Compilation on Linux without both libffi-dev and libncurses-dev results in an unexpected warning when using the new REPL

[plashchynski]

Bug report

Bug description:

Compilation on Ubuntu 24.04.2 with ./configure --with-pydebug without libffi-dev package installed cause the following unexpected warning while loading the resulting executable as ./python:

warning: can't use pyrepl: No module named 'msvcrt'

After installation of libffi-dev and recompilation the warning was resolved.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-131189: Fix "msvcrt" import warning on Linux when "_ctypes" is not available. #131201
• [3.14] gh-131189: Fix "msvcrt" import warning on Linux when "_ctypes" is not available. (GH-131201) #136668
• [3.13] gh-131189: Fix "msvcrt" import warning on Linux when "_ctypes" is not available. (GH-131201) #138934
• gh-131189: Remove curses mention from PYTHON_BASIC_REPL docs #140022
• [3.14] gh-131189: Remove curses mention from PYTHON_BASIC_REPL docs (GH-140022) #140027

Related

• pyrepl: restructure imports to give a meaningful error when curses is missing on POSIX #130046

[StanFromIreland]

Yes, its listed as a dependency. Behavior like this is to be expected. I don't really see a need for anything to be introduced (eg warnings).

[plashchynski]

After a quick exploration, I've found that the warning caused by an ImportError exception in the following code in pyrepl:

cpython/Lib/_pyrepl/readline.py

Lines 45 to 48 in e9d210b

	try:
	from .unix_console import UnixConsole as Console, _error
	except ImportError:
	from .windows_console import WindowsConsole as Console, _error

It's unable to load unix_console because of lack of libffi-dev (and _ctypes) package cause the following import error:

  File "/home/dzmitry/cpython/Lib/_pyrepl/readline.py", line 46, in <module>
    from .unix_console import UnixConsole as Console, _error
  File "/home/dzmitry/cpython/Lib/_pyrepl/unix_console.py", line 35, in <module>
    from . import curses
  File "/home/dzmitry/cpython/Lib/_pyrepl/curses.py", line 28, in <module>
    from . import _minimal_curses as _curses  # type: ignore[no-redef]
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dzmitry/cpython/Lib/_pyrepl/_minimal_curses.py", line 12, in <module>
    import ctypes
  File "/home/dzmitry/cpython/Lib/ctypes/__init__.py", line 8, in <module>
    from _ctypes import Union, Structure, Array
ModuleNotFoundError: No module named '_ctypes'

So it fallbacks to import of windows_console on Linux, resulting in another ModuleNotFoundError that results in the No module named 'msvcrt' warning.

In such situation, I'd expect to see a warning about lack of _ctypes. So, may we have here some platform checks instead of catching and suppressing import errors?

cpython/Lib/_pyrepl/readline.py

Lines 45 to 48 in e9d210b

	try:
	from .unix_console import UnixConsole as Console, _error
	except ImportError:
	from .windows_console import WindowsConsole as Console, _error

[picnixz]

We should warn at the make step or the configure step when libffi is not available I think.

[picnixz]

cc @encukou WDYT is the best approach? (and how do we actually check it?) I'm actually surprised that make correctly worked here (aren't we checking the necessary dependencies?)

[encukou]

ctypes is optional; libffi is not a build requirement.
I'm not sure anyone knows what “optional” means exactly; see the zlib discussion.

[picnixz]

Maybe we should warn once we're done with make? like, if ctypes is not available and we're on Linux, then the new REPL won't work (maybe the old REPL could work?) Having a simple warning: can't use pyrepl: No module named 'msvcrt' doesn't really help in this case.