[FIXED] How do I type hint a variable whose value is itself a type hint?

Issue

I have a function one of whose arguments is expected to be a type hint: something like typing.List, or typing.List[int], or even just int. Anything you would reasonably expect to see as a type annotation to an ordinary field.

What's the correct type hint to put on this argument?

(The context for this is that I'm writing a utility that works on classes that define fields using type annotations, a bit like the dataclass decorator.)

Solution

Answer for Python 3.10:

Almost complete but less readable answer:

type | types.GenericAlias | types.UnionType | typing._BaseGenericAlias | typing._SpecialForm

Here are the possibilities of all types of annotations I can think of:

• Type object itself, such as int, list, etc. Corresponds to type.
• Type hinting generics in standard collections, such as list[int]. Corresponds to types.GenericAlias.
• Types union, such as int | float (but typing.Union[int, float] is not, it corresponds to the next item). Corresponds to types.UnionType.
• Generic concrete collections in typing, such as typing.List, typing.List[int], etc. Here I choose the common base class typing._BaseGenericAlias that first appeared in their inheritance chain. (In fact, not only these, but almost all subscriptible annotation classes in typing inherit from this class, including typing.Literal[True], typing.Union[int, float], etc.)
• Special typing primitives in typing, such as typing.Any, typing.NoReturn, etc. Corresponds to typing._SpecialForm.

It should be noted that the last two types begin with a single underline, indicating that they are internal types, and should not be imported and used from typing. However, they should be indispensable if you insist on completely covering all type annotations.

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Python 3.11 update:

I noticed that typing.Any has changed from typing._SpecialForm to typing._AnyMeta (to allow inheritance of it), but it is now a type (which we have already included) and we don't need to add anything new to it.

typing.Self and typing.LiteralString have been added in 3.11, but they are both typing._SpecialForm, so there is also no need for anything new.

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Python 3.12 update:

We now have a type statement, which allows us to create typing.TypeAliasType object, this seems to be the only new content.

Answered By - Mechanic Pig