Unpacking arguments from argparse

Question

I've been writing some command line python programs and using argparse to do it. I've been structuring my code somewhat as follows.

def main(arg1, arg2):
    # magic
    pass

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('arg1')
    parser.add_argument('arg2')

    args = parser.parse_args()

    main(args.arg1, args.arg2)

It's really super irritating to have to call out arg1 and arg2 3 times. I understand having to do it twice.

Is there some way to treat the namespace returned by the parse_args function as a tuple? Or better yet as a tuple and a dict for optional args and do unpacking?

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('arg1')
    parser.add_argument('arg2')
    parser.add_argument('--opt-arg', default='default_value')

    args, kwargs = parser.magic_method_call_that_would_make_my_life_amazing()

    # I get goosebumps just thinking about this
    main(*args, **kwargs)

Answer 1

https://docs.python.org/3/library/argparse.html#the-namespace-object

This class is deliberately simple, just an object subclass with a readable string representation. If you prefer to have dict-like view of the attributes, you can use the standard Python idiom, vars():

>>>
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> args = parser.parse_args(['--foo', 'BAR'])
>>> vars(args)
{'foo': 'BAR'}

Note that one of the big advances, or changes at least, from optparse to argparse is that positional arguments, such as yours, are treated the same as optionals. They both appear in the args Namespace object. In optparse, positionals are just the left overs from parsing defined options. You could get the same effect in argparse by omiting your arguments and using parse_known_args:

parser = argparse.ArgumentParser()
args, extras = parser.parse_known_args()

args is now a Namespace, and extras a list. You could then call your function as:

myfoo(*extras, **vars(args))

For example:

In [994]: import argparse
In [995]: def foo(*args, **kwargs):
   .....:     print(args)
   .....:     print(kwargs)
   .....:     
In [996]: parser=argparse.ArgumentParser()
In [997]: parser.add_argument('-f','--foo')
Out[997]: _StoreAction(option_strings=['-f', '--foo'], dest='foo', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)
In [998]: args,extras = parser.parse_known_args(['-f','foobar','arg1','arg2'])
In [999]: args
Out[999]: Namespace(foo='foobar')
In [1000]: extras
Out[1000]: ['arg1', 'arg2']
In [1001]: foo(*extras, **vars(args))
('arg1', 'arg2')
{'foo': 'foobar'}

That same argparse paragraph shows that you can define your own Namespace class. It wouldn't be hard to define one that behaves like a dictionary (for use as **args) and as namespace. All argparse requires is that it works with getattr and setattr.

In [1002]: getattr(args,'foo')
Out[1002]: 'foobar'
In [1004]: setattr(args,'bar','ugg')
In [1005]: args
Out[1005]: Namespace(bar='ugg', foo='foobar')

another standard Python feature lets me pass vars(args) as a tuple:

In [1013]: foo(*vars(args).items())
(('foo', 'foobar'), ('bar', 'ugg'))
{}

For a similar answer from last January: https://stackoverflow.com/a/34932478/901925

Neatly pass positional arguments as args and optional arguments as kwargs from argpase to a function

There I give ideas on how to separate out 'positionals' from 'optionals' after parsing.

---

Here's a custom namespace class that includes, in its API, a means of returning itself as a dictionary:

In [1014]: class MyNameSpace(argparse.Namespace):
   ......:     def asdict(self):
   ......:         return vars(self)
   ......:     
In [1015]: args = parser.parse_args(['-f','foobar'], namespace=MyNameSpace())
In [1016]: args
Out[1016]: MyNameSpace(foo='foobar')
In [1017]: foo(**args.asdict())
()
{'foo': 'foobar'}

---

Another idea - use one of the multiple nargs (2,'*','+') for the positional argument. Then you have only one name to type when passing it to your function.

parser.add_argument('pos',nargs='+')
args = ...
args.pos # a list, possibly empty
foo(*args.pos, **vars(args))