Accessing the choices passed to argument in argparser?

Question

Is it possible to access the tuple of choices passed to an argument? If so, how do I go about it

for example if I have

parser = argparse.ArgumentParser(description='choose location')
parser.add_argument(
    "--location",
    choices=('here', 'there', 'anywhere')
)
args = parser.parse_args()

can I access the tuple ('here', 'there', 'anywhere')?

Answer 1

It turns out that parser.add_argument actually returns the associated Action. You can pick the choices off of that:

>>> import argparse
>>> parser = argparse.ArgumentParser(description='choose location')
>>> action = parser.add_argument(
...     "--location",
...     choices=('here', 'there', 'anywhere')
... )
>>> action.choices
('here', 'there', 'anywhere')

Note that (AFAIK) this isn't documented anywhere and may be considered an "implementation detail" and therefore subject to change without notice, etc. etc.

There also isn't any publicly accessible way to get at the actions stored on an ArgumentParser after they've been added. I believe that they are available as parser._actions if you're willing to go mucking about with implementation details (and assume any risks involved with that)...

---

Your best bet is to probably create a constant for the location choices and then use that in your code:

LOCATION_CHOICES = ('here', 'there', 'anywhere')

parser = argparse.ArgumentParser(description='choose location')
parser.add_argument(
    "--location",
    choices=LOCATION_CHOICES
)
args = parser.parse_args()

# Use LOCATION_CHOICES down here...

Answer 2

There might be a better way, but I don't see any in the documentation. If you know the parser option you should be able to do:

parser = argparse.ArgumentParser()
parser.add_argument("--location", choices=("here", "there", "everywhere"))

storeaction = next(a for a in parser._actions if "--location" in a.option_strings)

storeaction.choices
# ('here', 'there', 'everywhere')

As in mgilson's answer, accessing the _actions attribute is undocumented and the underscored prefix means "Hey, you probably shouldn't be messing with me." Don't be surprised if this breaks between versions of Python.