What is a type? #923

amacfie · 2021-11-04T23:59:48Z

amacfie
Nov 4, 2021

In Python how do you tell if an object is a type? Are types well defined? Is an instance of TypeVar a type?

Answered by JelleZijlstra

"Type" can have two different plausible meanings:

A runtime type object, such as a user-defined class or a builtin type like int. At runtime you can check for these with isinstance(obj, type). In the type system they are represented as e.g. type[int] (or Type[int] in older versions). An instance of TypeVar is not a type in this definition.
Something that is valid as an annotation in the type system. That includes all runtime types, but also some other objects, such as TypeVars and special forms like Any. There is no runtime function that tells you whether something is valid as a type annotation.

JelleZijlstra · 2021-11-05T00:29:51Z

"Type" can have two different plausible meanings:

A runtime type object, such as a user-defined class or a builtin type like int. At runtime you can check for these with isinstance(obj, type). In the type system they are represented as e.g. type[int] (or Type[int] in older versions). An instance of TypeVar is not a type in this definition.
Something that is valid as an annotation in the type system. That includes all runtime types, but also some other objects, such as TypeVars and special forms like Any. There is no runtime function that tells you whether something is valid as a type annotation.

1 reply

There is no runtime function that tells you whether something is valid as a type annotation.

However, there are some 3rd party runtime type checkers that can do that. List: https://github.com/typeddjango/awesome-python-typing/#dynamic-type-checkers