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Fix MutableMapping.update to require string keys #14099

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May 19, 2025
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Fix MutableMapping.update to require string keys #14099

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Fix MutableMapping.update to require string keys 
Currently with most type checkers you can do:

```python
d: dict[int, int] = {}
d.update(a=2)
```

The reason for that is that `update` merely requires `MutableMapping[_KT, _VT]`, whereas as soon as you are passing kwargs, you must have a str key type.
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@ndmitchell
ndmitchell authored May 19, 2025
commit 7680e2ef77853c927d834ae560225760c390e391
10 changes: 7 additions & 3 deletions 10 stdlib/typing.pyi
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Original file line number Diff line number Diff line change
Expand Up @@ -797,11 +797,15 @@ class MutableMapping(Mapping[_KT, _VT]):
# -- weakref.WeakValueDictionary.__ior__
# -- weakref.WeakKeyDictionary.__ior__
@overload
def update(self, m: SupportsKeysAndGetItem[_KT, _VT], /, **kwargs: _VT) -> None: ...
def update(self, m: SupportsKeysAndGetItem[_KT, _VT], /) -> None: ...
@overload
def update(self, m: Iterable[tuple[_KT, _VT]], /, **kwargs: _VT) -> None: ...
def update(self: Mapping[str, _VT], m: SupportsKeysAndGetItem[_KT, _VT], /, **kwargs: _VT) -> None: ...
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Should this be:

def update(self: Mapping[str, _VT], m: SupportsKeysAndGetItem[str, _VT], /, **kwargs: _VT) -> None: ...

It's possible that _KT is constrained to str anyway via the self mapping, but I still think being explicit would be better (and easier on the type checkers).

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I think SupportsKeysAndGetItem is invariant in the first arg so that might cause trouble?

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#14100 for these cleanups.

@overload
def update(self, **kwargs: _VT) -> None: ...
def update(self, m: Iterable[tuple[_KT, _VT]], /) -> None: ...
@overload
def update(self: Mapping[str, _VT], m: Iterable[tuple[_KT, _VT]], /, **kwargs: _VT) -> None: ...
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Same here.

@overload
def update(self: Mapping[str, _VT], **kwargs: _VT) -> None: ...

Text = str

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