As type guards become more popular, this one may become extremely common: ``` _T = TypeVar("_T") class Collection: def contains_only(self, type_: Type[_T]) -> TypeGuard[Collection[_T]]: return all(isinstance(x, type_) for x in self) # usage: usernames = settings_dict['usernames'] assert isinstance(usernames, list) assert usernames.contains_only(str) # Ensure it contains only strings. assert_type(usernames, list[str]) # Should pass, but see note. ``` Would it make sense to add it to the `Collection` mixin so that it's available on all collections? Maybe `is_collection_of` would be a better name? The final check may not be easy to get working without an extra ``` assert isinstance(usernames, list) ``` in today's Pyright/MyPy. Or maybe it would be better to have this work on a `GenericAlias` of a collection? Like ```python assert list[str].typeguard(usernames) ``` The benefit of this being that generic class `G[C]` could use `C.typeguard(vaue)` in its code without knowing what `C` is. E.g. ``` class SupportsTypeguard(Protocol): @classmethod # this needs to be available on the generic alias, though def typeguard(cls) -> TypeGuard[Self]: ... C = TypeVar(C, bound=SupportsTypeguard) class G(Generic[C]): def f(self, ...) -> C: # generate a return value of unknown type assert C.typeguard(value) # Works if C is int, or list[str], or whatever else implements this interface. return value