Better support for defaultdict(lambda)

[mihneagiurgea]

On this code:

from collections import defaultdict

from typing import Any, Dict, Tuple, Union

class Point(object):
    def __init__(self, x, y):
        # type: (int, int) -> None
        self.x = x
        self.y = y

x = defaultdict(lambda: Point(0, 0))

mypy complains with:

sandbox.py:12: error: Need type annotation for variable

[JukkaL]

There isn't enough context to infer the defaultdict key type in your example, as nobody assigns anything to the dictionary.

However, mypy could do a better job in cases like this:

...
x = defaultdict(lambda: Point(0, 0))  # enough context to infer type defaultdict[int, Point]
x[1] = Point(2, 3)

[JukkaL]

This is a little harder but could also be nice (though probably not high priority):

x = defaultdict(list)  # inferred type should be defaultdict[int, List[str]]
x[0].append('x')

[AlexWaygood]

It seems like mypy's type inference has improved a great deal. Here's the output on mypy 0.941:

from collections import defaultdict
from typing import Any, Dict, Tuple, Union

class Point(object):
    def __init__(self, x, y):
        # type: (int, int) -> None
        self.x = x
        self.y = y

x = defaultdict(lambda: Point(0, 0))
x[1] = Point(2, 3)
reveal_type(x)  # Revealed type is "collections.defaultdict[builtins.int, __main__.Point]"

y = defaultdict(list)
y[0].append('x')
reveal_type(y)  # Revealed type is "collections.defaultdict[builtins.int, builtins.list[builtins.str]]"