iPDB loads readline despite claiming not to

[jdtsmith]

Despite its code indicating that iPDB doesn't load readline:

% ipython 
Python 3.10.6 (main, Aug 30 2022, 05:12:36) [Clang 13.1.6 (clang-1316.0.21.2.5)]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.5.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: 1/0
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
Cell In [1], line 1
----> 1 1/0

ZeroDivisionError: division by zero

In [2]: import sys

In [3]: sys.modules.get('readline') # Nothing!

In [4]: %debug
> <ipython-input-1-9e1622b385b6>(1)<module>()
----> 1 1/0

ipdb>   # Ctrl-d to exit                                                                        


In [5]: sys.modules.get('readline')
Out[5]: <module 'readline' from '/usr/local/Cellar/python@3.10/3.10.6_2/Frameworks/Python.framework/Versions/3.10/lib/python3.10/lib-dynload/readline.cpython-310-darwin.so'>

In [6]: 

Can also be seen by running under python -v -m IPython:

In [1]: 1/0
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
Cell In [1], line 1
----> 1 1/0

ZeroDivisionError: division by zero

In [2]: %debug
# extension module 'readline' loaded from '/usr/local/Cellar/python@3.10/3.10.6_2/Frameworks/Python.framework/Versions/3.10/lib/python3.10/lib-dynload/readline.cpython-310-darwin.so'
# extension module 'readline' executed from '/usr/local/Cellar/python@3.10/3.10.6_2/Frameworks/Python.framework/Versions/3.10/lib/python3.10/lib-dynload/readline.cpython-310-darwin.so'
import 'readline' # <_frozen_importlib_external.ExtensionFileLoader object at 0x10d14b070>
> <ipython-input-1-9e1622b385b6>(1)<module>()
----> 1 1/0

ipdb> 

This leads to issues accepting raw control characters (see #13775).

[jdtsmith]

I tracked this down: Pdb itself (which iPDB inherits from) loads readline unconditionally. So nothing IPython can do about it (aside maybe from patching builtins.__import__).

[jdtsmith]

OK, the real issue is that the new iPython Pdb (confusingly also called "Pdb"), initializes OldPdb:

        OldPdb.__init__(self, completekey, stdin, stdout, **kwargs)

which, due to the above referenced Pdb itself code, imports readline. So the docstring statement for new Pdb of:

"""Modified Pdb class, does not load readline."""

is incorrect. It does not directly load readline, but it causes it to be loaded via the parent class. This is only valid when TerminalPdb is not used, i.e. with simpleprompt.