dict.__init__() and dict.update() don't match

[Akuli]

typeshed/stdlib/builtins.pyi

Lines 807 to 818 in 196f69b

	@overload
	def __init__(self: dict[_KT, _VT]) -> None: ...
	@overload
	def __init__(self: dict[str, _VT], **kwargs: _VT) -> None: ...
	@overload
	def __init__(self, map: SupportsKeysAndGetItem[_KT, _VT], **kwargs: _VT) -> None: ...
	@overload
	def __init__(self, iterable: Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
	# Next overload is for dict(string.split(sep) for string in iterable)
	# Cannot be Iterable[Sequence[_T]] or otherwise dict(["foo", "bar", "baz"]) is not an error
	@overload
	def __init__(self: dict[str, str], iterable: Iterable[list[str]]) -> None: ...

typeshed/stdlib/builtins.pyi

Lines 824 to 829 in 196f69b

	@overload
	def update(self, __m: Mapping[_KT, _VT], **kwargs: _VT) -> None: ...
	@overload
	def update(self, __m: Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
	@overload
	def update(self, **kwargs: _VT) -> None: ...

In stubs, __init__ has 5 overloads and update() has 3 overloads. But at runtime, dict.update() accepts everything that dict.__init__ accepts. For example:

>>> d={}
>>> d.update((s.split('=') for s in ['a=b', 'c=d']), lol='wut')
>>> d
{'a': 'b', 'c': 'd', 'lol': 'wut'}

[JelleZijlstra]

Concrete changes that could be made are:

• Make the mapping variant of .update take SupportsKeysAndGetItem instead of Mapping (Widen MutableMapping.update type #6653)
• Allow **kwargs to update() only if the key type is str. I haven't tried this yet and it may run into tricky variance issues. (What if the key type is a Literal over particular strings?)