mypy confused by defaultdict(list)

[agentydragon]

Source:

from collections import defaultdict
x = defaultdict(list)

Error:

test.py:2: error: Argument 1 to "defaultdict" has incompatible type List[_T]; expected Callable[[], List[_T]]
test.py:2: error: Need type annotation for variable

[o11c]

What happens if you add # type: defaultdict[List[int]] or whatever the actual type of the list is?

[agentydragon]

x = defaultdict(list) # type: defaultdict[str, List[str]] ==> test.py:2: error: Argument 1 to "defaultdict" has incompatible type List[_T]; expected Callable[[], List[str]]

[agentydragon]

Also, this is an issue on the hack branch -- master might or might not have this issue.

[JukkaL]

Also happens on master. Here is a self-contained repro suitable for a unit test:

from typing import Generic, Callable, TypeVar

T = TypeVar('T')

class C(Generic[T]):
    def __init__(self, x: Callable[[], T]) -> None: ...

class D(Generic[T]): pass

a = C(D)
b = C(D) # type: C[D[str]]

[gvanrossum]

How hard would this be? The majority of uses of defaultdict that I see have one of the builtins set or list as argument. (Also the error message seems wrong -- the argument is not a value of type List[_T], it's a class representing that type.)

[JukkaL]

I haven't look into this yet but my guess would be that I could fix this pretty quickly.

[gvanrossum]

One quick hack would be to just add another overload to collections.pyi:

    @overload
    def __init__(self, default_factory: type) -> None: ...

But this feels wrong -- it should just know that these types are callables of 0 arguments.

[gvanrossum]

If you decide to fix this in typeshed, also note that two of the overloads
of defaultdict.__init__ are wrong -- it does not take a map or iterable
first parameter (those must be the second). Also the default_factory
parameter should be Optional. Keyword args are treated like dict's
keyword args.

On Sun, Dec 13, 2015 at 8:49 PM, Jukka Lehtosalo [email protected]
wrote:

I haven't look into this yet but my guess would be that I could fix this
pretty quickly.

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[JukkaL]

I'll give this a try tomorrow or Tuesday.