python-peps/pep-0673.rst

PEP: 673
Title: Self Type
Version: $Revision$
Last-Modified: $Date$
Author: Pradeep Kumar Srinivasan <gohanpra@gmail.com>,
        James Hilton-Balfe <gobot1234yt@gmail.com>
Sponsor: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Discussions-To: typing-sig@python.org
Status: Accepted
Type: Standards Track
Content-Type: text/x-rst
Created: 10-Nov-2021
Python-Version: 3.11
Post-History: 17-Nov-2021
Resolution: https://mail.python.org/archives/list/python-dev@python.org/thread/J7BWL5KWOPQQK5KFWKENVLXW6UGSPTGI/

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
<https://github.com/pradeep90/annotation_collector/#self-type-stats>`_ 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
<https://github.com/pradeep90/annotation_collector/#overall-stats-in-typeshed>`_.
This suggests that a ``Self`` type will be used quite often and users will
benefit a lot from the simpler approach above.

Users of Python types have also frequently requested this feature,
both on the `proposal doc
<https://docs.google.com/document/d/1ujuSMXDmSIOJpiZyV7mvBEC8P-y55AgSzXcvhrZciuI/edit?disco=AAAARP_cNdc>`_
and on `GitHub <https://github.com/python/mypy/issues/11871>`_.

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):
        def circumference(self) -> float: ...

    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]):
        value: T
        next: Self | None = None

    @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 ShapeProtocol(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 ShapeProtocol(Protocol):
        scale: float

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


See :pep:`PEP 544
<544#self-types-in-protocols>` 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

    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(Bar[Self]): ...  # Rejected

We reject type aliases containing ``Self``. Supporting ``Self``
outside class definitions can require a lot of special-handling in
type checkers. Given that it also goes against the rest of the PEP to
use ``Self`` outside a class definition, we believe the added
convenience of aliases is not worth it:

::

    TupleSelf = Tuple[Self, Self]  # Rejected

    class Alias:
        def return_tuple(self) -> TupleSelf:  # Rejected
            return (self, self)

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
<https://github.com/Gobot1234/mypy>`_.

Pyright: v1.1.184

Runtime implementation of ``Self``: `PR
<https://github.com/python/typing/pull/933>`_.

Resources
=========

Similar discussions on a ``Self`` type in Python started in Mypy around 2016:
`Mypy issue #1212 <https://github.com/python/mypy/issues/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
<https://github.com/python/mypy/issues/2354>`_ - Self types in generic
classes.

Pradeep made a concrete proposal at the PyCon Typing Summit 2021:
 `recorded talk <https://youtu.be/ld9rwCvGdhc?t=3260>`_, `slides
 <https://drive.google.com/file/d/1x-qoDVY_OvLpIV1EwT7m3vm4HrgubHPG/view>`_.

James brought up the proposal independently on typing-sig:
`Typing-sig thread <https://mail.python.org/archives/list/typing-sig@python.org/thread/SJAANGA2CWZ6D6TJ7KOPG7PZQC56K73S/#B2CBLQDHXQ5HMFUMS4VNY2D4YDCFT64Q>`_.

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

+ TypeScript has the ``this`` type (`TypeScript docs
  <https://typescriptlang.org/docs/handbook/2/classes.html#this-types>`_)
+ Rust has the ``Self`` type (`Rust docs
  <https://doc.rust-lang.org/std/keyword.SelfTy.html>`_)

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

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

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.



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