Problem matching dictionaries into Protocols

[dmoisset]

I'm exploring capturing in a Protocol some of the readable interface of a Mapping where values have different types for keys (like a lightweight Mapping version of TypedDict. I expected the snippet below to work but it doesn't:

To Reproduce

from typing import Literal, Protocol, overload


class MyMap(Protocol):

    @overload
    def __getitem__(self, key: Literal["a"]) -> int: ...
    @overload
    def __getitem__(self, key: Literal["b"]) -> str: ...    

m: MyMap = {"a": 1, "b": "hello"}

Expected Behavior

I expected the assignment above to type check properly, given that the dictionary object used satisfies the constraints given in the MyMap protocol.

Actual Behavior

map.py:11: error: Incompatible types in assignment (expression has type "Dict[Literal['a'], int]", variable has type "MyMap")
map.py:11: note: Following member(s) of "Dict[Literal['a'], int]" have conflicts:
map.py:11: note:     Expected:
map.py:11: note:         @overload
map.py:11: note:         def __getitem__(self, Literal['a']) -> int
map.py:11: note:         @overload
map.py:11: note:         def __getitem__(self, Literal['b']) -> str
map.py:11: note:     Got:
map.py:11: note:         def __getitem__(self, Literal['a']) -> int
map.py:11: error: Dict entry 1 has incompatible type "Literal['b']": "str"; expected "Literal['a']": "int"
Found 2 errors in 1 file (checked 1 source file)

Even if my expected behaviour is incorrect, there's something wrong with this error message, where it says that expression has type "Dict[Literal['a'], int]" (which clearly isn't the type of the expression)

Your Environment

• Mypy version used: 0.982 (compiled: yes)
• Mypy command-line flags: just the filename
• Python version used: 3.10.8

[tmke8]

Doesn't address your protocol problem but there was discussion about a read-only TypedDict on the mailing list: https://mail.python.org/archives/list/typing-sig@python.org/thread/6FR6RKNUZU4UY6B6RXC2H4IAHKBU3UKV/

[dmoisset]

Doesn't address your protocol problem but there was discussion about a read-only TypedDict on the mailing list: https://mail.python.org/archives/list/typing-sig@python.org/thread/6FR6RKNUZU4UY6B6RXC2H4IAHKBU3UKV/

I'm aware of that thread (I wrote this code after looking at it trying to explore alternative implementations); still I think the issue goes beyond the original problem

[erictraut]

The signature of dict.__getitem__ doesn't match the protocol in your code sample, so mypy correct reports a type compatibility error. I think mypy is doing the right thing here. Recommend closing.