[issue23487] argparse: add_subparsers 'action' broken

[paul j3]

paul j3 added the comment:

I'm going to close this.  At most it calls for a documentation change.  But 
saying something like

   It must be compatible with argparse._SubParsersAction.

will just confuse the average user.  Any customization of this action class is 
an advanced topic, that requires familiarity with the code, not just the 
documentation.

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stage:  -> resolved
status: open -> closed

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[issue23487] argparse: add_subparsers 'action' broken

[paul j3]

paul j3 added the comment:

OK, so you are thinking about what happens to the subparsers `dest` when the 
user names a command.  That isn't handled by the `store` action, but by the 
call of the subparsers action

class _SubParsersAction(Action):

def __call__

# set the parser name if requested
if self.dest is not SUPPRESS:
setattr(namespace, self.dest, parser_name)
...
namespace, arg_strings = parser.parse_known_args(arg_strings, 
namespace)

Storing the parser_name is a minor part of this action.  The important, 
distinctive part is passing the parsing action to that subparser.

That storing could be done in an append way, but why?  There can only one 
`subparsers`, and it can be invoked only once.

None of the existing Action classes can replace _SubParsersAction, which is 
entered in the `registry` as `parsers`:

In [11]: parser._registry_get('action','parsers')
Out[11]: argparse._SubParsersAction
 
If you want to write your own version of this action, you can use it, either by 
replacing the existing class, or by changing this registry entry.

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[issue23487] argparse: add_subparsers 'action' broken

[paul j3]

paul j3 added the comment:

As to the nature of the error when 'add_subparsers' is given an 'action' 
parameter:

'add_subparsers' does several things to 'kwargs' before it passes them to the 
relevant Action class.

 def add_subparsers(self, **kwargs):
# adds 'parser_class'
# removes 'title', 'description' (used in an argument group)
# add 'prog'
parsers_class = self._pop_action_class(kwargs, 'parsers')
action = parsers_class(option_strings=[], **kwargs)

What I wrote earlier about using the registry is partly wrong.  The Action 
class is determined by either the 'action' parameter or the registry entry.

 In [17]: p._pop_action_class({}, 'parsers')
 Out[17]: argparse._SubParsersAction

 In [18]: p._pop_action_class({'action':'test'}, 'parsers')
 Out[18]: 'test'

So the 'action' parameter works - if you specify a compatible Action class.

sp=p.add_subparsers(dest='cmd',action=argparse._SubParsersAction)

Such a class must have the same __init__ signature, otherwise you'll get errors 
such the OP's.

It might be worth rewriting the documentation line so this is clearer.  
Otherwise I recommend closing this issue.






 action = parsers_class(option_strings=[], **kwargs)

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[issue23487] argparse: add_subparsers 'action' broken

[paul j3]

Changes by paul j3 ajipa...@gmail.com:


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assignee:  - docs@python
components: +Documentation
nosy: +docs@python

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[issue23487] argparse: add_subparsers 'action' broken

[Berker Peksag]

Changes by Berker Peksag berker.pek...@gmail.com:


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[issue23487] argparse: add_subparsers 'action' broken

[SpaceOne]

SpaceOne added the comment:

In replay to msg238931 from paul j3 (paul.j3) *
 And specifying something other than the default 'store' action class for the 
 arguments of the parsers doesn't make sense.
Of course it makes sense. If you e.g. want the action to be 'append' so that 
the subparser-name is appended to a already existing list / or to create a new 
list with the subparser-name as first argument.
E.g.:

parser.add_subparser(dest='foo', action='append')
parser.parse_args('bar').__dict__ == {'foo': ['bar']}

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[issue23487] argparse: add_subparsers 'action' broken

[paul j3]

paul j3 added the comment:

It certainly looks like a documentation issue.  `action` doesn't make sense 
here.  `add_subparsers` normally creates an Action of type `_SubParsersAction`. 
 

'action' for normal 'add_argument' command defines the Action type created at 
that time.  Conceivably a user might want to use a customized 
'_SubParsersAction', but it shouldn't be as easy as just specifying 'action' in 
the 'add_subparsers' command.

And specifying something other than the default 'store' action class for the 
arguments of the parsers doesn't make sense.

But I'm puzzled by the error message you got.  I'll have to run some tests to 
figure out what is going on.

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[issue23487] argparse: add_subparsers 'action' broken

[BJ Dierkes]

New submission from BJ Dierkes:

Related: http://bugs.python.org/issue9253

I came across issue9253 in trying to implement a default action for a subparser 
namespace.  In the absence of a 'default' option, I thought that it may be 
possible by adding an 'action' to 'add_subparsers'.  Per the documentation this 
should be possible:

https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser.add_subparsers

[QUOTE]
action - the basic type of action to be taken when this argument is 
encountered at the command line
[/QUOTE]


That said, custom actions on 'add_subparsers' doesn't appear to work at all:

import argparse

class CustomAction(argparse.Action):
def __call__(self, parser, namespace, values, option_string=None):
print('Inside CustomAction')
setattr(namespace, self.dest, values)

root_parser = argparse.ArgumentParser(prog='myapp')
sub_parser = root_parser.add_subparsers(dest='commands', 
action=CustomAction)
args = root_parser.parse_args()


Produces:

$ python argtest.py --help
Traceback (most recent call last):
  File argtest.py, line 46, in module
sub_parser = root_parser.add_subparsers(dest='commands', 
action=CustomAction)
  File 
/usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/argparse.py,
 line 1661, in add_subparsers
action = parsers_class(option_strings=[], **kwargs)
TypeError: __init__() got an unexpected keyword argument 'prog'



Erroneous documentation maybe?  Tested the same on Python 2.7 and 3.3.

--
components: Library (Lib)
messages: 236254
nosy: derks
priority: normal
severity: normal
status: open
title: argparse: add_subparsers 'action' broken
type: behavior
versions: Python 2.7, Python 3.3

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