argparse mutually exclusive group does not exclude in some cases #85531

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1. kkarbowiak commented on Jul 21, 2020

   

   kkarbowiakmannequin

   on Jul 21, 2020

   MannequinAuthor

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   The documentation for ArgumentParser.add_mutually_exclusive_group states: "argparse will make sure that only one of the arguments in the mutually exclusive group was present on the command line".

   This is not the case in certain circumstances:

   import argparse
   
   parser = argparse.ArgumentParser()
   group = parser.add_mutually_exclusive_group()
   group.add_argument('-a')
   group.add_argument('-b', nargs='?')
   
   parser.parse_args('-a a -b'.split())

   The above code does not produce any error, even though both exclusive arguments are present.

   My guess is that the check for mutual exclusion is not done during processing of each command line argument, but rather afterwards. It seems the check only ensures at most one argument from group is not None.

   The issue exists at least on Python 2.7.13, 3.6, 3.7.5, 3.8, and 3.10.
2. kkarbowiak

   added

   3.10 (EOL)end of lifeend of life

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Jul 21, 2020
3. paulj3 commented on Jul 21, 2020

   

   paulj3mannequin

   on Jul 21, 2020

   Mannequin

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   This is the result of how default values are handled with '?' (and '*') nargs.

   At the start of nested take_action function (which handles all actions) is this code:

               argument_values = self._get_values(action, argument_strings)
   
               # error if this argument is not allowed with other previously
               # seen arguments, assuming that actions that use the default
               # value don't really count as "present"
               if argument_values is not action.default:
                   seen_non_default_actions.add(action)
                   for conflict_action in action_conflicts.get(action, []):
                       if conflict_action in seen_non_default_actions:
                           msg = _('not allowed with argument %s')
                           action_name = _get_action_name(conflict_action)
                           raise ArgumentError(action, msg % action_name)

   'get_values' gets the values for this action. A bare '-b' will be given its 'default'.

   An optional positional is always 'seen', since an empty list satisfies its 'nargs'. 'get_values' assigns the default instead. This code in take_action allows us to include such positionals in a mutually exclusive group.

   This handing of the '?' optional is a byproduct of that code. Normally we provide both a 'default' and a 'const' with such an argument, giving us a 3-way switch (default, const or user-value). If either 'default' or 'const' is provided, your '-b' will behave as expected.

   I have a feeling that trying to document this edge case will just make things more confusing.
4. ezio-melotti

   transferred this issue fromon Apr 10, 2022
5. erlend-aasland

   added this to Argparse issueson May 19, 2022
6. erlend-aasland

   moved this to Bugs in Argparse issueson May 19, 2022
7. serhiy-storchaka

   added a commit that references this issue on Sep 21, 2024

   Add test for pythongh-85531.

   82a56b6
8. serhiy-storchaka commented on Sep 21, 2024

   

   serhiy-storchaka

   on Sep 21, 2024

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   This looks similar to #63143, and #124307 fixes also this issue.
9. serhiy-storchaka

   closed this as not plannedon Sep 25, 2024
10. github-project-automation

    moved this from Bugs to Doc issues in Argparse issueson Sep 25, 2024