Dict[AbstractSet, Any] weird type error

[berdario]

I get Argument 1 to "f" has incompatible type Dict[frozenset[str], int]; expected Dict[AbstractSet[str], int]

with the following code:

from typing import AbstractSet, Dict

def f(v: Dict[AbstractSet[str], int]):
    pass

f({frozenset(['a']): 1})  # this works
x = {frozenset(['a']): 1}
f(x) # this fails

[rwbarton]

In the first call to f, mypy infers the type Dict[AbstractSet[str], int] for the expression {frozenset(['a']): 1}, because that's what is needed for the argument to f. In the definition of x, mypy infers the type Dict[frozenset[str], int] for the variable x based on its initial value of {frozenset(['a']): 1}, not taking the later call to f into account.

Dict[frozenset[str], int] is not a subtype of Dict[AbstractSet[str], int] (even though frozenset[str] is a subtype of AbstractSet[str]) because a Dict[AbstractSet[str], int] is a mutable object into which you can insert arbitrary AbstractSet[str] keys, and that is not true of a Dict[frozenset[str], int]. Here f could modify its argument and then x would no longer really be a Dict[frozenset[str], int].

If f doesn't actually modify its argument, then it's better to give the argument a type like Mapping[AbstractSet[str], int]. This won't actually let your program type check yet, but it should after python/typeshed#278 is fixed.

[JukkaL]

This might be worth explaining in the Common issues section of mypy documentation.

[berdario]

FWIW, even after the work on #278 and #1044, by defining f as

def f(v: Mapping[AbstractSet[str], int]):
    pass

There'll be a Argument 1 to "f" has incompatible type "Dict[FrozenSet[str], int]"; expected "Mapping[AbstractSet[str], int]" yielded

[ilevkivskyi]

Because Mapping is invariant in key type.