Errors when using defaultdict

[rowillia]

defaultdict's documentation suggests passing list as an argument to defaultdict - https://docs.python.org/2/library/collections.html#defaultdict-examples

mypy complains when I do this however

from collections import defaultdict

foo = defaultdict(list)
bar = defaultdict(dict)

Results in:

default_dict_example.py, line 3: Argument 1 to "defaultdict" has incompatible type List[_T]; expected Callable[[], List[_T]]
default_dict_example.py, line 3: Need type annotation for variable
default_dict_example.py, line 4: Argument 1 to "defaultdict" has incompatible type Dict[_KT, _VT]; expected Callable[[], Dict[_KT, _VT]]
default_dict_example.py, line 4: Need type annotation for variable

[gvanrossum]

This seems a symptom of a general problem where classes don't seen to be considered compatible with callables. I've found a few other instances of this too.

[rowillia]

Changing this to defaultdict(lambda: dict()) can be used as a work around, but then items coming out of the defaultdict aren't being type checked -

bar = defaultdict(lambda: dict())
bar.foobar() # works
bar['0'].foobar() # also works

[rowillia]

Hmm, weird. In a standalone file I get an error about bar needing a type (which is equally strange). In the real example it's ignoring it's type.

[gvanrossum]

Hm, this works for me:

from typing import MutableMapping, Any, Dict
bar = defaultdict(dict)  # type: MutableMapping[Any, Dict]

[JukkaL]

Ah this is a dupe of #815.