Why is this an issue?

Python 3.9 introduced built-in generic types such as list[T], dict[T], set[T] to make type hints more concise and easier to read. These built-in types have the same functionality as their counterparts in the typing module, but are more readable and idiomatic.

Using types such as typing.List is confusing in the presence of the already existing built-in types. This can also create inconsistencies when different parts of the codebase use different syntaxes for the same type.

How to fix it

Replace the generic type from the typing module with its built-in counterpart:

Legacy type

Replacement

typing.List[int]

list[int]

typing.Dict[str, int]

dict[str, int]

typing.Set[str]

set[str]

typing.FrozenSet[str]

frozenset[str]

typing.Tuple[int, int]

tuple[int, int]

typing.Tuple[int, ...]

tuple[int, ...]

typing.Iterable[int]

collections.abc.Iterable[int]

typing.Sequence[bool]

collections.abc.Sequence[bool]

typing.Mapping[str, int]

collections.abc.Mapping[str, int]

typing.Type[T]

type[T]

Refer to PEP-585 in the Resources section for the full list of replacements.

Code examples

Noncompliant code example

import typing

def print_numbers(numbers: typing.List[int]) -> None:
    for n in numbers:
        print(n)

Compliant solution

def print_numbers(numbers: list[int]) -> None:
    for n in numbers:
        print(n)

Resources

Documentation