User input to argparse raises Index_Error: "-a=" on a 'store_true' action

[paulj3]

BPO	36267
Nosy	@rhettinger, @tirkarthi, @shihai1991
PRs	gh-80448: argparse: Fix IndexError on store_true action #15656
Files	parse_v1.patch

^{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 2019-03-12.01:36:10.610>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.7', 'library']
title = 'User input to argparse raises Index_Error:  "-a=" on a \'store_true\' action'
updated_at = <Date 2020-06-06.22:10:34.797>
user = 'https://bugs.python.org/paulj3'

bugs.python.org fields:

activity = <Date 2020-06-06.22:10:34.797>
actor = 'remi.lapeyre'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-03-12.01:36:10.610>
creator = 'paul.j3'
dependencies = []
files = ['48582']
hgrepos = []
issue_num = 36267
keywords = ['patch']
message_count = 7.0
messages = ['337709', '351031', '351035', '351047', '351048', '351050', '351061']
nosy_count = 4.0
nosy_names = ['rhettinger', 'paul.j3', 'xtreak', 'shihai1991']
pr_nums = ['15656']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36267'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

• PR: [3.10] gh-80448: argparse: Fix IndexError on store_true action (GH-15656) #99404

• PR: [3.11] gh-80448: argparse: Fix IndexError on store_true action (GH-15656) #99405

[paulj3]

Test case:

    parser = argparse.ArgumentParser()
    parser.add_argument("-a", action="store_true")
    args = parser.parse_args("-a=".split())

raises an Index_Error, which is not caught by the argparse error mechanism.

This is an unusual case, the coincidence of several user errors - 'store_true' which shouldn't take an argument and a short optional with a bare =. So it's unlikely to occur. Still, argparse shouldn't ever respond to a command line value with an uncaught error.

The traceback shows that it occurs during the handling of the explicit_arg in consume_optional.

Traceback (most recent call last):
  File "argparseRaiseIndexError.py", line 5, in <module>
    args = parser.parse_args("-a=".split())
  File "/usr/lib/python3.6/argparse.py", line 1743, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/lib/python3.6/argparse.py", line 1775, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib/python3.6/argparse.py", line 1981, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/usr/lib/python3.6/argparse.py", line 1881, in consume_optional
    option_string = char + explicit_arg[0]
IndexError: string index out of range

The issue was raised in a Stackoverflow question, where I and the poster explore why it occurs, and possible fixes.

https://stackoverflow.com/questions/54662609/python-argparse-indexerror-for-passing-a

[shihai1991]

I do something such as:
p = argparse.ArgumentParser()
p.add_argument('-a', action='store_true')
p.add_argument('-b', action='store_true')
p.parse_args('-ab='.split())

thos code code jump in 1903, and the explicit_arg's value is: 'b='

1901 action_tuples.append((action, [], option_string))
1902 char = option_string[0]
1903 option_string = char + explicit_arg[0]

IMHO, we should judge this explicit_arg's value before jump this judgment statement.

[shihai1991]

Adding a judgment of explicit_args in judgment statement in the patch.

[rhettinger]

FWIW, raising an uncaught exception isn't what we normally consider a "crash". We typically save that designation for segfaults.

Ideally, a good patch/pr will also have tests.

Also, given the rarity of the exception, I would be reserved about backporting, keeping it to 3.8 and 3.9.

[shihai1991]

Thanks, Raymond. This patch not a formal patch, just show what i want to do. if everything agree this behavior, I would add a normal PR.

[rhettinger]

Why did you switch back to "crash"?

[shihai1991]

oh, sorry, pls ignore it. I misunderstand your opinion.