[issue24419] In argparse action append_const doesn't work for positional arguments

[paul j3]

Changes by paul j3 :


--
status: open -> closed

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[paul j3]

paul j3 added the comment:

To wrap this up, the correct way to specify that 2 or more positionals share a 
'dest' is to supply that dest as the first parameter.  If the help should have 
something else, use the `metavar`.

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('x', action='append_const', const=42, metavar='foo')
parser.add_argument('x', action='append_const', const=43, metavar='bar')
parser.print_help()
args=parser.parse_args([])
print(args)

produces

usage: issue24419.py [-h]

positional arguments:
  foo
  bar

optional arguments:
  -h, --help  show this help message and exit
Namespace(x=[42, 43])

(I think this issue can be closed).

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[py.user]

py.user added the comment:

Tested on argdest.py:

#!/usr/bin/env python3

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('x', action='append')
parser.add_argument('x', action='append_const', const=42, metavar='foo')
parser.add_argument('x', action='append_const', const=43, metavar='bar')
parser.add_argument('-x', action='append_const', const=44)

args = parser.parse_args()
print(args)


Run:

[guest@localhost debug]$ ./argdest.py -h
usage: argdest.py [-h] [-x] x

positional arguments:
  x
  foo
  bar

optional arguments:
  -h, --help  show this help message and exit
  -x
[guest@localhost debug]$ ./argdest.py -x 1 -x
Namespace(x=[44, '1', 42, 43, 44])
[guest@localhost debug]$


LGTM.

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[paul j3]

paul j3 added the comment:

You can give the positional any custom name (the first parameter).  You just 
can't reuse it (by giving 2 positionals the same name).  And if you don't like 
what the 'help' shows, you can set the 'metavar'.  That way only you see the 
positional's name.

The name of a positional can be the 'dest' of an optional.  But wouldn't that 
be confusing?  Setting the same attribute with a required postional and one or 
more optional optionals?

'nargs' is another way of assigning more than one value to a Namespace 
attribute.  You just can't put an optional between two such values.

`argparse` is a parser, a way of identifying what the user gives you.  It is 
better to err on the side of preserving information.  Different argument dests 
does that.   You can always combine values after parsing.

args.foo.append(args.bar)   # or .extend()
args.x = [args.foo, args.bar]

Don't try to force argparse to do something special when you can just as easily 
do that later with normal Python expressions.

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[py.user]

py.user added the comment:

paul j3 wrote:
 You can give the positional any custom name (the first parameter).

The dest argument is not required for giving name for an optional.
You can either make it automatically or set by dest, it's handy and clear.

 import argparse
 
 parser = argparse.ArgumentParser()
 _ = parser.add_argument('-a', '--aa')
 _ = parser.add_argument('-b', '--bb', dest='x')
 args = parser.parse_args([])
 print(args)
Namespace(aa=None, x=None)


But if you do the same thing with a positional, it throws an exception. Why?
(I'm a UNIX user and waiting predictable behaviour.)

And the situation with another action (not only append_const, but future 
extensions) shows that dest may be required.

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[paul j3]

paul j3 added the comment:

(Important correction at the end of this post)

The test that you are complaining about occurs at the start of the 
'add_argument' method:

def add_argument(self, *args, **kwargs):

add_argument(dest, ..., name=value, ...)
add_argument(option_string, option_string, ..., name=value, ...)


# if no positional args are supplied or only one is supplied and
# it doesn't look like an option string, parse a positional
# argument
chars = self.prefix_chars
if not args or len(args) == 1 and args[0][0] not in chars:
if args and 'dest' in kwargs:
raise ValueError('dest supplied twice for positional argument')
kwargs = self._get_positional_kwargs(*args, **kwargs)

# otherwise, we're adding an optional argument
else:
kwargs = self._get_optional_kwargs(*args, **kwargs)
...

and the 2 methods it calls:

def _get_positional_kwargs(self, dest, **kwargs):
# code to deduce the 'required' parameter ...
# return the keyword arguments with no option strings
return dict(kwargs, dest=dest, option_strings=[])

def _get_optional_kwargs(self, *args, **kwargs):
# determine short and long option strings

# infer destination, '--foo-bar' - 'foo_bar' and '-x' - 'x'
dest = kwargs.pop('dest', None)
if dest is None:
if long_option_strings:
dest_option_string = long_option_strings[0]
else:
dest_option_string = option_strings[0]
dest = dest_option_string.lstrip(self.prefix_chars)
if not dest:
msg = _('dest= is required for options like %r')
raise ValueError(msg % option_string)
dest = dest.replace('-', '_')

# return the updated keyword arguments
return dict(kwargs, dest=dest, option_strings=option_strings)

At the 'add_argument' stage, a big difference between positionals and optionals 
is in how 'dest' is deduced.  Note the doc string.

During parsing, positionals are distinguished from optionals by the 
'option_strings' attribute (empty or not).  'dest' is not used during parsing, 
except by the Action '__call__'.

-

I just thought of another way around this constraint - set 'dest' after the 
action is created:

p=argparse.ArgumentParser()
a1=p.add_argument('foo',action='append')
a2=p.add_argument('bar',action='append')
a1.dest='x'
a2.dest='x'
args=p.parse_args(['one','two'])

produces

Namespace(x=['one', 'two'])

This works because after the action has been created, no one checks whether the 
'dest' value is duplicated or even looks pretty (except when trying to format 
it for the help.

You could also write a custom Action class, one that mangles the 'dest' to your 
heart's delight.  The primary use of 'self.dest' is in the expression:

setattr(namespace, self.dest, items)

you could replace this line in the Action '__call__' with

setattr(namespace, 'secret#dest', items)

-

I was mistaken on one thing - you can reuse positional 'names':

 a1=p.add_argument('foo',action='append')
 a2=p.add_argument('foo',action='append',type=int)
 p.parse_args(['a','3'])

produces:

 Namespace(foo=['a', 3])

There is a 'name' conflict handler, but it only pays attention to the option 
strings (flags for optionals).  You can't have two arguments using '-f' or 
'--foo'.  But you can have 2 or more positionals with the same 'dest'.  You 
just have to set the dest the right way.

This last point renders the whole issue moot.  But I'll leave it at the end to 
reflect my train of thought.

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[py.user]

py.user added the comment:

paul j3 wrote:
 What are you trying to accomplish in the examples with a 'dest'?

To append all that constants to one list.

From this:
Namespace(bar=[43], foo=[42])

To this:
Namespace(x=[43, 42])

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[paul j3]

paul j3 added the comment:

What are you trying to accomplish in the examples with a 'dest'?  For a 
positional, 'dest' is derived from the 'foo' name.  There is no need to supply 
'dest', in fact produces the error you get.  It has nothing to with this action 
type (as your last example demonstrates).

Your case with 'foo' and 'bar' shows that 'append_const' works fine.  

But be ware that such an action does not make much sense for a positional.  
Such an action takes no arguments, i.e. 'nargs=0'.  So such a positional is 
always present, since it does not consume any `argv` strings.  You might even 
get an error if you supply a string.  (same would be true of 'store_true' and 
related actions).

--
nosy: +paul.j3

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[py.user]

py.user added the comment:

paul j3 wrote:
 The name (and hence the 'dest') must be unique.

The problem is in the dest argument of add_argument(). Why user can't set a 
custom name for a positional?

We can use this list not only for positionals but for optionals too.

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[paul j3]

paul j3 added the comment:

None of the `append` actions makes sense with positionals.  The name (and hence 
the 'dest') must be unique.  And positionals can't be repeated.

There are other ways to put a pair of values in the Namespace.  For example, 
after parsing

args.x = [42, 43]

or before

ns = argparse.Namespace(x=[42,43])
parser.parse_args(namespace=ns)

or the `const` (or better the default) could be `[42, 43]`.

Plain `append` might let you put `[42,43]` in the dest via the default, and 
then append further values to that list (from the user).  I'd have to test that.

It might be instructive to look at the `test_argparse.py` file, and search for 
test cases that use `append` or `const`.

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24419] In argparse action append_const doesn't work for positional arguments

[py.user]

New submission from py.user:

Action append_const works for options:

 import argparse
 
 parser = argparse.ArgumentParser()
 _ = parser.add_argument('--foo', dest='x', action='append_const', const=42)
 _ = parser.add_argument('--bar', dest='x', action='append_const', const=43)
 parser.parse_args('--foo --bar'.split())
Namespace(x=[42, 43])


Action append_const works for single positionals:

 import argparse
 
 parser = argparse.ArgumentParser()
 _ = parser.add_argument('foo', action='append_const', const=42)
 _ = parser.add_argument('bar', action='append_const', const=43)
 parser.parse_args([])
Namespace(bar=[43], foo=[42])


Action append_const doesn't work for positionals in one list:

 import argparse
 
 parser = argparse.ArgumentParser()
 _ = parser.add_argument('foo', dest='x', action='append_const', const=42)
Traceback (most recent call last):
  File stdin, line 1, in module
  File /usr/lib/python3.3/site-packages/argparse-1.1-py3.3.egg/argparse.py, 
line 1282, in add_argument

ValueError: dest supplied twice for positional argument
 _ = parser.add_argument('bar', dest='x', action='append_const', const=43)
Traceback (most recent call last):
  File stdin, line 1, in module
  File /usr/lib/python3.3/site-packages/argparse-1.1-py3.3.egg/argparse.py, 
line 1282, in add_argument

ValueError: dest supplied twice for positional argument
 parser.parse_args([])
Namespace()



The reason is that a positional argument can't accept dest:

 import argparse
 
 parser = argparse.ArgumentParser()
 parser.add_argument('foo', dest='x')
Traceback (most recent call last):
  File stdin, line 1, in module
  File /usr/lib/python3.3/site-packages/argparse-1.1-py3.3.egg/argparse.py, 
line 1282, in add_argument

ValueError: dest supplied twice for positional argument


--
components: Library (Lib)
messages: 245099
nosy: py.user
priority: normal
severity: normal
status: open
title: In argparse action append_const doesn't work for positional arguments
type: behavior
versions: Python 3.3, Python 3.6

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue24419
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com