[issue16878] argparse: positional args with nargs='*' defaults to []

[Irit Katriel]

Change by Irit Katriel :


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3.3, Python 3.4

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[issue16878] argparse: positional args with nargs='*' defaults to []

[sebix]

Change by sebix :


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[issue16878] argparse: positional args with nargs='*' defaults to []

[paul j3]

paul j3 added the comment:

The documentation patch here should note that a positional '*' string default 
is not parsed (i.e. does not pass through _get_value).

(see http://bugs.python.org/issue17250)

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[issue16878] argparse: positional args with nargs='*' defaults to []

[paul j3]

paul j3 added the comment:

On a related point, the 'action.required' value is set differently for '?' and 
'*' positionals.  

if kwargs.get('nargs') not in [OPTIONAL, ZERO_OR_MORE]:
kwargs['required'] = True
if kwargs.get('nargs') == ZERO_OR_MORE and 'default' not in kwargs:
kwargs['required'] = True

OPTIONAL is always not required, ZERO_OR_MORE is not required if it has a 
default.  But for reasons discussed here, that 'required' value makes little 
difference.  

`parse_args` checks that all 'required' arguments have been seen, but a 
ZERO_OR_MORE positional is always seen (i.e. it matches an empty string).  

Usage formatting always uses '[%s [%s ...]]' with ZERO_OR_MORE, regardless of 
the 'required' attribute.

The only place where this 'required' value seems to matter is when adding such 
an argument to a mutually exclusive group.  But if an unused '*' positional is 
going to get a '[]' value anyways, why should it be excluded from such a use?

If I remove the 

if kwargs.get('nargs') == ZERO_OR_MORE and 'default' not in kwargs:

test, test_argparse.py still runs fine.

http://bugs.python.org/issue18943 is a possibly related issue, involving  a 'is 
not action.default' test in a mutually exclusive group.

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[issue16878] argparse: positional args with nargs='*' defaults to []

[paul j3]

paul j3 added the comment:

In this example:

 p.add_argument('--foo', nargs='*', default=None)
 p.parse_args([])
Namespace(foo=None)
 p.parse_args(['--foo'])
Namespace(foo=[])

'p.parse_args([])' just assigns the default to 'foo' in the Namespace.

p.parse_args(['--foo']) invokes 'take_action(dest='foo',[]).  That is, it 
'assigns' an empty array to 'foo'.   The same thing would happen if 'foo' was a 
positional.  

'take_action' then passes these arguments to '_get_values'.  That is where the 
differences between '?' and '*' arise.

The key pieces of code in '_get_values' when arg_strings==[] are:

# optional argument produces a default when not present
if not arg_strings and action.nargs == OPTIONAL:
value = action.default
# and evaluate 'value' if is a string

# when nargs='*' on a positional, if there were no command-line
# args, use the default if it is anything other than None
elif (not arg_strings and action.nargs == ZERO_OR_MORE ...):
if action.default is not None:
value = action.default
else:
value = arg_strings # i.e. []

In other words, if nargs='?', the attribute gets its default value.  But for 
'*', this is true only if the default is not None.

So in:

parse([], nargs='?')# get the default value: None
parse([], nargs='*')# default is None, get arg_strings []
parse([], nargs='*', default=None)  # same case
parse([], nargs='*', default=False) # default is not None, get default
parse([], nargs='*', default=0) # same case

I tried changing the _get_values() so '*' got the default (like '?' does), and 
got 54 failures when running test_argparse.py.

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[issue16878] argparse: positional args with nargs='*' defaults to []

[Chris Jerdonek]

Chris Jerdonek added the comment:

It turns out that there is already a test case:

http://hg.python.org/cpython/file/05183ce544be/Lib/test/test_argparse.py#l799

(at the line ('', NS(foo=[])),)

I've updated the patch with a note to reflect this.

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Added file: http://bugs.python.org/file28664/issue-16878-2.patch

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[issue16878] argparse: positional args with nargs='*' defaults to []

[Chris Jerdonek]

Chris Jerdonek added the comment:

Attached is a doc patch.  I also improved some other aspects of the *default* 
section while I was there.

We should probably make sure a test exists for the newly-documented behavior 
(i.e. for passing no arguments for a positional argument with nargs='*' and 
default=None).

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keywords: +patch
Added file: http://bugs.python.org/file28623/issue-16878-1.patch

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[issue16878] argparse: positional args with nargs='*' defaults to []

[Chris Jerdonek]

New submission from Chris Jerdonek:

In argparse, positional arguments with nargs='*' default to [] rather None, 
even if default=None is passed explicitly.  The documentation says otherwise:

The default keyword argument of add_argument(), whose value defaults to None, 
specifies what value should be used if the command-line argument is not 
present. ... For positional arguments with nargs equal to ? or *, the default 
value is used when no command-line argument was present:

(from http://docs.python.org/dev/library/argparse.html#default )

import argparse

def parse(args, **kwargs):
parser = argparse.ArgumentParser()
parser.add_argument('foo', **kwargs)
ns = parser.parse_args(args)
print(repr(ns.foo))

parse([], nargs='?')# None
parse([], nargs='*')# []--
parse([], nargs='*', default=None)  # []--
parse([], nargs='*', default=False) # False
parse([], nargs='*', default=0) # 0

Three options include (there may be more):

(1) document the behavior
(2) make a default of None yield None
(3) do (2), but change the default to [] instead of None when nargs='*'

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components: Library (Lib)
messages: 179174
nosy: bethard, chris.jerdonek
priority: normal
severity: normal
status: open
title: argparse: positional args with nargs='*' defaults to []
type: behavior
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4

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[issue16878] argparse: positional args with nargs='*' defaults to []

[R. David Murray]

R. David Murray added the comment:

I'd prefer to fix it, since having the action='append' and nargs='*' behaviors 
be different looks like a bug.  However, fixing it to match 'append' (default 
really is None) would be likely to break working code, so we certainly couldn't 
backport that, and I'd be really reluctant to fix it that way in 3.4, either.  
(3) is thus preferable to (2), even though it leaves us with an inconsistency.  
It would still be a judgment call on whether or not to backport it.  It seems 
like it would be reasonably safe, but such changes always worry me :)

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[issue16878] argparse: positional args with nargs='*' defaults to []

[Chris Jerdonek]

Chris Jerdonek added the comment:

I agree it would be very likely to break working code.  Can you elaborate on 
your point about 'append' though?  I'm not sure I see it.

Aside from consistency, I'm wondering if there is ever a case where it would 
help to return None for positional arguments.  For example, unlike with 
optional arguments, it doesn't seem like it would ever make sense to 
distinguish between the option being present and the option being present with 
no values (which is why const is needed in the optional case).  In other words, 
there is no loss of information by returning [].

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[issue16878] argparse: positional args with nargs='*' defaults to []

[R. David Murray]

R. David Murray added the comment:

 p.add_argument('-a', action=append, default=None)
 p.parse_args([])
Namespace(a=None)
 p.parse_args(['-a', '1', '-a', '2'])
Namespace(a=['1', '2'])

So there's a logical correspondence there (repeated option vs multiple values, 
each producing a list).

I'm not sure what you mean by the difference between options and positionals.  
I note that you can use nargs with an optional, and in that case the default 
works, which is even more unfortunate than the difference with 'append'.  But 
if you can articulate a logical difference between optionals and positional 
here, maybe we can make it look reasonable :)

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[issue16878] argparse: positional args with nargs='*' defaults to []

[Chris Jerdonek]

Chris Jerdonek added the comment:

I was referring to the fact that optionals have an additional case that 
positionals don't have: Note that for optional arguments, there is an 
additional case -- the option string is present but not followed by a 
command-line argument.

(from http://docs.python.org/dev/library/argparse.html#nargs )

 p.add_argument('--foo', nargs='*', default=None)
 p.parse_args([])
Namespace(foo=None)
 p.parse_args(['--foo'])
Namespace(foo=[])

So it could be argued that positionals (at least by default) are behaving like 
the second case.  But that's as far as the parallel goes apparently.  *default* 
affects the first case and not the second case for optional arguments:

 p.add_argument('--foo', nargs='*', default=False)
 p.parse_args([])
Namespace(foo=False)
 p.parse_args(['--foo'])
Namespace(foo=[])

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