[issue9348] Calling argparse's add_argument with the wrong number of metavars causes delayed error message

[daniel hahler]

daniel hahler  added the comment:

This adds overhead, since it creates a formatter and uses it for formatting 
only for validation purposes.

I think it is better to only have the error when the formatter is actually used 
(i.e. the help is displayed - which is not the typical use case, and it should 
be optimized for not displaying the help (i.e. only parsing args)).
The error could be more specific than before/initially though, of course.

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[issue9348] Calling argparse's add_argument with the wrong number of metavars causes delayed error message

[Roundup Robot]

Roundup Robot devnull@devnull added the comment:

New changeset c89db9b36ea6 by Steven Bethard in branch '3.2':
Issue #9348: Raise an early error if argparse nargs and metavar don't match.
http://hg.python.org/cpython/rev/c89db9b36ea6

New changeset b93a50bb74f2 by Steven Bethard in branch 'default':
Issue #9348: Raise an early error if argparse nargs and metavar don't match. 
(Merge from 3.2.)
http://hg.python.org/cpython/rev/b93a50bb74f2

New changeset 4bb651eb7539 by Steven Bethard in branch '2.7':
Issue #9348: Raise an early error if argparse nargs and metavar don't match. 
(Merge from 3.2.)
http://hg.python.org/cpython/rev/4bb651eb7539

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[issue9348] Calling argparse's add_argument with the wrong number of metavars causes delayed error message

[Steven Bethard]

Steven Bethard steven.beth...@gmail.com added the comment:

Thanks for the patch. I used something similar to what you proposed, but 
instead of creating a local formatter, I just call self._get_formatter() if it 
exists.

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assignee:  - bethard
nosy:  -python-dev
resolution:  - fixed
stage: needs patch - committed/rejected
status: open - closed
versions: +Python 3.3

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[issue9348] Calling argparse's add_argument with the wrong number of metavars causes delayed error message

[Denver Coneybeare]

Denver Coneybeare denver.coneybe...@gmail.com added the comment:

Awesome, thanks for committing the patch.  Glad I could help.

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[issue9348] Calling argparse's add_argument with the wrong number of metavars causes delayed error message

[Denver Coneybeare]

Denver Coneybeare denver.coneybe...@gmail.com added the comment:

For kicks, I just took a look at this old, forgotten issue.  I agree with the 
submitter that add_argument() should fail if nargs and metavar do not match, 
instead of having format_help() raise the exception later on.

I've attached a patch (issue9348_patch_v01.txt) with a proposed fix and 
associated test cases.  The drawback of my approach is that if a custom 
HelpFormatter is used which has different semantics for metavar it could cause 
add_argument() to incorrectly reject the metavar value.  I'm not sure how 
common that is though.

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Added file: http://bugs.python.org/file21294/issue9348_patch_v01.txt

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[issue9348] Calling argparse's add_argument with the wrong number of metavars causes delayed error message

[Steven Bethard]

Steven Bethard steven.beth...@gmail.com added the comment:

You can specify either 1 or N. So for n=3, you can specify metavar=X or 
metavar=(X, Y, Z) but not metavar=(X, Y). The special nargs value ? 
always takes only one, while * and + always take two. (This makes sense if 
you think about how they're formatted, e.g. X [X ...].)

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[issue9348] Calling argparse's add_argument with the wrong number of metavars causes delayed error message

[Catherine Devlin]

Catherine Devlin fredv8vi...@liquidid.net added the comment:

Thanks for the correction, Steven.  This unit test matches your description.  
Use it instead of my earlier submission.

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Added file: http://bugs.python.org/file18390/wrong_metavars_test_corrected.patch

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[issue9348] Calling argparse's add_argument with the wrong number of metavars causes delayed error message

[Catherine Devlin]

Catherine Devlin fredv8vi...@liquidid.net added the comment:

I'm attaching a unit test patch that does something vaguely like Steven 
suggests.  It definitely needs review.

Also, I'm assuming that it's OK to specify *fewer* metavars than nargs, just 
not more... but I'm not sure about that.

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keywords: +patch
nosy: +catherine
Added file: http://bugs.python.org/file18327/wrong_metavars_test.patch

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[issue9348] Calling argparse's add_argument with the wrong number of metavars causes delayed error message

[Steven Bethard]

New submission from Steven Bethard steven.beth...@gmail.com:

What steps will reproduce the problem?

parser = argparse.ArgumentParser()  
parser.add_argument('--foo', nargs=2, metavar=('X','Y','Z'))
parser.parse_args(['-h'])

The error dosn't show up until help is formatted.

Giving any incorrect length of metavar will produce the problem, which
includes a tuple whos length doesn't match a numerical value, more than two 
metavars to '*' or '+', and more than one metavar to '?'. Furthermore, a tuple 
of length one causes the error when nargs is greater than 1, '*', or '+'.

What is the expected output? What do you see instead?

When the help is displayed, you get:

TypeError: not all arguments converted during string formatting

Or a similar error message for other cases.

It would be expected that the error message would be more specific. The
error should definitely be raised when add_argument is called, rather than 
later.

There should be a test that does something like:

for meta in ('X', ('X',), ('X','Y'), ('X','Y','Z')):
for n in (1, 2, 3, '?', '+', '*'):
parser = argparse.ArgumentParser()
parser.add_argument('--foo', nargs=n, metavar=meta)
parser.format_help()

and makes sure that the error shows up in add_argument, not format_help.

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components: Library (Lib)
messages: 111317
nosy: bethard
priority: normal
severity: normal
stage: needs patch
status: open
title: Calling argparse's add_argument with the wrong number of metavars causes 
delayed error message
type: behavior
versions: Python 2.7, Python 3.2

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