[issue36267] User input to argparse raises Index_Error: "-a=" on a 'store_true' action

[paul j3]

New submission from paul j3 <ajipanca at gmail.com>:

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

----------
components: Library (Lib)
messages: 337709
nosy: paul.j3
priority: normal
severity: normal
stage: needs patch
status: open
title: User input to argparse raises Index_Error:  "-a=" on a 'store_true' action
type: crash

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36267>
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