zsh filename completion when using "python foo.py" rather than "./foo.py"

[FlorianPommerening]

I ran into one more issue with the FilesCompleter, this one zsh-specific:

To reproduce

Test environment:

• Ubuntu 24.04
• Python 3.12.3
• argcomplete 3.1.4-1ubuntu0.1 (this is 3.1.4 with a patch back-ported from 3.3.0 to make it compatible with Python 12.3)
• zsh 5.9

I tested this with the following file foo.py

#!/usr/bin/env python3
# PYTHON_ARGCOMPLETE_OK

import argcomplete, argparse

parser = argparse.ArgumentParser()
arg = parser.add_argument("file")
arg.completer = argcomplete.completers.FilesCompleter()

argcomplete.autocomplete(parser)

./foo.py /e<TAB>        # completes to /etc as expected
python ./foo.py /e<TAB> # completes to /etc as expected
python foo.py /e<TAB>   # deletes /e instead

Likely Cause

It looks like the root cause for this is in line 154, where [[ "$executable" == python* ]] checks if the executable is python or the script.
In this context, $executable was set earlier (line 148) as executable="${words[1]}". By this time zsh probably already did some preprocessing because words is ( foo.py /e), not (python foo.py /e). This then means that the test fails and no completions are generated there. The elif case in line 172 also fails because __python_argcomplete_which "$executable" executes whence -p "foo.py" which fails because the current working directory is not on the PATH. This also explains why python ./foo.py /e<TAB> works while python foo.py /e<TAB> doesn't.