_fn have old_fn's doc string. This is particularly useful
     for the ``do_...`` commands that hook into the help system.
     Adapted from from a comp.lang.python posting
     by Duncan Booth."""
     def wrapper(*args, **kw):
         return new_fn(*args, **kw)
     if old_fn.__doc__:
         wrapper.__doc__ = strip_indentation(old_fn.__doc__) + additional_text
     return wrapper
 
 
 class Pdb(OldPdb):
     """Modified Pdb class, does not load readline.
 
     for a standalone version that uses prompt_toolkit, see
     `IPython.terminal.debugger.TerminalPdb` and
     `IPython.terminal.debugger.set_trace()`
     """
 
     def __init__(self, color_scheme=None, completekey=None,
                  stdin=None, stdout=None, context=5, **kwargs):
         """Create a new IPython debugger.
         
         :param color_scheme: Deprecated, do not use.
         :param completekey: Passed to pdb.Pdb.
         :param stdin: Passed to pdb.Pdb.
         :param stdout: Passed to pdb.Pdb.
         :param context: Number of lines of source code context to show when
             displaying stacktrace information.
         :param kwargs: Passed to pdb.Pdb.
             The possibilities are python version dependent, see the python
             docs for more info.
         """
 
         # Parent constructor:
         try:
             self.context = int(context)
             if self.context <= 0:
                 raise ValueError("Context must be a positive integer")
         except (TypeError, ValueError):
                 raise ValueError("Context must be a positive integer")
 
         # `kwargs` ensures full compatibility with stdlib's `pdb.Pdb`.
         OldPdb.__init__(self, completekey, stdin, stdout, **kwargs)
 
         # IPython changes...
         self.shell = get_ipython()
 
         if self.shell is None:
             save_main = sys.modules['__main__']
             # No IPython instance running, we must create one
             from IPython.terminal.interactiveshell import \
                 TerminalInteractiveShell
             self.shell = TerminalInteractiveShell.instance()
             # needed by any code which calls __import__("__main__") after
             # the debugger was entered. See also #9941.
             sys.modules['__main__'] = save_main 
 
         if color_scheme is not None:
             warnings.warn(
                 "The `color_scheme` argument is deprecated since version 5.1",
                 DeprecationWarning, stacklevel=2)
         else:
             color_scheme = self.shell.colors
 
         self.aliases = {}
 
         # Create color table: we copy the default one from the traceback
         # module and add a few attributes needed for debugging
         self.color_scheme_table = exception_colors()
 
         # shorthands
         C = coloransi.TermColors
         cst = self.color_scheme_table
 
         cst['NoColor'].colors.prompt = C.NoColor
         cst['NoColor'].colors.breakpoint_enabled = C.NoColor
         cst['NoColor'].colors.breakpoint_disabled = C.NoColor
 
         cst['Linux'].colors.prompt = C.Green
         cst['Linux'].colors.breakpoint_enabled = C.LightRed
         cst['Linux'].colors.breakpoint_disabled = C.Red
 
         cst['LightBG'].colors.prompt = C.Blue
         cst['LightBG'].colors.breakpoint_enabled = C.LightRed
         cst['LightBG'].colors.breakpoint_disabled = C.Red
 
         cst['Neutral'].colors.prompt = C.Blue
         cst['Neutral'].colors.breakpoint_enabled = C.LightRed
         cst['Neutral'].colors.breakpoint_disabled = C.Red
 
 
         # Add a python parser so we can syntax highlight source while
         # debugging.
         self.parser = PyColorize.Parser(style=color_scheme)
         self.set_colors(color_scheme)
 
         # Set the prompt - the default prompt is '(Pdb)'
         self.prompt = prompt
         self.skip_hidden = True
 
     def set_colors(self, scheme):
         """Shorthand access to the color table scheme selector method."""
         self.color_scheme_table.set_active_scheme(scheme)
         self.parser.style = scheme
 
     def set_trace(self, frame=None):
         if frame is None:
             frame = sys._getframe().f_back
         self.initial_frame = frame
         return super().set_trace(frame)
 
     def hidden_frames(self, stack):
         """
         Given an index in the stack return wether it should be skipped.
 
         This is used in up/down and where to skip frames.
         """
         # The f_locals dictionary is updated from the actual frame
         # locals whenever the .f_locals accessor is called, so we
         # avoid calling it here to preserve self.curframe_locals.
         # Futhermore, there is no good reason to hide the current frame.
         ip_hide = [
             False
             if s[0] in (self.curframe, getattr(self, "initial_frame", None))
             else s[0].f_locals.get("__tracebackhide__", False)
             for s in stack
         ]
         ip_start = [i for i, s in enumerate(ip_hide) if s == "__ipython_bottom__"]
         if ip_start:
             ip_hide = [h if i > ip_start[0] else True for (i, h) in enumerate(ip_hide)]
         return ip_hide
 
     def interaction(self, frame, traceback):
         try:
             OldPdb.interaction(self, frame, traceback)
         except KeyboardInterrupt:
             self.stdout.write("\n" + self.shell.get_exception_only())
 
     def new_do_frame(self, arg):
         OldPdb.do_frame(self, arg)
 
     def new_do_quit(self, arg):
 
         if hasattr(self, 'old_all_completions'):
             self.shell.Completer.all_completions=self.old_all_completions
 
         return OldPdb.do_quit(self, arg)
 
     do_q = do_quit = decorate_fn_with_doc(new_do_quit, OldPdb.do_quit)
 
     def new_do_restart(self, arg):
         """Restart command. In the context of ipython this is exactly the same
         thing as 'quit'."""
         self.msg("Restart doesn't make sense here. Using 'quit' instead.")
         return self.do_quit(arg)
 
     def print_stack_trace(self, context=None):
         Colors = self.color_scheme_table.active_colors
         ColorsNormal = Colors.Normal
         if context is None:
             context = self.context
         try:
             context=int(context)
             if context <= 0:
                 raise ValueError("Context must be a positive integer")
         except (TypeError, ValueError):
                 raise ValueError("Context must be a positive integer")
         try:
             skipped = 0
             for hidden, frame_lineno in zip(self.hidden_frames(self.stack), self.stack):
                 if hidden and self.skip_hidden:
                     skipped += 1
                     continue
                 if skipped:
                     print(
                         f"{Colors.excName}    [... skipping {skipped} hidden frame(s)]{ColorsNormal}\n"
                     )
                     skipped = 0
                 self.print_stack_entry(frame_lineno, context=context)
             if skipped:
                 print(
                     f"{Colors.excName}    [... skipping {skipped} hidden frame(s)]{ColorsNormal}\n"
                 )
         except KeyboardInterrupt:
             pass
 
     def print_stack_entry(self, frame_lineno, prompt_prefix='\n-> ',
                           context=None):
         if context is None:
             context = self.context
         try:
             context=int(context)
             if context <= 0:
                 raise ValueError("Context must be a positive integer")
         except (TypeError, ValueError):
                 raise ValueError("Context must be a positive integer")
         print(self.format_stack_entry(frame_lineno, '', context), file=self.stdout)
 
         # vds: >>
         frame, lineno = frame_lineno
         filename = frame.f_code.co_filename
         self.shell.hooks.synchronize_with_editor(filename, lineno, 0)
         # vds: <<
 
     def format_stack_entry(self, frame_lineno, lprefix=': ', context=None):
         if context is None:
             context = self.context
         try:
             context=int(context)
             if context <= 0:
                 print("Context must be a positive integer", file=self.stdout)
         except (TypeError, ValueError):
                 print("Context must be a positive integer", file=self.stdout)
         try:
             import reprlib  # Py 3
         except ImportError:
             import repr as reprlib  # Py 2
 
         ret = []
 
         Colors = self.color_scheme_table.active_colors
         ColorsNormal = Colors.Normal
         tpl_link = u'%s%%s%s' % (Colors.filenameEm, ColorsNormal)
         tpl_call = u'%s%%s%s%%s%s' % (Colors.vName, Colors.valEm, ColorsNormal)
         tpl_line = u'%%s%s%%s %s%%s' % (Colors.lineno, ColorsNormal)
         tpl_line_em = u'%%s%s%%s %s%%s%s' % (Colors.linenoEm, Colors.line,
                                             ColorsNormal)
 
         frame, lineno = frame_lineno
 
         return_value = ''
         if '__return__' in frame.f_locals:
             rv = frame.f_locals['__return__']
             #return_value += '->'
             return_value += reprlib.repr(rv) + '\n'
         ret.append(return_value)
 
         #s = filename + '(' + `lineno` + ')'
         filename = self.canonic(frame.f_code.co_filename)
         link = tpl_link % py3compat.cast_unicode(filename)
 
         if frame.f_code.co_name:
             func = frame.f_code.co_name
         else:
             func = "<lambda>"
 
         call = ''
         if func != '?':
             if '__args__' in frame.f_locals:
                 args = reprlib.repr(frame.f_locals['__args__'])
             else:
                 args = '()'
             call = tpl_call % (func, args)
 
         # The level info should be generated in the same format pdb uses, to
         # avoid breaking the pdbtrack functionality of python-mode in *emacs.
         if frame is self.curframe:
             ret.append('> ')
         else:
             ret.append('  ')
         ret.append(u'%s(%s)%s\n' % (link,lineno,call))
 
         start = lineno - 1 - context//2
         lines = linecache.getlines(filename)
         start = min(start, len(lines) - context)
         start = max(start, 0)
         lines = lines[start : start + context]
 
         for i,line in enumerate(lines):
             show_arrow = (start + 1 + i == lineno)
             linetpl = (frame is self.curframe or show_arrow) \
                       and tpl_line_em \
                       or tpl_line
             ret.append(self.__format_line(linetpl, filename,
                                           start + 1 + i, line,
                                           arrow = show_arrow) )
         return ''.join(ret)
 
     def __format_line(self, tpl_line, filename, lineno, line, arrow = False):
         bp_mark = ""
         bp_mark_color = ""
 
         new_line, err = self.parser.format2(line, 'str')
         if not err:
             line = new_line
 
         bp = None
         if lineno in self.get_file_breaks(filename):
             bps = self.get_breaks(filename, lineno)
             bp = bps[-1]
 
         if bp:
             Colors = self.color_scheme_table.active_colors
             bp_mark = str(bp.number)
             bp_mark_color = Colors.breakpoint_enabled
             if not bp.enabled:
                 bp_mark_color = Colors.breakpoint_disabled
 
         numbers_width = 7
         if arrow:
             # This is the line with the error
             pad = numbers_width - len(str(lineno)) - len(bp_mark)
             num = '%s%s' % (make_arrow(pad), str(lineno))
         else:
             num = '%*s' % (numbers_width - len(bp_mark), str(lineno))
 
         return tpl_line % (bp_mark_color + bp_mark, num, line)
 
 
     def print_list_lines(self, filename, first, last):
         """The printing (as opposed to the parsing part of a 'list'
         command."""
         try:
             Colors = self.color_scheme_table.active_colors
             ColorsNormal = Colors.Normal
             tpl_line = '%%s%s%%s %s%%s' % (Colors.lineno, ColorsNormal)
             tpl_line_em = '%%s%s%%s %s%%s%s' % (Colors.linenoEm, Colors.line, ColorsNormal)
             src = []
             if filename == "<string>" and hasattr(self, "_exec_filename"):
                 filename = self._exec_filename
 
             for lineno in range(first, last+1):
                 line = linecache.getline(filename, lineno)
                 if not line:
                     break
 
                 if lineno == self.curframe.f_lineno:
                     line = self.__format_line(tpl_line_em, filename, lineno, line, arrow = True)
                 else:
                     line = self.__format_line(tpl_line, filename, lineno, line, arrow = False)
 
                 src.append(line)
                 self.lineno = lineno
 
             print(''.join(src), file=self.stdout)
 
         except KeyboardInterrupt:
             pass
 
     def do_skip_hidden(self, arg):
         """
         Change whether or not we should skip frames with the
         __tracebackhide__ attribute.
         """
         if arg.strip().lower() in ("true", "yes"):
             self.skip_hidden = True
         elif arg.strip().lower() in ("false", "no"):
             self.skip_hidden = False
 
     def do_list(self, arg):
         """Print lines of code from the current stack frame
         """
         self.lastcmd = 'list'
         last = None
         if arg:
             try:
                 x = eval(arg, {}, {})
                 if type(x) == type(()):
                     first, last = x
                     first = int(first)
                     last = int(last)
                     if last < first:
                         # Assume it's a count
                         last = first + last
                 else:
                     first = max(1, int(x) - 5)
             except:
                 print('*** Error in argument:', repr(arg), file=self.stdout)
                 return
         elif self.lineno is None:
             first = max(1, self.curframe.f_lineno - 5)
         else:
             first = self.lineno + 1
         if last is None:
             last = first + 10
         self.print_list_lines(self.curframe.f_code.co_filename, first, last)
 
         # vds: >>
         lineno = first
         filename = self.curframe.f_code.co_filename
         self.shell.hooks.synchronize_with_editor(filename, lineno, 0)
         # vds: <<
 
     do_l = do_list
 
     def getsourcelines(self, obj):
         lines, lineno = inspect.findsource(obj)
         if inspect.isframe(obj) and obj.f_globals is obj.f_locals:
             # must be a module frame: do not try to cut a block out of it
             return lines, 1
         elif inspect.ismodule(obj):
             return lines, 1
         return inspect.getblock(lines[lineno:]), lineno+1
 
     def do_longlist(self, arg):
         """Print lines of code from the current stack frame.
 
         Shows more lines than 'list' does.
         """
         self.lastcmd = 'longlist'
         try:
             lines, lineno = self.getsourcelines(self.curframe)
         except OSError as err:
             self.error(err)
             return
         last = lineno + len(lines)
         self.print_list_lines(self.curframe.f_code.co_filename, lineno, last)
     do_ll = do_longlist
 
     def do_debug(self, arg):
         """debug code
         Enter a recursive debugger that steps through the code
         argument (which is an arbitrary expression or statement to be
         executed in the current environment).
         """
         trace_function = sys.gettrace()
         sys.settrace(None)
         globals = self.curframe.f_globals
         locals = self.curframe_locals
         p = self.__class__(completekey=self.completekey,
                            stdin=self.stdin, stdout=self.stdout)
         p.use_rawinput = self.use_rawinput
         p.prompt = "(%s) " % self.prompt.strip()
         self.message("ENTERING RECURSIVE DEBUGGER")
         sys.call_tracing(p.run, (arg, globals, locals))
         self.message("LEAVING RECURSIVE DEBUGGER")
         sys.settrace(trace_function)
         self.lastcmd = p.lastcmd
 
     def do_pdef(self, arg):
         """Print the call signature for any callable object.
 
         The debugger interface to %pdef"""
         namespaces = [
             ("Locals", self.curframe_locals),
             ("Globals", self.curframe.f_globals),
         ]
         self.shell.find_line_magic("pdef")(arg, namespaces=namespaces)
 
     def do_pdoc(self, arg):
         """Print the docstring for an object.
 
         The debugger interface to %pdoc."""
         namespaces = [
             ("Locals", self.curframe_locals),
             ("Globals", self.curframe.f_globals),
         ]
         self.shell.find_line_magic("pdoc")(arg, namespaces=namespaces)
 
     def do_pfile(self, arg):
         """Print (or run through pager) the file where an object is defined.
 
         The debugger interface to %pfile.
         """
         namespaces = [
             ("Locals", self.curframe_locals),
             ("Globals", self.curframe.f_globals),
         ]
         self.shell.find_line_magic("pfile")(arg, namespaces=namespaces)
 
     def do_pinfo(self, arg):
         """Provide detailed information about an object.
 
         The debugger interface to %pinfo, i.e., obj?."""
         namespaces = [
             ("Locals", self.curframe_locals),
             ("Globals", self.curframe.f_globals),
         ]
         self.shell.find_line_magic("pinfo")(arg, namespaces=namespaces)
 
     def do_pinfo2(self, arg):
         """Provide extra detailed information about an object.
 
         The debugger interface to %pinfo2, i.e., obj??."""
         namespaces = [
             ("Locals", self.curframe_locals),
             ("Globals", self.curframe.f_globals),
         ]
         self.shell.find_line_magic("pinfo2")(arg, namespaces=namespaces)
 
     def do_psource(self, arg):
         """Print (or run through pager) the source code for an object."""
         namespaces = [
             ("Locals", self.curframe_locals),
             ("Globals", self.curframe.f_globals),
         ]
         self.shell.find_line_magic("psource")(arg, namespaces=namespaces)
 
     def do_where(self, arg):
         """w(here)
         Print a stack trace, with the most recent frame at the bottom.
         An arrow indicates the "current frame", which determines the
         context of most commands. 'bt' is an alias for this command.
 
         Take a number as argument as an (optional) number of context line to
         print"""
         if arg:
             try:
                 context = int(arg)
             except ValueError as err:
                 self.error(err)
                 return
             self.print_stack_trace(context)
         else:
             self.print_stack_trace()
 
     do_w = do_where
 
     def stop_here(self, frame):
         """Check if pdb should stop here"""
         if not super().stop_here(frame):
             return False
         if self.skip_hidden and frame.f_locals.get("__tracebackhide__", False):
             if self._wait_for_mainpyfile:
                 return False
             Colors = self.color_scheme_table.active_colors
             ColorsNormal = Colors.Normal
             print(f"{Colors.excName}    [... skipped 1 hidden frame]{ColorsNormal}\n")
             return False
         return True
 
     def do_up(self, arg):
         """u(p) [count]
         Move the current frame count (default one) levels up in the
         stack trace (to an older frame).
 
         Will skip hidden frames.
         """
         ## modified version of upstream that skips
         # frames with __tracebackide__
         if self.curindex == 0:
             self.error("Oldest frame")
             return
         try:
             count = int(arg or 1)
         except ValueError:
             self.error("Invalid frame count (%s)" % arg)
             return
         skipped = 0
         if count < 0:
             _newframe = 0
         else:
             _newindex = self.curindex
             counter = 0
             hidden_frames = self.hidden_frames(self.stack)
             for i in range(self.curindex - 1, -1, -1):
                 frame = self.stack[i][0]
                 if hidden_frames[i] and self.skip_hidden:
                     skipped += 1
                     continue
                 counter += 1
                 if counter >= count:
                     break
             else:
                 # if no break occured.
                 self.error("all frames above hidden")
                 return
 
             Colors = self.color_scheme_table.active_colors
             ColorsNormal = Colors.Normal
             _newframe = i
         self._select_frame(_newframe)
         if skipped:
             print(
                 f"{Colors.excName}    [... skipped {skipped} hidden frame(s)]{ColorsNormal}\n"
             )
 
     def do_down(self, arg):
         """d(own) [count]
         Move the current frame count (default one) levels down in the
         stack trace (to a newer frame).
 
         Will skip hidden frames.
         """
         if self.curindex + 1 == len(self.stack):
             self.error("Newest frame")
             return
         try:
             count = int(arg or 1)
         except ValueError:
             self.error("Invalid frame count (%s)" % arg)
             return
         if count < 0:
             _newframe = len(self.stack) - 1
         else:
             _newindex = self.curindex
             counter = 0
             skipped = 0
             hidden_frames = self.hidden_frames(self.stack)
             for i in range(self.curindex + 1, len(self.stack)):
                 frame = self.stack[i][0]
                 if hidden_frames[i] and self.skip_hidden:
                     skipped += 1
                     continue
                 counter += 1
                 if counter >= count:
                     break
             else:
                 self.error("all frames bellow hidden")
                 return
 
             Colors = self.color_scheme_table.active_colors
             ColorsNormal = Colors.Normal
             if skipped:
                 print(
                     f"{Colors.excName}    [... skipped {skipped} hidden frame(s)]{ColorsNormal}\n"
                 )
             _newframe = i
 
         self._select_frame(_newframe)
 
     do_d = do_down
     do_u = do_up
 
     def do_context(self, context):
         """context number_of_lines
         Set the number of lines of source code to show when displaying
         stacktrace information.
         """
         try:
             new_context = int(context)
             if new_context <= 0:
                 raise ValueError()
+            self.context = new_context
         except ValueError:
             self.error("The 'context' command requires a positive integer argument.")
-        self.context = new_context
 
 
 class InterruptiblePdb(Pdb):
     """Version of debugger where KeyboardInterrupt exits the debugger altogether."""
 
     def cmdloop(self):
         """Wrap cmdloop() such that KeyboardInterrupt stops the debugger."""
         try:
             return OldPdb.cmdloop(self)
         except KeyboardInterrupt:
             self.stop_here = lambda frame: False
             self.do_quit("")
             sys.settrace(None)
             self.quitting = False
             raise
 
     def _cmdloop(self):
         while True:
             try:
                 # keyboard interrupts allow for an easy way to cancel
                 # the current command, so allow them during interactive input
                 self.allow_kbdint = True
                 self.cmdloop()
                 self.allow_kbdint = False
                 break
             except KeyboardInterrupt:
                 self.message('--KeyboardInterrupt--')
                 raise
 
 
 def set_trace(frame=None):
     """
     Start debugging from `frame`.
 
     If frame is not specified, debugging starts from caller's frame.
     """
     Pdb().set_trace(frame or sys._getframe().f_back)