V = TypeVar('V', covariant=True)

Type System & Annotations

Meaning

Defines a covariant type variable named V using typing.TypeVar, allowing it to be used in generic types where subtyping preserves subtype relationships.

Primary Function

Declare a covariant type variable for use in generic type annotations.

Communicative Purpose

Declare a generic type variable that preserves subtype relationships when used as a type parameter.

Pattern

V = TypeVar('V', covariant=True)

Core Structure

TypeVar with covariant=True

Função primária

Declare a covariant type variable for use in generic type annotations.

Propósito comunicativo

Declare a generic type variable that preserves subtype relationships when used as a type parameter.

Situações de gatilho

When defining a generic class or function that should accept subtypes of a type parameter (e.g., Container[Cat] is a subtype of Container[Animal] when Cat is a subtype of Animal).

Contextos

Used in type annotations for generic classes, functions, or protocols where covariance is desired, e.g., Iterable[Cat] is a subtype of Iterable[Animal].

Padrão

V = TypeVar('V', covariant=True)

Estrutura central

TypeVar with covariant=True

Slots de substituição

V: type variable name; covariant=True: flag indicating covariance

Colocados típicos

typing
Generic
Protocol
covariant
contravariant
invariant

Substituições comuns

T
U
K
V (type variable names)
covariant=False (invariant)
contravariant=True

Erros comuns

Using covariant=True with mutable types leading to type safety issues; forgetting to import TypeVar from typing.

Similar / contraste

Invariant TypeVar (covariant=False, contravariant=False); Contravariant TypeVar (contravariant=True)

Família do chunk

Python typing
TypeVar
generics

Tipo de construção: type_variable_declarationTag de espaçamento: Unknown

Examples

from typing import TypeVar, Generic\n\nV = TypeVar('V', covariant=True)\n\nclass Container(Generic[V]):\n def __init__(self, value: V):\n self.value = value\n\n# Container[Cat] is a subtype of Container[Animal] if Cat <: Animal

from typing import TypeVar, Sequence\n\nV = TypeVar('V', covariant=True)\n\ndef first(seq: Sequence[V]) -> V:\n return seq[0]\n\n# works with List[Cat] where V is Cat

from typing import TypeVar, Generic, Protocol\n\nclass Drawable(Protocol):\n def draw(self) -> None: ...\n\nV = TypeVar('V', covariant=True)\n\nclass DrawableContainer(Generic[V]):\n def __init__(self, items: list[V]):\n self.items = items\n def draw_all(self):\n for item in self.items:\n item.draw()\n\n# DrawableContainer[Cat] is subtype of DrawableContainer[Animal]