bug#58389: 29.0.50; Python completion fails under certain conditions

[kobarity]

Hi,

Completion in Python mode buffer fails under certain conditions:

- non-native completion is used,
- ElDoc mode is used, or
- FFAP is used.

Here is how to reproduce the problem of non-native completion:

1. emacs -Q
2. Open the following Python file.

#+begin_src python
import sys
#+end_src

3. Use M-x customize-variable to disable
   python-shell-completion-native-enable
4. C-u -1 M-x eldoc-mode (Disable ElDoc mode)
5. C-c C-p (M-x run-python)
6. C-x o (Back to the Python mode buffer)
7. C-c C-c (M-x python-shell-send-buffer)
8. C-n (M-x next-line)
9. Type "sys."
10. C-M-i (M-x completion-at-point)
11. Completions are shown in *Completions* buffer.
12. Type "e"
13. C-M-i (M-x completion-at-point)
14. "No match" is shown in the minibuffer.

This does not happen when native completion is enabled and ElDoc mode
is disabled.

The following steps show the problem of ElDoc mode:

1. emacs -Q
2. Open the following Python file.

#+begin_src python
import sys
#+end_src

3. Make sure that ElDoc mode is enabled.
4. C-c C-p (M-x run-python)
5. Make sure that "Shell native completion is enabled." is shown in
   the minibuffer.
6. C-x o (Back to the Python mode buffer)
7. C-c C-c (M-x python-shell-send-buffer)
8. C-n (M-x next-line)
9. Type "sys."
10. Wait for 1-2 seconds to make sure ElDoc function is run by the
    idle timer.
11. C-M-i (M-x completion-at-point)
12. "No match" is shown in the minibuffer.

The problem with FFAP can be reproduced with similar steps.

The direct cause of these problems is that
`python-util-comint-last-prompt' returns nil, which is not supposed in
`python-shell-completion-at-point'.

`python-util-comint-last-prompt' is a wrapper to return
`comint-last-prompt'.  `comint-last-prompt' is initialized to nil in
`comint-snapshot-last-prompt' and is set in `comint-output-filter'.
However, `python-shell-send-string-no-output' which is used by the
features mentioned above temporarily changes the `process-filter' to
redirect the output.  This causes `comint-output-filter' to not be
called and `comint-last-prompt' to remain nil.

I think `comint-snapshot-last-prompt' should not be called as long as 
`comint-output-filter' is not called and the Inferior Python Shell
buffer is not changed.  Attached is a patch to fix this issue.

Best Regards,

--

In GNU Emacs 29.0.50 (build 5, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw scroll bars) of 2022-10-09 built on ubuntu
Repository revision: 9d47364634f96b2f2651a7f3942f8f87b920f02d
Repository branch: master
System Description: Ubuntu 22.04.1 LTS

Configured using:
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Configured features:
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Important settings:
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Major mode: Lisp Interaction

Minor modes in effect:
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Memory information:
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0001-Fix-Python-completion-failure-under-certain-conditio.patch
Description: Binary data