argparse mx_group is required, when action value equal default will be ignore #74349

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1. mlouielu commented on Apr 26, 2017

   

   mlouielumannequin

   on Apr 26, 2017

   MannequinAuthor

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   When adding mutually exclusive group and required is True, and the group argument has default value. If we type its default value, argparse will ignore the input and return argument is required

   ------- PoC --------

   import argparse
   
   parser = argparse.ArgumentParser()
   group = parser.add_mutually_exclusive_group(required=True)
   group.add_argument('-v', type=int, default=10)
   
   print(parser.parse_args())

   ---

   $ python tests.py -v 10
   usage: tests.py [-h] -v V
   tests.py: error: one of the arguments -v is required
   $ python tests.py -v 11
   Namespace(v=11)
2. mlouielu

   added

   3.7 (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 Apr 26, 2017
3. mlouielu commented on Apr 26, 2017

   

   mlouielumannequin

   on Apr 26, 2017

   MannequinAuthor

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   Strange, this will only trigger when that argument type is int.
4. mlouielu commented on Apr 26, 2017

   

   mlouielumannequin

   on Apr 26, 2017

   MannequinAuthor

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   Maybe documentation should note that:
   """
   # 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"
   """
5. paulj3 commented on Apr 26, 2017

   

   paulj3mannequin

   on Apr 26, 2017

   Mannequin

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   And only for integers smaller than 257!

   The problem is with this test in take_action function

               # 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)

   Specifically with the

   argument_values is not action.default
   

   test. In CPython integers below 257 are unique, so that

   In [33]: int('10') is 10
   Out[33]: True
   In [34]: int('257') is 257
   Out[34]: False
   

   (and strings from 'sys.argv' do not match in the 'is' sense with strings set via 'default'. So the problem is unique to small integers.)

   The test is really meant to catch the default that is added in '_get_values()' for positionals with optional nargs ('?').

   This issue was raised by some PyPy developers several years ago.
   PyPy does not treat the small integers as unique. I'll have to dig around to see what the resolution was, if any.

   For now the solution is to use a string '10' as the default.
6. paulj3 commented on Apr 26, 2017

   

   paulj3mannequin

   on Apr 26, 2017

   Mannequin

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   This issue was raised in 2013. The proposed patch is still pending

   http://bugs.python.org/issue18943

   I'm going to close this new one, though it is a good reminder of a rare, but persistent bug.
7. paulj3

   closed this as completedon Apr 26, 2017
8. mlouielu commented on Apr 26, 2017

   

   mlouielumannequin

   on Apr 26, 2017

   MannequinAuthor

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   Thanks, paul. Your msg help a lot.

   Will you work on bpo-18943? I can help for this or review the pending patch.
9. ezio-melotti

   transferred this issue fromon Apr 10, 2022