Python 3.9+Built-in typesterm

types.GenericAlias

annotation types

The type of an expression like list[int]: a parameterised built-in class used in annotations that keeps the original class and its type arguments.

Syntax

list[int]  # → types.GenericAlias

list[int] creates no new class and checks nothing at run time: list[int]([1, 'a']) happily returns a list with a string. It cannot be passed to isinstance — that raises TypeError, so check isinstance(x, list). typing.get_origin and typing.get_args extract the original class and the arguments. Since Python 3.9 built-in collections support this syntax directly, without typing.List.

Examples

import types
import typing
alias = list[int]
print(type(alias) is types.GenericAlias)   # → True
print(typing.get_origin(alias), typing.get_args(alias))   # → <class 'list'> (<class 'int'>,)
print(alias([1, 'a']))   # → [1, 'a'] — тип элементов не проверяется
isinstance([], list[int])   # → TypeError: параметризованный тип в isinstance нельзя

See also

Common errors

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