Legacy type variables

The tests in this file focus on how type variables are defined using the legacy notation. Most uses of type variables are tested in other files in this directory; we do not duplicate every test for both type variable syntaxes.

Unless otherwise specified, all quotations come from the Generics section of the typing spec.

Type variables

Defining legacy type variables

Generics can be parameterized by using a factory available in typing called TypeVar.

This was the only way to create type variables prior to PEP 695/Python 3.12. It is still available in newer Python releases.

from typing import TypeVar

T = TypeVar("T")
reveal_type(type(T))  # revealed: <class 'TypeVar'>
reveal_type(T)  # revealed: typing.TypeVar
reveal_type(T.__name__)  # revealed: Literal["T"]

Directly assigned to a variable

A TypeVar() expression must always directly be assigned to a variable (it should not be used as part of a larger expression).

from typing import TypeVar

T = TypeVar("T")
# TODO: no error
# error: [invalid-legacy-type-variable]
U: TypeVar = TypeVar("U")

# error: [invalid-legacy-type-variable] "A legacy `typing.TypeVar` must be immediately assigned to a variable"
# error: [invalid-type-form] "Function calls are not allowed in type expressions"
TestList = list[TypeVar("W")]

`TypeVar` parameter must match variable name

The argument to TypeVar() must be a string equal to the variable name to which it is assigned.

from typing import TypeVar

# error: [invalid-legacy-type-variable] "The name of a legacy `typing.TypeVar` (`Q`) must match the name of the variable it is assigned to (`T`)"
T = TypeVar("Q")

No redefinition

Type variables must not be redefined.

from typing import TypeVar

T = TypeVar("T")

# TODO: error
T = TypeVar("T")

Type variables with a default

Note that the __default__ property is only available in Python ≥3.13.

[environment]
python-version = "3.13"

from typing import TypeVar

T = TypeVar("T", default=int)
reveal_type(T.__default__)  # revealed: int
reveal_type(T.__bound__)  # revealed: None
reveal_type(T.__constraints__)  # revealed: tuple[()]

S = TypeVar("S")
reveal_type(S.__default__)  # revealed: NoDefault

Using other typevars as a default

from typing import Generic, TypeVar, Union

T = TypeVar("T")
U = TypeVar("U", default=T)
V = TypeVar("V", default=Union[T, U])

class Valid(Generic[T, U, V]): ...

reveal_type(Valid())  # revealed: Valid[Unknown, Unknown, Unknown]
reveal_type(Valid[int]())  # revealed: Valid[int, int, int]
reveal_type(Valid[int, str]())  # revealed: Valid[int, str, int | str]
reveal_type(Valid[int, str, None]())  # revealed: Valid[int, str, None]

# TODO: error, default value for U isn't available in the generic context
class Invalid(Generic[U]): ...

Type variables with an upper bound

from typing import TypeVar

T = TypeVar("T", bound=int)
reveal_type(T.__bound__)  # revealed: int
reveal_type(T.__constraints__)  # revealed: tuple[()]

S = TypeVar("S")
reveal_type(S.__bound__)  # revealed: None

Type variables with constraints

from typing import TypeVar

T = TypeVar("T", int, str)
reveal_type(T.__constraints__)  # revealed: tuple[int, str]

S = TypeVar("S")
reveal_type(S.__constraints__)  # revealed: tuple[()]

Cannot have only one constraint

TypeVar supports constraining parametric types to a fixed set of possible types...There should be at least two constraints, if any; specifying a single constraint is disallowed.

from typing import TypeVar

# TODO: error: [invalid-type-variable-constraints]
T = TypeVar("T", int)

Cannot be both covariant and contravariant

To facilitate the declaration of container types where covariant or contravariant type checking is acceptable, type variables accept keyword arguments covariant=True or contravariant=True. At most one of these may be passed.

from typing import TypeVar

# error: [invalid-legacy-type-variable]
T = TypeVar("T", covariant=True, contravariant=True)

Variance parameters must be unambiguous

from typing import TypeVar

def cond() -> bool:
    return True

# error: [invalid-legacy-type-variable]
T = TypeVar("T", covariant=cond())

# error: [invalid-legacy-type-variable]
U = TypeVar("U", contravariant=cond())

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