ruff/crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md

Generic classes: Legacy syntax

Defining a generic class

At its simplest, to define a generic class using the legacy syntax, you inherit from the typing.Generic special form, which is "specialized" with the generic class's type variables.

from ty_extensions import generic_context
from typing import Generic, TypeVar

T = TypeVar("T")
S = TypeVar("S")

class SingleTypevar(Generic[T]): ...
class MultipleTypevars(Generic[T, S]): ...

reveal_type(generic_context(SingleTypevar))  # revealed: tuple[T]
reveal_type(generic_context(MultipleTypevars))  # revealed: tuple[T, S]

You cannot use the same typevar more than once.

# TODO: error
class RepeatedTypevar(Generic[T, T]): ...

You can only specialize typing.Generic with typevars (TODO: or param specs or typevar tuples).

# error: [invalid-argument-type] "`Literal[int]` is not a valid argument to `typing.Generic`"
class GenericOfType(Generic[int]): ...

You can also define a generic class by inheriting from some other generic class, and specializing it with typevars.

class InheritedGeneric(MultipleTypevars[T, S]): ...
class InheritedGenericPartiallySpecialized(MultipleTypevars[T, int]): ...
class InheritedGenericFullySpecialized(MultipleTypevars[str, int]): ...

reveal_type(generic_context(InheritedGeneric))  # revealed: tuple[T, S]
reveal_type(generic_context(InheritedGenericPartiallySpecialized))  # revealed: tuple[T]
reveal_type(generic_context(InheritedGenericFullySpecialized))  # revealed: None

If you don't specialize a generic base class, we use the default specialization, which maps each typevar to its default value or Any. Since that base class is fully specialized, it does not make the inheriting class generic.

class InheritedGenericDefaultSpecialization(MultipleTypevars): ...

reveal_type(generic_context(InheritedGenericDefaultSpecialization))  # revealed: None

When inheriting from a generic class, you can optionally inherit from typing.Generic as well. But if you do, you have to mention all of the typevars that you use in your other base classes.

class ExplicitInheritedGeneric(MultipleTypevars[T, S], Generic[T, S]): ...

# error: [invalid-generic-class] "`Generic` base class must include all type variables used in other base classes"
class ExplicitInheritedGenericMissingTypevar(MultipleTypevars[T, S], Generic[T]): ...
class ExplicitInheritedGenericPartiallySpecialized(MultipleTypevars[T, int], Generic[T]): ...
class ExplicitInheritedGenericPartiallySpecializedExtraTypevar(MultipleTypevars[T, int], Generic[T, S]): ...

# error: [invalid-generic-class] "`Generic` base class must include all type variables used in other base classes"
class ExplicitInheritedGenericPartiallySpecializedMissingTypevar(MultipleTypevars[T, int], Generic[S]): ...

reveal_type(generic_context(ExplicitInheritedGeneric))  # revealed: tuple[T, S]
reveal_type(generic_context(ExplicitInheritedGenericPartiallySpecialized))  # revealed: tuple[T]
reveal_type(generic_context(ExplicitInheritedGenericPartiallySpecializedExtraTypevar))  # revealed: tuple[T, S]

Specializing generic classes explicitly

The type parameter can be specified explicitly:

from typing import Generic, TypeVar

T = TypeVar("T")

class C(Generic[T]):
    x: T

reveal_type(C[int]())  # revealed: C[int]

The specialization must match the generic types:

# error: [too-many-positional-arguments] "Too many positional arguments to class `C`: expected 1, got 2"
reveal_type(C[int, int]())  # revealed: Unknown

If the type variable has an upper bound, the specialized type must satisfy that bound:

from typing import Union

BoundedT = TypeVar("BoundedT", bound=int)
BoundedByUnionT = TypeVar("BoundedByUnionT", bound=Union[int, str])

class Bounded(Generic[BoundedT]): ...
class BoundedByUnion(Generic[BoundedByUnionT]): ...
class IntSubclass(int): ...

reveal_type(Bounded[int]())  # revealed: Bounded[int]
reveal_type(Bounded[IntSubclass]())  # revealed: Bounded[IntSubclass]

# TODO: update this diagnostic to talk about type parameters and specializations
# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `str`"
reveal_type(Bounded[str]())  # revealed: Unknown

# TODO: update this diagnostic to talk about type parameters and specializations
# error:  [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `int | str`"
reveal_type(Bounded[int | str]())  # revealed: Unknown

reveal_type(BoundedByUnion[int]())  # revealed: BoundedByUnion[int]
reveal_type(BoundedByUnion[IntSubclass]())  # revealed: BoundedByUnion[IntSubclass]
reveal_type(BoundedByUnion[str]())  # revealed: BoundedByUnion[str]
reveal_type(BoundedByUnion[int | str]())  # revealed: BoundedByUnion[int | str]

If the type variable is constrained, the specialized type must satisfy those constraints:

ConstrainedT = TypeVar("ConstrainedT", int, str)

class Constrained(Generic[ConstrainedT]): ...

reveal_type(Constrained[int]())  # revealed: Constrained[int]

# TODO: error: [invalid-argument-type]
# TODO: revealed: Constrained[Unknown]
reveal_type(Constrained[IntSubclass]())  # revealed: Constrained[IntSubclass]

reveal_type(Constrained[str]())  # revealed: Constrained[str]

# TODO: error: [invalid-argument-type]
# TODO: revealed: Unknown
reveal_type(Constrained[int | str]())  # revealed: Constrained[int | str]

# TODO: update this diagnostic to talk about type parameters and specializations
# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int | str`, found `object`"
reveal_type(Constrained[object]())  # revealed: Unknown

Inferring generic class parameters

We can infer the type parameter from a type context:

from typing import Generic, TypeVar

T = TypeVar("T")

class C(Generic[T]):
    x: T

c: C[int] = C()
# TODO: revealed: C[int]
reveal_type(c)  # revealed: C[Unknown]

The typevars of a fully specialized generic class should no longer be visible:

# TODO: revealed: int
reveal_type(c.x)  # revealed: Unknown

If the type parameter is not specified explicitly, and there are no constraints that let us infer a specific type, we infer the typevar's default type:

DefaultT = TypeVar("DefaultT", default=int)

class D(Generic[DefaultT]): ...

reveal_type(D())  # revealed: D[int]

If a typevar does not provide a default, we use Unknown:

reveal_type(C())  # revealed: C[Unknown]

Inferring generic class parameters from constructors

If the type of a constructor parameter is a class typevar, we can use that to infer the type parameter. The types inferred from a type context and from a constructor parameter must be consistent with each other.

__new__ only

from typing import Generic, TypeVar

T = TypeVar("T")

class C(Generic[T]):
    def __new__(cls, x: T) -> "C[T]":
        return object.__new__(cls)

reveal_type(C(1))  # revealed: C[Literal[1]]

# error: [invalid-assignment] "Object of type `C[Literal["five"]]` is not assignable to `C[int]`"
wrong_innards: C[int] = C("five")

__init__ only

from typing import Generic, TypeVar

T = TypeVar("T")

class C(Generic[T]):
    def __init__(self, x: T) -> None: ...

reveal_type(C(1))  # revealed: C[Literal[1]]

# error: [invalid-assignment] "Object of type `C[Literal["five"]]` is not assignable to `C[int]`"
wrong_innards: C[int] = C("five")

Identical __new__ and __init__ signatures

from typing import Generic, TypeVar

T = TypeVar("T")

class C(Generic[T]):
    def __new__(cls, x: T) -> "C[T]":
        return object.__new__(cls)

    def __init__(self, x: T) -> None: ...

reveal_type(C(1))  # revealed: C[Literal[1]]

# error: [invalid-assignment] "Object of type `C[Literal["five"]]` is not assignable to `C[int]`"
wrong_innards: C[int] = C("five")

Compatible __new__ and __init__ signatures

from typing import Generic, TypeVar

T = TypeVar("T")

class C(Generic[T]):
    def __new__(cls, *args, **kwargs) -> "C[T]":
        return object.__new__(cls)

    def __init__(self, x: T) -> None: ...

reveal_type(C(1))  # revealed: C[Literal[1]]

# error: [invalid-assignment] "Object of type `C[Literal["five"]]` is not assignable to `C[int]`"
wrong_innards: C[int] = C("five")

class D(Generic[T]):
    def __new__(cls, x: T) -> "D[T]":
        return object.__new__(cls)

    def __init__(self, *args, **kwargs) -> None: ...

reveal_type(D(1))  # revealed: D[Literal[1]]

# error: [invalid-assignment] "Object of type `D[Literal["five"]]` is not assignable to `D[int]`"
wrong_innards: D[int] = D("five")

Both present, __new__ inherited from a generic base class

If either method comes from a generic base class, we don't currently use its inferred specialization to specialize the class.

from typing import Generic, TypeVar

T = TypeVar("T")
U = TypeVar("U")
V = TypeVar("V")

class C(Generic[T, U]):
    def __new__(cls, *args, **kwargs) -> "C[T, U]":
        return object.__new__(cls)

class D(C[V, int]):
    def __init__(self, x: V) -> None: ...

reveal_type(D(1))  # revealed: D[Literal[1]]

__init__ is itself generic

from typing import Generic, TypeVar

S = TypeVar("S")
T = TypeVar("T")

class C(Generic[T]):
    def __init__(self, x: T, y: S) -> None: ...

reveal_type(C(1, 1))  # revealed: C[Literal[1]]
reveal_type(C(1, "string"))  # revealed: C[Literal[1]]
reveal_type(C(1, True))  # revealed: C[Literal[1]]

# error: [invalid-assignment] "Object of type `C[Literal["five"]]` is not assignable to `C[int]`"
wrong_innards: C[int] = C("five", 1)

Generic subclass

When a generic subclass fills its superclass's type parameter with one of its own, the actual types propagate through:

from typing import Generic, TypeVar

T = TypeVar("T")

class Base(Generic[T]):
    x: T | None = None

class ExplicitlyGenericSub(Base[T], Generic[T]): ...
class ImplicitlyGenericSub(Base[T]): ...

reveal_type(Base[int].x)  # revealed: int | None
reveal_type(ExplicitlyGenericSub[int].x)  # revealed: int | None
reveal_type(ImplicitlyGenericSub[int].x)  # revealed: int | None

Generic methods

Generic classes can contain methods that are themselves generic. The generic methods can refer to the typevars of the enclosing generic class, and introduce new (distinct) typevars that are only in scope for the method.

from typing import Generic, TypeVar

T = TypeVar("T")
U = TypeVar("U")

class C(Generic[T]):
    def method(self, u: U) -> U:
        return u

c: C[int] = C[int]()
reveal_type(c.method("string"))  # revealed: Literal["string"]

Cyclic class definitions

F-bounded quantification

A class can use itself as the type parameter of one of its superclasses. (This is also known as the curiously recurring template pattern or F-bounded quantification.)

In a stub file

Here, Sub is not a generic class, since it fills its superclass's type parameter (with itself).

from typing import Generic, TypeVar

T = TypeVar("T")

class Base(Generic[T]): ...
class Sub(Base[Sub]): ...

reveal_type(Sub)  # revealed: Literal[Sub]

With string forward references

A similar case can work in a non-stub file, if forward references are stringified:

from typing import Generic, TypeVar

T = TypeVar("T")

class Base(Generic[T]): ...
class Sub(Base["Sub"]): ...

reveal_type(Sub)  # revealed: Literal[Sub]

Without string forward references

In a non-stub file, without stringified forward references, this raises a NameError:

from typing import Generic, TypeVar

T = TypeVar("T")

class Base(Generic[T]): ...

# error: [unresolved-reference]
class Sub(Base[Sub]): ...

Cyclic inheritance as a generic parameter

from typing import Generic, TypeVar

T = TypeVar("T")

class Derived(list[Derived[T]], Generic[T]): ...

Direct cyclic inheritance

Inheritance that would result in a cyclic MRO is detected as an error.

from typing import Generic, TypeVar

T = TypeVar("T")

# error: [unresolved-reference]
class C(C, Generic[T]): ...

# error: [unresolved-reference]
class D(D[int], Generic[T]): ...