argparse.ArgumentParser does not raise on duplicated subparsers, even though it does on duplicated flags

[anntzer]

BPO	39716
Nosy	@rhettinger, @anntzer
PRs	gh-83897: Raise on conflicting subparser names. #18605

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-02-21.18:20:54.496>
labels = ['library', '3.9']
title = 'argparse.ArgumentParser does not raise on duplicated subparsers, even though it does on duplicated flags'
updated_at = <Date 2020-02-22.11:04:34.546>
user = 'https://github.com/anntzer'

bugs.python.org fields:

activity = <Date 2020-02-22.11:04:34.546>
actor = 'Antony.Lee'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-02-21.18:20:54.496>
creator = 'Antony.Lee'
dependencies = []
files = []
hgrepos = []
issue_num = 39716
keywords = ['patch']
message_count = 3.0
messages = ['362421', '362428', '362458']
nosy_count = 3.0
nosy_names = ['rhettinger', 'paul.j3', 'Antony.Lee']
pr_nums = ['18605']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue39716'
versions = ['Python 3.9']

[anntzer]

If one tries to add twice the same flag to an ArgumentParser, one gets a helpful exception:

    from argparse import ArgumentParser
    p = ArgumentParser()
    p.add_argument("--foo")
    p.add_argument("--foo")

results in

argparse.ArgumentError: argument --foo: conflicting option string: --foo

However, adding twice the same subparser raises no exception:

    from argparse import ArgumentParser
    p = ArgumentParser()
    sp = p.add_subparsers()
    sp.add_parser("foo")
    sp.add_parser("foo")

even though the two subparsers shadow one another in the same way as two identical flags.

[rhettinger]

Would you like to work-up a patch to fix this?

[anntzer]

Sure, #18605 it is.

[ppolewicz]

Please note that this issue is labeled as 3.9 and fix for it was submitted in February 2020, but the fix was actually released with 3.11

[terryjreedy]

@ppolewicz PRs are nearly always merged into main and only sometimes backported. Tracker version labels indicate the version(s) reported for, not necessarily the versions patched. Since we moved the repository to github, PR titles indicate the version(s) actually patched (with 'main' the default). The tracker move to github messed up merge reporting for old PRs. I repointed the PR for this issue to the gh number so it is listed immediately above.

[serhiy-storchaka]

This was actually a duplicate of #59061.