[issue9352] argparse eats characters when parsing multiple merged short options

[Steven Bethard]

New submission from Steven Bethard <steven.bethard at gmail.com>:

[Moved from http://code.google.com/p/argparse/issues/detail?id=73]

What steps will reproduce the problem?
parser = ArgumentParser(prefix_chars="-+")
parser.add_argument("-a",action="store_true")
parser.add_argument("+b",action="store_true")
parser.add_argument("+c",action="store_true")
print parser.parse_args("-abc".split())

What is the expected output? What do you see instead?
I would expect Namespace(a=True, b=True, c=True)
You get Namespace(a=True, b=False, c=True)

because in the loop that iterates through the prefix characters it builds an option_string to try by pulling the first character off explicit_arg. If it doesn't match any option then it will try the next prefix character, but it will have lost that character from the explicit_arg. This is also a problem even with only one prefix character because if it doesn't match an option the error message will be missing that one character.

The above match -a, tried -b and fails, but instead of trying +b, it tries +c because the b has been lost.

I've included multiple_short.patch which fixes just this bug. However, I also don't agree with the behavior of trying all prefix characters. I would expect that when merging short options together they would have to all share the same prefix character. If I have prefix_chars="-+" and I have options -a, +a, -b, and +b and I type +ab I would expect my action to be called with option_strings +a and +b, not +a and -b. The patch to fix the above bug *and* only try the same prefix character as the first option is multiple_short_same_prefix.patch.

[Only the latter patch (which sounds like the right behavior) is attached. It will need to be updated to work against Python trunk.]

----------
components: Library (Lib)
files: multiple_short_same_prefix.patch
keywords: patch
messages: 111328
nosy: bethard
priority: normal
severity: normal
stage: needs patch
status: open
title: argparse eats characters when parsing multiple merged short options
type: behavior
versions: Python 2.7, Python 3.2
Added file: http://bugs.python.org/file18147/multiple_short_same_prefix.patch

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<http://bugs.python.org/issue9352>
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