Command parsing does not handle options that conflict with `--settings`/`--pythonpath`

[Ryan P Kilby]

Argparse performs ​prefix matching when arguments are unambiguous (e.g., --set=my.settings and --settings=my.settings are both valid). Since --settings and --pythonpath are ​parsed before the command is fetched, conflicting/ambiguous command options are not checked. For example, if I have a command for getting/setting config values, the result of using the --set option is:

$ python manage.py config --set foo bar
Traceback (most recent call last):
  File "manage.py", line 20, in <module>
    execute_from_command_line(sys.argv)
  File ".venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File ".venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 308, in execute
    settings.INSTALLED_APPS
  File ".venv/lib/python3.6/site-packages/django/conf/__init__.py", line 56, in __getattr__
    self._setup(name)
  File ".venv/lib/python3.6/site-packages/django/conf/__init__.py", line 41, in _setup
    self._wrapped = Settings(settings_module)
  File ".venv/lib/python3.6/site-packages/django/conf/__init__.py", line 110, in __init__
    mod = importlib.import_module(self.SETTINGS_MODULE)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 978, in _gcd_import
  File "<frozen importlib._bootstrap>", line 961, in _find_and_load
  File "<frozen importlib._bootstrap>", line 948, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'foo'

One solution would be to set ​allow_abbrev to False, which would disable prefix matching. However, this option is only available in Python 3.5 and above. Also, this might be considered a breaking change if users are expecting to be able to use the abbreviated option names, but I don't know if that's really a concern.

[Carlton Gibson]

Hi Ryan. Thanks for the report. This seems reasonable.

Interestingly, with the patch, if you pass an abbreviation to second parser you get the error you'd expect:

manage.py config: error: ambiguous option: --se could match --set, --settings

...this option is only available in Python 3.5 and above.

Right. For that reason lets call this a New Feature and roll it into 2.1, where we don't have to worry about compat. (This has existed for ≈forever so I can't see the harm in holding off slightly.)

Also, this might be considered a breaking change if users are expecting to be able to use the abbreviated option names, but I don't know if that's really a concern.

Yes, probably not a biggie but could you add a note to the Breaking changes section in 2.1.txt.

[Ryan P Kilby]

Thanks for the review Carlton. Added a note to the misc backwards incompatibility section.

[Tim Graham <timograham@…>]

In 2dcc5d62:

Fixed #29392 -- Disallowed use of abbreviated forms of --settings and --pythonpath management command options.