typing --- Support for type hints — User-defined generic types
A user-defined class can be defined as a generic class. from logging import Logger class LoggedVar[T]: def init(self, value: T, name: str, logger: Logger) -> None: self.name = name self.logger = logger self.value = value This syntax indicates that the class LoggedVar is parameterised around a single
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A user-defined class can be defined as a generic class.
from logging import Logger
class LoggedVar[T]: def init(self, value: T, name: str, logger: Logger) -> None: self.name = name self.logger = logger self.value = value
This syntax indicates that the class LoggedVar is parameterised around a single type variable T . This also makes T valid as a type within the class body.
Generic classes implicitly inherit from Generic. For compatibility with Python 3.11 and lower, it is also possible to inherit explicitly from Generic to indicate a generic class
from typing import TypeVar, Generic
class LoggedVar(Generic[T]): ...
Generic classes have ~object.class_getitem methods, meaning they can be parameterised at runtime (e.g. LoggedVar[int] below)
from collections.abc import Iterable
def zero_all_vars(vars: Iterable[LoggedVar[int]]) -> None: for var in vars: var.set(0)
A generic type can have any number of type variables. All varieties of TypeVar are permissible as parameters for a generic type
from typing import TypeVar, Generic, Sequence
class WeirdTrio[T, B: Sequence[bytes], S: (int, str)]: ...
OldT = TypeVar('OldT', contravariant=True) OldB = TypeVar('OldB', bound=Sequence[bytes], covariant=True) OldS = TypeVar('OldS', int, str)
class OldWeirdTrio(Generic[OldT, OldB, OldS]): ...
Each type variable argument to Generic must be distinct. This is thus invalid
from typing import TypeVar, Generic ...
class Pair[M, M]: # SyntaxError ...
class Pair(Generic[T, T]): # INVALID ...
Generic classes can also inherit from other classes
from collections.abc import Sized
When inheriting from generic classes, some type parameters could be fixed
In this case MyDict has a single parameter, T.
Using a generic class without specifying type parameters assumes Any for each position. In the following example, MyIterable is not generic but implicitly inherits from Iterable[Any]
from collections.abc import Iterable
class MyIterable(Iterable): # Same as Iterable[Any] ...
User-defined generic type aliases are also supported. Examples
from collections.abc import Iterable
type Response[S] = Iterable[S] | int
# Return type here is same as Iterable[str] | int def response(query: str) -> Response[str]: ...
type Vec[T] = Iterable[tuple[T, T]]
def inproductT: (int, float, complex) -> T: # Same as Iterable[tuple[T, T]] return sum(xy for x, y in v) …
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Python Documentation — Doc/library/typing.rst :: User-defined generic types ↗Revision f10166035d60 · PSF-2.0 and attribution