dict.update has incorrect variance for one of its overloads

[lazytype]

from typing import Dict, Union

d: Dict[Union[str, int], None] = {}

e: Dict[str, None] = {}

# not ok
d.update(e)

# ok
d.update(e.items())

# ok
for key, value in e.items():
    d[key] = value

[ilevkivskyi]

It looks like a possible solution here is to define a hidden typeshed-only supertype of Mapping (maybe call it _Mapping) that will be covariant in key type. We can then use it to annotate some built-in function and method arguments where we know it is safe.

[rohitkg98]

Most Mutable containers are invariant. You can read more about that in details here: generics. For Dict(Mapping), it has been discussed in detail here. That said, you may try out the Protocol: SupportsItems, to have a covariant mapping of a dictionary.

[JelleZijlstra]

Just to clarify, mypy produces this error on the original sample:

main.py:8: error: Argument 1 to "update" of "dict" has incompatible type "Dict[str, None]"; expected "Mapping[Union[str, int], None]"

The problem (as Ivan said) is that dict-like types are invariant in their key type, but here we want it to be covariant.