@ Koos Zevenhoven > and there should at least *exist* a well-defined answer to whether an object is in 'instance' of a given type. (Not sure if 'instance' should be world used here)
Let me illustrate why being an "instance" (or any other word) does not apply well to runtime objects. Consider a list [1, 2, 3], then is it an "instance" of List[int]? Probably yes. Is it an "instance" of List[Union[str, int]]? Probably also yes. However, List[int] and List[Union[str, int]] are mutually incompatible i.e. the latter is not a subtype of the former and the former is not a subtype of the latter. (This is due to lists being mutable and therefore invariant in its type variable.) The next important point is that static type checkers never decide (or at least I have never seen this) whether a given literal (since there are no objects before runtime) is an "instance" of a type. Static type checkers (roughly speaking) verify that the semantics represented by an AST is consistent with declared/inferred types. Concerning the above example with [1, 2, 3], static type checkers can infer List[int] for such literal, or refuse to do so and require an explicit annotation, or a user can overrule the inference by an explicit annotation. This decision (whether to use List[int] or any other acceptable type for this literal) will influence type checking outcome (i.e. are there errors or not) even _earlier_ in the program, this is something that is not possible at runtime. > Ignoring that *types* are also a runtime concept seems dangerous to me. It is not ignored. Annotations are structured type metadata accessible both at static and runtime, there is even typing_inspect module on PyPI designed to provide some runtime introspection of types (it is at an early stage of development) and some elements of it might end up in typing. Also checking subtyping between two types (without mixing them with classes) at runtime is entirely possible, but this is very complicated task with lots of corner cases, therefore I don't think it will be in stdlib. stdlib was always kept simple and easy to maintain. -- Ivan
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