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PEP 673 -- Self Type

PEP:673
Title:Self Type
Author:Pradeep Kumar Srinivasan <gohanpra at gmail.com>, James Hilton-Balfe <gobot1234yt at gmail.com>
Sponsor:Jelle Zijlstra <jelle.zijlstra at gmail.com>
Discussions-To:Typing-Sig <typing-sig at python.org>
Status:Draft
Type:Standards Track
Created:10-Nov-2021
Python-Version:3.11
Post-History:

Abstract

This PEP introduces a simple and intuitive way to annotate methods that return an instance of their class. This behaves the same as the TypeVar-based approach specified in PEP 484 but is more concise and easier to follow.

Motivation

A common use case is to write a method that returns an instance of the same class, usually by returning self.

class Shape:
    def set_scale(self, scale: float):
        self.scale = scale
        return self

Shape().set_scale(0.5)  # => should be Shape

One way to denote the return type is to specify it as the current class, say, Shape. Using the method makes the type checker infer the type Shape, as expected.

class Shape:
    def set_scale(self, scale: float) -> Shape:
        self.scale = scale
        return self

Shape().set_scale(0.5)  # => Shape

However, when we call set_scale on a subclass of Shape, the type checker still infers the return type to be Shape. This is problematic in situations such as the one shown below, where the type checker will return an error because we are trying to use attributes or methods not present on the base class.

class Circle(Shape):
    def set_radius(self, r: float) -> Circle:
        self.radius = r
        return self

Circle().set_scale(0.5)  # *Shape*, not Circle
Circle().set_scale(0.5).set_radius(2.7)
# => Error: Shape has no attribute set_radius

The present workaround for such instances is to define a TypeVar with the base class as the bound and use it as the annotation for the self parameter and the return type:

from typing import TypeVar

TShape = TypeVar("TShape", bound="Shape")

class Shape:
    def set_scale(self: TShape, scale: float) -> TShape:
        self.scale = scale
        return self


class Circle(Shape):
    def set_radius(self, radius: float) -> Circle:
        self.radius = radius
        return self

Circle().set_scale(0.5).set_radius(2.7)  # => Circle

Unfortunately, this is verbose and unintuitive. Because self is usually not explicitly annotated, the above solution doesn't immediately come to mind, and even if it does, it is very easy to go wrong by forgetting either the bound on the TypeVar(bound="Shape") or the annotation for self.

This difficulty means that users often give up and either use fallback types like Any or just omit the type annotation completely, both of which make the code less safe.

We propose a more intuitive and succinct way of expressing the above intention. We introduce a special form Self that stands for a type variable bound to the encapsulating class. For situations such as the one above, the user simply has to annotate the return type as Self:

from typing import Self

class Shape:
    def set_scale(self, scale: float) -> Self:
        self.scale = scale
        return self


class Circle(Shape):
    def set_radius(self, radius: float) -> Self:
        self.radius = radius
        return self

By annotating the return type as Self, we no longer have to declare a TypeVar with an explicit bound on the base class. The return type Self mirrors the fact that the function returns self and is easier to understand.

As in the above example, the type checker will correctly infer the type of Circle().set_scale(0.5) to be Circle, as expected.

Usage statistics

We analyzed popular open-source projects and found that patterns like the above were used about 40% as often as popular types like dict or Callable. For example, in typeshed alone, such “Self” types are used 523 times, compared to 1286 uses of dict and 1314 uses of Callable as of October 2021. This suggests that a Self type will be used quite often and users will benefit a lot from the simpler approach above.

Specification

Use in Method Signatures

Self used in the signature of a method is treated as if it were a TypeVar bound to the class.

from typing import Self

class Shape:
    def set_scale(self, scale: float) -> Self:
        self.scale = scale
        return self

is treated equivalently to:

from typing import TypeVar

SelfShape = TypeVar("SelfShape", bound="Shape")

class Shape:
    def set_scale(self: SelfShape, scale: float) -> SelfShape:
        self.scale = scale
        return self

This works the same for a subclass too:

class Circle(Shape):
    def set_radius(self, radius: float) -> Self:
        self.radius = radius
        return self

which is treated equivalently to:

SelfCircle = TypeVar("SelfCircle", bound="Circle")

class Circle(Shape):
    def set_radius(self: SelfCircle, radius: float) -> SelfCircle:
        self.radius = radius
        return self

One implementation strategy is to simply desugar the former to the latter in a preprocessing step. If a method uses Self in its signature, the type of self within a method will be Self. In other cases, the type of self will remain the enclosing class.

Use in Classmethod Signatures

The Self type annotation is also useful for classmethods that return an instance of the class that they operate on. For example, from_config in the following snippet builds a Shape object from a given config.

class Shape:
    def __init__(self, scale: float) -> None: ...

    @classmethod
    def from_config(cls, config: dict[str, float]) -> Shape:
        return cls(config["scale"])

However, this means that Circle.from_config(...) is inferred to return a value of type Shape, when in fact it should be Circle:

class Circle(Shape): ...

shape = Shape.from_config({"scale": 7.0})
# => Shape

circle = Circle.from_config({"scale": 7.0})
# => *Shape*, not Circle

circle.circumference()
# Error: `Shape` has no attribute `circumference`

The current workaround for this is unintuitive and error-prone:

Self = TypeVar("Self", bound="Shape")

class Shape:
    @classmethod
    def from_config(
        cls: type[Self], config: dict[str, float]
    ) -> Self:
        return cls(config["scale"])

We propose using Self directly:

from typing import Self

class Shape:
    @classmethod
    def from_config(cls, config: dict[str, float]) -> Self:
        return cls(config["scale"])

This avoids the complicated cls: type[Self] annotation and the TypeVar declaration with a bound. Once again, the latter code behaves equivalently to the former code.

Use in Parameter Types

Another use for Self is to annotate parameters that expect instances of the current class:

Self = TypeVar("Self", bound="Shape")

class Shape:
    def difference(self: Self, other: Self) -> float: ...

    def apply(self: Self, f: Callable[[Self], None]) -> None: ...

We propose using Self directly to achieve the same behavior:

from typing import Self

class Shape:
    def difference(self, other: Self) -> float: ...

    def apply(self, f: Callable[[Self], None]) -> None: …

Note that specifying self: Self is harmless, so some users may find it more readable to write the above as:

class Shape:
    def difference(self: Self, other: Self) -> float: ...

Use in Attribute Annotations

Another use for Self is to annotate attributes. One example is where we have a LinkedList whose elements must be subclasses of the current class.

from dataclasses import dataclass
from typing import Generic, TypeVar

T = TypeVar("T")

@dataclass
class LinkedList(Generic[T]):
    value: T
    next: LinkedList[T] | None = None

# OK
LinkedList[int](value=1, next=LinkedList[int](value=2))
# Not OK
LinkedList[int](value=1, next=LinkedList[str](value=”hello”))

However, annotating the next attribute as LinkedList[T] allows invalid constructions with subclasses:

@dataclass
class OrdinalLinkedList(LinkedList[int]):
    def ordinal_value(self) -> str:
        return as_ordinal(self.value)

# Should not be OK because LinkedList[int] is not a subclass of
# OrdinalLinkedList, # but the type checker allows it.
xs = OrdinalLinkedList(value=1, next=LinkedList[int](value=2))

if xs.next:
    print(xs.next.ordinal_value())  # Runtime Error.

We propose expressing this constraint using next: Self | None:

from typing import Self

@dataclass
class LinkedList(Generic[T]):
    next: Self | None = None
    value: T


@dataclass
class OrdinalLinkedList(LinkedList[int]):
    def ordinal_value(self) -> str:
        return as_ordinal(self.value)

xs = OrdinalLinkedList(value=1, next=LinkedList[int](value=2))
# Type error: Expected OrdinalLinkedList, got LinkedList[int].

if xs.next is not None:
    xs.next = OrdinalLinkedList(value=3, next=None)  # OK
    xs.next = LinkedList[int](value=3, next=None)  # Not OK

The code above is semantically equivalent to treating each attribute containing a Self type as a property that returns that type:

from dataclasses import dataclass
from typing import Any, Generic, TypeVar

T = TypeVar("T")
Self = TypeVar("Self", bound="LinkedList")


class LinkedList(Generic[T]):
    value: T

    @property
    def next(self: Self) -> Self | None:
        return self._next

    @next.setter
    def next(self: Self, next: Self | None) -> None:
        self._next = next

class OrdinalLinkedList(LinkedList[int]):
    def ordinal_value(self) -> str:
        return str(self.value)

Use in Generic Classes

Self can also be used in generic class methods:

class Container(Generic[T]):
    value: T
    def set_value(self, value: T) -> Self: ...

This is equivalent to writing:

Self = TypeVar(“Self”, bound=”Container[Any]”)

class Container(Generic[T]):
    value: T
    def set_value(self: Self, value: T) -> Self: ...

The behavior is to preserve the type argument of the object on which the method was called. When called on an object with concrete type Container[int], Self is bound to Container[int]. When called with an object of generic type Container[T], Self is bound to Container[T]:

def object_with_concrete_type() -> None:
    int_container: Container[int]
    str_container: Container[str]
    reveal_type(int_container.set_value(42))  # => Container[int]
    reveal_type(str_container.set_value(“hello”))  # => Container[str]

def object_with_generic_type(
    container: Container[T], value: T,
) -> Container[T]:
    return container.set_value(value)  # => Container[T]

The PEP doesn’t specify the exact type of self.value within the method set_value. Some type checkers may choose to implement Self types using class-local type variables with Self = TypeVar(“Self”, bound=Container[T]), which will infer a precise type T. However, given that class-local type variables are not a standardized type system feature, it is also acceptable to infer Any for self.value. We leave this up to the type checker.

Note that we reject using Self with type arguments, such as Self[int]. This is because it creates ambiguity about the type of the self parameter and introduces unnecessary complexity:

class Container(Generic[T]):
    def foo(
        self, other: Self[int], other2: Self,
    ) -> Self[str]:  # Rejected
        ...

In such cases, we recommend using an explicit type for self:

class Container(Generic[T]):
    def foo(
        self: Container[T],
        other: Container[int],
        other2: Container[T]
    ) -> Container[str]: ...

Use in Protocols

Self is valid within Protocols, similar to its use in classes:

from typing import Protocol, Self

class Shape(Protocol):
    scale: float

    def set_scale(self, scale: float) -> Self:
        self.scale = scale
        return self

is treated equivalently to:

from typing import TypeVar

SelfShape = TypeVar("SelfShape", bound="ShapeProtocol")

class Shape(Protocol):
    scale: float

    def set_scale(self: SelfShape, scale: float) -> SelfShape:
        self.scale = scale
        return self

See PEP 544 for details on the behavior of TypeVars bound to protocols.

Checking a class for compatibility with a protocol: If a protocol uses Self in methods or attribute annotations, then a class Foo is considered compatible with the protocol if its corresponding methods and attribute annotations use either Self or Foo or any of Foo’s subclasses. See the examples below:

from typing import Protocol

class ShapeProtocol(Protocol):
    def set_scale(self, scale: float) -> Self: ...

class ReturnSelf:
    scale: float = 1.0

    def set_scale(self, scale: float) -> Self:
        self.scale = scale
        return self

class ReturnConcreteShape:
    scale: float = 1.0

    def set_scale(self, scale: float) -> ReturnConcreteShape:
        self.scale = scale
        return self

class BadReturnType:
    scale: float = 1.0

    def set_scale(self, scale: float) -> int:
        self.scale = scale
        return 42

class ReturnDifferentClass:
    scale: float = 1.0

    def set_scale(self, scale: float) -> ReturnConcreteShape:
        return ReturnConcreteShape(...)

def accepts_shape(shape: ShapeProtocol) -> None:
    y = shape.set_scale(0.5)
    reveal_type(y)

def main() -> None:
    return_self_shape: ReturnSelf
    return_concrete_shape: ReturnConcreteShape
    bad_return_type: BadReturnType
    return_different_class: ReturnDifferentClass

    accepts_shape(return_self_shape)  # OK
    accepts_shape(return_concrete_shape)  # OK
    accepts_shape(bad_return_type)  # Not OK
    # Not OK because it returns a non-subclass.
    accepts_shape(return_different_class)

Valid Locations for Self

A Self annotation is only valid in class contexts, and will always refer to the encapsulating class. In contexts involving nested classes, Self will always refer to the innermost class.

The following uses of Self are accepted:

class ReturnsSelf:
    def foo(self) -> Self: ... # Accepted

    @classmethod
    def bar(cls) -> Self:  # Accepted
        return cls()

    def __new__(cls, value: int) -> Self: ...  # Accepted

    def explicitly_use_self(self: Self) -> Self: ...  # Accepted

    # Accepted (Self can be nested within other types)
    def returns_list(self) -> list[Self]: ...

    # Accepted (Self can be nested within other types)
    @classmethod
    def return_cls(cls) -> type[Self]:
        return cls

class Child(ReturnsSelf):
    # Accepted (we can override a method that uses Self annotations)
    def foo(self) -> Self: ...

class TakesSelf:
    def foo(self, other: Self) -> bool: ...  # Accepted

class Recursive:
    # Accepted (treated as an @property returning ``Self | None``)
    next: Self | None

class CallableAttribute:
    def foo(self) -> int: ...

    # Accepted (treated as an @property returning the Callable type)
    bar: Callable[[Self], int] = foo

TupleSelf = Tuple[Self, Self]
class Alias:
    def return_tuple(self) -> TupleSelf:
        return (self, self)

class HasNestedFunction:
    x: int = 42

    def foo(self) -> None:

        # Accepted (Self is bound to HasNestedFunction).
        def nested(z: int, inner_self: Self) -> Self:
            print(z)
            print(inner_self.x)
            return inner_self

        nested(42, self)  # OK


class Outer:
    class Inner:
        def foo(self) -> Self: ...  # Accepted (Self is bound to Inner)

The following uses of Self are rejected.

def foo(bar: Self) -> Self: ...  # Rejected (not within a class)

bar: Self  # Rejected (not within a class)

class Foo:
    # Rejected (Self is treated as unknown).
    def has_existing_self_annotation(self: T) -> Self: ...

class Foo:
    def return_concrete_type(self) -> Self:
        return Foo()  # Rejected (see FooChild below for rationale)

class FooChild(Foo):
    child_value: int = 42

    def child_method(self) -> None:
        # At runtime, this would be Foo, not FooChild.
        y = self.return_concrete_type()

        y.child_value
        # Runtime error: Foo has no attribute child_value

class Bar(Generic[T]):
    def bar(self) -> T: ...

class Baz(Foo[Self]): ...  # Rejected

Note that we reject Self in staticmethods. Self does not add much value since there is no self or cls to return. The only possible use cases would be to return a parameter itself or some element from a container passed in as a parameter. These don’t seem worth the additional complexity.

class Base:
    @staticmethod
    def make() -> Self:  # Rejected
        ...

    @staticmethod
    def return_parameter(foo: Self) -> Self:  # Rejected
        ...

Likewise, we reject Self in metaclasses. Self in this PEP consistently refers to the same type (that of self). But in metaclasses, it would have to refer to different types in different method signatures. For example, in __mul__, Self in the return type would refer to the implementing class Foo, not the enclosing class MyMetaclass. But, in __new__, Self in the return type would refer to the enclosing class MyMetaclass. To avoid confusion, we reject this edge case.

class MyMetaclass(type):
    def __new__(cls, *args: Any) -> Self:  # Rejected
        return super().__new__(cls, *args)

    def __mul__(cls, count: int) -> list[Self]:  # Rejected
        return [cls()] * count

class Foo(metaclass=MyMetaclass): ...

Runtime behavior

Because Self is not subscriptable, we propose an implementation similar to typing.NoReturn.

@_SpecialForm
def Self(self, params):
    """Used to spell the type of "self" in classes.

    Example::

      from typing import Self

      class ReturnsSelf:
          def parse(self, data: bytes) -> Self:
              ...
              return self

    """
    raise TypeError(f"{self} is not subscriptable")

Rejected Alternatives

Allow the Type Checker to Infer the Return Type

One proposal is to leave the Self type implicit and let the type checker infer from the body of the method that the return type must be the same as the type of the self parameter:

class Shape:
    def set_scale(self, scale: float):
        self.scale = scale
        return self  # Type checker infers that we are returning self

We reject this because Explicit Is Better Than Implicit. Beyond that, the above approach will fail for type stubs, which don’t have method bodies to analyze.

Reference Implementations

Mypy: Proof of concept implementation in Mypy.

Pyright: v1.1.184

Runtime implementation of Self: PR.

Resources

Similar discussions on a Self type in Python started in Mypy around 2016: Mypy issue #1212 - SelfType or another way to spell "type of self". However, the approach ultimately taken there was the bounded TypeVar approach shown in our "before" examples. Other issues that discuss this include Mypy issue #2354 - Self types in generic classes.

Pradeep made a concrete proposal at the PyCon Typing Summit 2021:
recorded talk, slides.

James brought up the proposal independently on typing-sig: Typing-sig thread.

Other languages have similar ways to express the type of the enclosing class:

Thanks to the following people for their feedback on the PEP:

Jia Chen, Rebecca Chen, Sergei Lebedev, Kaylynn Morgan, Tuomas Suutari, Alex Waygood, Shannon Zhu, and Никита Соболев

Source: https://github.com/python/peps/blob/master/pep-0673.rst