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python argparse
Updated Mon, 06 Jun 2022 08:44:13 GMT

Python argparse - mandatory argument - either positional or optional


I want the user to be able to pass a mandatory argument to 'argparse', with either positional, or optional argument.

I.e., both following forms are valid:

my_prog arg
my_prog -m arg

I've seen Argparse optional positional arguments?

But the suggestions there make both form optional. I want that one of them will be mandatory.

Of course, I can add a manual checking after parsing that at least one of them has been set. But I got a hunch that there must be a better solution.

(And even with my manual approach, the 'help' section shows both of them as optional)

Solution

The mutually exclusive group mechanism can take a required parameter. It also works with one ? positional along with the optionals (flagged arguments). (more than one '?' positional doesn't make sense).

As for the help display there are 2 default groups, positonal and optional. So even if an optional (flagged) is set to required it is, by default, displayed in the optional group. The usage line is a better guide as to whether an argument is required or not. If you don't like the group labels in the help section, define your own argument groups.

In [146]: import argparse
In [147]: parser = argparse.ArgumentParser()
In [148]: gp = parser.add_mutually_exclusive_group(required=True)
In [149]: gp.add_argument('pos', nargs='?', default='foo');
In [150]: gp.add_argument('-f','--foo', default='bar');
In [151]: parser.parse_args('arg'.split())
Out[151]: Namespace(foo='bar', pos='arg')
In [152]: parser.parse_args('-f arg'.split())
Out[152]: Namespace(foo='arg', pos='foo')
In [153]: parser.parse_args('arg -f arg'.split())
usage: ipython3 [-h] [-f FOO] [pos]
ipython3: error: argument -f/--foo: not allowed with argument pos
In [154]: parser.parse_args(''.split())
usage: ipython3 [-h] [-f FOO] [pos]
ipython3: error: one of the arguments pos -f/--foo is required
In [155]: parser.parse_args('-h'.split())
usage: ipython3 [-h] [-f FOO] [pos]
positional arguments:
  pos
optional arguments:
  -h, --help         show this help message and exit
  -f FOO, --foo FOO

Oops, usage isn't showing the -f and pos in a mutually exlusive group. Sometimes that usage formatting is brittle.

Switching the order in which the arguments are defined gives a better usage

In [156]: parser = argparse.ArgumentParser()
In [157]: gp = parser.add_mutually_exclusive_group(required=True)
In [158]: gp.add_argument('-f','--foo', default='bar');
In [159]: gp.add_argument('pos', nargs='?', default='foo');
In [160]: 
In [160]: parser.parse_args('-h'.split())
usage: ipython3 [-h] (-f FOO | pos)
positional arguments:
  pos
optional arguments:
  -h, --help         show this help message and exit
  -f FOO, --foo FOO

With a user defined argument group:

In [165]: parser = argparse.ArgumentParser()
In [166]: gp = parser.add_argument_group('Mutually exclusive')
In [167]: gpm = gp.add_mutually_exclusive_group(required=True)
In [168]: gpm.add_argument('-f','--foo', default='bar');
In [169]: gpm.add_argument('pos', nargs='?', default='foo');
In [170]: 
In [170]: parser.parse_args('-h'.split())
usage: ipython3 [-h] (-f FOO | pos)
optional arguments:
  -h, --help         show this help message and exit
Mutually exclusive:
  -f FOO, --foo FOO
  pos

This is the one exception to the general rule argument_groups and mutually_exclusive_groups aren't designed for nesting.

The m-x-group was not required, usage would use []

usage: ipython3 [-h] [-f FOO | pos]




Comments (2)

  • +0 – Great. Just one caveat: these group definitions must be after all other regular arguments, otherwise it won't show the nice (-f FOO | pos) in the help — Nov 15, 2017 at 20:34  
  • +0 – Yes, 2 things affect the usage. Positionals are shown after optionals, and a group only shows up if its arguments match that order. Correcting that brittleness will require a major rewrite of the usage formatter. — Nov 15, 2017 at 20:58  

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