[New-bugs-announce] [issue45235] argparse does not preserve namespace with subparser defaults

[Adam Schwalm]

New submission from Adam Schwalm <adamschwalm at gmail.com>:

The following snippet demonstrates the problem. If a subparser flag has a default set, argparse will override the existing value in the provided 'namespace' if the flag does not appear (e.g., if the default is used):

    import argparse
    parser = argparse.ArgumentParser()
    sub = parser.add_subparsers()
    example_subparser = sub.add_parser("example")
    example_subparser.add_argument("--flag", default=10)
    print(parser.parse_args(["example"], argparse.Namespace(flag=20)))

This should return 'Namespace(flag=20)' because 'flag' already exists in the namespace, but instead it returns 'Namespace(flag=10)'. This intended behavior is described and demonstrated in the second example here: https://docs.python.org/3/library/argparse.html#default

Lib's behavior is correct for the non-subparser cause.

----------
components: Library (Lib)
messages: 402060
nosy: ALSchwalm
priority: normal
severity: normal
status: open
title: argparse does not preserve namespace with subparser defaults
type: behavior
versions: Python 3.10, Python 3.11, Python 3.6, Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45235>
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