argparse.Namespace is more complicated than it seems

[gvanrossum]

The argparse.Namespace class stub doesn't reflect its actual complexity -- it has dynamically created attributes based on the parser specification. For example:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('-d', '--database', type=str, default='data.db')
def foo(args: argparse.Namespace):
    print(args.database)
def main():
    args = parser.parse_args()
    assert args.__class__ is argparse.Namespace
    foo(args)

This gives these errors:

t.py: note: In function "foo":
t.py:5: error: "Namespace" has no attribute "database"

Now, ideally there would be some kind of plugin that understands the parser specification. But I'd be happy if it just didn't complain about using unknown attributes of Namespace objects, while still verifying that the caller indeed passes something of that type (so I don't want to just use Any).

[matthiaskramm]

It sounds like you'd want something like

class Namespace:
  def __getattr__(self, s: str) -> Any

?

[gvanrossum]

Would that work?

[matthiaskramm]

It works in mypy (I just tried it). It will also work in pytype as soon as I get around to implementing __getattr__ support.

[o11c]

There are already two types that must necessarily support per-instance attributes with full typechecking: classes and modules.

[gvanrossum]

This was closed by 9ae0c0b.

[mo22]

For anyone reaching this from google with the same problem as me (namespace is Any), I've build a small script around this where you define arguments as class:

    class ArgsImportYml(args2.Command):
        _name = 'import-yml'
        group = Argument(Optional[List[str]], help='name of group(s)')
        zone = Argument(Optional[List[str]], help='name of zone(s)')
        test = Argument(Optional[str])
        dry_run = Argument(bool)
        create = Argument(bool, help='create route53 zone if needed')
        files = Positional(List[args2.ReadableTextFile], help='path to yml file(s)')

https://gist.github.com/mo22/3c5cc239b6237d2a3ee2b86eb6960325