Incorrect interaction between argparse nargs='*' and choices #92445

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1. rhettinger

   added

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

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

   3.11only security fixesonly security fixes

   easy

   3.10 (EOL)end of lifeend of life

   3.9 (EOL)end of lifeend of life

   3.12only security fixesonly security fixes

   on May 8, 2022
2. AbhigyanBose commented on May 9, 2022

   

   AbhigyanBose

   on May 9, 2022

   Contributor

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   @rhettinger

   Could you please elaborate the expected behavior here ?

   I tested this on 3.8 and the main branch. In both p.parse_args([]) gives us the same result as calling the script without any arguments.

   Could you tell me the expected output should the code snippet you shared(also pasted below) be run without any arguments?

   from argparse import ArgumentParser
   
   p = ArgumentParser()
   p.add_argument('dessert', nargs='*', choices=('cake', 'pie'))
   p.parse_args([])
   
3. Harry-Lees commented on May 9, 2022

   

   Harry-Lees

   on May 9, 2022

   Contributor

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   Could you please elaborate the expected behavior here ?

   I believe the expected behaviour for passing an empty list with nargs="*" should be the same behaviour as passing an empty list with nargs="?" since * is 0 or more arguments whereas ? is 0 or 1 arguments.

   >>> from argparse import ArgumentParser
   >>> p = ArgumentParser()
   >>> p.add_argument('dessert', nargs="?", choices=('cake', 'pie'))
   >>> p.parse_args([])
   Namespace(dessert=None)
   
4. hpaulj commented on May 10, 2022

   

   hpaulj

   on May 10, 2022

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   This only happens for a positional. nargs='*' (and '?') gets some special handling in get_values, when an emply list satisfies its nargs. The default is assigned in place of the empty list.

   This is part of the logic that allows us to use these positionals in a mutually_exclusive_group.
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. rhettinger

   closed this as completedon Aug 25, 2022
8. Repository owner moved this from Bugs to Doc issues in Argparse issueson Aug 25, 2022
9. rhettinger

   added a commit that references this issue on Aug 25, 2022

   gh-92445 Improve interaction between nargs="*" and choices() (GH-92565)

   ad7340e