PEP 695 Generics

Class Declarations

Basic PEP 695 generics

class MyBox[T]:
    data: T
    box_model_number = 695

    def __init__(self, data: T):
        self.data = data

box: MyBox[int] = MyBox(5)

# TODO should emit a diagnostic here (str is not assignable to int)
wrong_innards: MyBox[int] = MyBox("five")

# TODO reveal int
reveal_type(box.data)  # revealed: @Todo(instance attributes)

reveal_type(MyBox.box_model_number)  # revealed: Literal[695]

Subclassing

class MyBox[T]:
    data: T

    def __init__(self, data: T):
        self.data = data

# TODO not error on the subscripting
# error: [non-subscriptable]
class MySecureBox[T](MyBox[T]): ...

secure_box: MySecureBox[int] = MySecureBox(5)
reveal_type(secure_box)  # revealed: MySecureBox
# TODO reveal int
reveal_type(secure_box.data)  # revealed: @Todo(instance attributes)

Cyclical class definition

In type stubs, classes can reference themselves in their base class definitions. For example, in typeshed, we have class str(Sequence[str]): ....

This should hold true even with generics at play.

class Seq[T]: ...

# TODO not error on the subscripting
class S[T](Seq[S]): ...  # error: [non-subscriptable]

reveal_type(S)  # revealed: Literal[S]

Type params

A PEP695 type variable defines a value of type typing.TypeVar.

def f[T]():
    reveal_type(T)  # revealed: T
    reveal_type(T.__name__)  # revealed: Literal["T"]

Minimum two constraints

A typevar with less than two constraints emits a diagnostic:

# error: [invalid-typevar-constraints] "TypeVar must have at least two constrained types"
def f[T: (int,)]():
    pass

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