PEP: 767

PEP: 767
Title: Annotating Read-Only Attributes
Author: Eneg <eneg at discuss.python.org>
Sponsor: Carl Meyer <[email protected]>
Discussions-To: https://discuss.python.org/t/pep-767-annotating-read-only-attributes/73408
Status: Draft
Type: Standards Track
Topic: Typing
Created: 18-Nov-2024
Python-Version: 3.14
Post-History: `09-Oct-2024 <https://discuss.python.org/t/expanding-readonly-to-normal-classes-protocols/67359>`__

Abstract
========

:pep:`705` introduced the :external+py3.13:data:`typing.ReadOnly` type qualifier
to allow defining read-only :class:`typing.TypedDict` items.

This PEP proposes using ``ReadOnly`` in :term:`annotations <annotation>` of class and protocol
:term:`attributes <attribute>`, as a single concise way to mark them read-only.

Akin to :pep:`705`, it makes no changes to setting attributes at runtime. Correct
usage of read-only attributes is intended to be enforced only by static type checkers.

Motivation
==========

The Python type system lacks a single concise way to mark an attribute read-only.
This feature is present in other statically and gradually typed languages
(such as `C# <https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/readonly>`_
or `TypeScript <https://www.typescriptlang.org/docs/handbook/2/objects.html#readonly-properties>`_),
and is useful for removing the ability to reassign or ``del``\ ete an attribute
at a type checker level, as well as defining a broad interface for structural subtyping.

. _classes:

Classes
-------

Today, there are three major ways of achieving read-only attributes, honored by type checkers:

* annotating the attribute with :data:`typing.Final`::

class Foo:
number: Final[int]

def __init__(self, number: int) -> None:
self.number = number

class Bar:
def __init__(self, number: int) -> None:
self.number: Final = number

- Supported by :mod:`dataclasses` (and type checkers since `typing#1669 <https://github.com/python/typing/pull/1669>`_).
- Overriding ``number`` is not possible - the specification of ``Final``
imposes that the name cannot be overridden in subclasses.

* read-only proxy via ``@property``::

class Foo:
_number: int

def __init__(self, number: int) -> None:
self._number = number

@property
def number(self) -> int:
return self._number

- Overriding ``number`` is possible. *Type checkers disagree about the specific rules*. [#overriding_property]_
- Read-only at runtime. [#runtime]_
- Requires extra boilerplate.
- Supported by :mod:`dataclasses`, but does not compose well - the synthesized
``__init__`` and ``__repr__`` will use ``_number`` as the parameter/attribute name.

* using a "freezing" mechanism, such as :func:`dataclasses.dataclass` or :class:`typing.NamedTuple`::

@dataclass(frozen=True)
class Foo:
number: int # implicitly read-only

class Bar(NamedTuple):
number: int # implicitly read-only

- Overriding ``number`` is possible in the ``@dataclass`` case.
- Read-only at runtime. [#runtime]_
- No per-attribute control - these mechanisms apply to the whole class.
- Frozen dataclasses incur some runtime overhead.
- ``NamedTuple`` is still a ``tuple``. Most classes do not need to inherit
indexing, iteration, or concatenation.

. _protocols:

Protocols
---------

A read-only attribute ``name: T`` on a :class:`~typing.Protocol` in principle
defines two requirements:

1. ``hasattr(obj, "name")``
2. ``isinstance(obj.name, T)``

Those requirements are satisfiable at runtime by all of the following:

* an object with an attribute ``name: T``,
* a class with a class variable ``name: ClassVar[T]``,
* an instance of the class above,
* an object with a ``@property`` ``def name(self) -> T``,
* an object with a custom descriptor, such as :func:`functools.cached_property`.

The current `typing spec <https://typing.readthedocs.io/en/latest/spec/protocol.html#protocol-members>`_
allows creation of such protocol members using (abstract) properties::

class HasName(Protocol):
@property
def name(self) -> T: ...

This syntax has several drawbacks:

* It is somewhat verbose.
* It is not obvious that the quality conveyed here is the read-only character of a property.
* It is not composable with :external+typing:term:`type qualifiers <type qualifier>`.
* Not all type checkers agree [#property_in_protocol]_ that all of the above five
objects are assignable to this structural type.

Rationale
=========

These problems can be resolved by an attribute-level type qualifier.
``ReadOnly`` has been chosen for this role, as its name conveys the intent well,
and the newly proposed changes complement its semantics defined in :pep:`705`.

A class with a read-only instance attribute can now be defined as::

from typing import ReadOnly

class Member:
def __init__(self, id: int) -> None:
self.id: ReadOnly[int] = id

..and the protocol described in :ref:`protocols` is now just::

from typing import Protocol, ReadOnly

class HasName(Protocol):
name: ReadOnly[str]

def greet(obj: HasName, /) -> str:
return f"Hello, {obj.name}!"

* A subclass of ``Member`` can redefine ``.id`` as a writable attribute or a
:term:`descriptor`. It can also :external+typing:term:`narrow` the type.
* The ``HasName`` protocol has a more succinct definition, and is agnostic
to the writability of the attribute.
* The ``greet`` function can now accept a wide variety of compatible objects,
while being explicit about no modifications being done to the input.

Specification
=============

The :external+py3.13:data:`typing.ReadOnly` :external+typing:term:`type qualifier`
becomes a valid annotation for :term:`attributes <attribute>` of classes and protocols.
It can be used at class-level or within ``__init__`` to mark individual attributes read-only::

class Book:
id: ReadOnly[int]

def __init__(self, id: int, name: str) -> None:
self.id = id
self.name: ReadOnly[str] = name

Type checkers should error on any attempt to reassign or ``del``\ ete an attribute
annotated with ``ReadOnly``.
Type checkers should also error on any attempt to delete an attribute annotated as ``Final``.
(This is not currently specified.)

Use of ``ReadOnly`` in annotations at other sites where it currently has no meaning
(such as local/global variables or function parameters) is considered out of scope
for this PEP.

Akin to ``Final`` [#final_mutability]_, ``ReadOnly`` does not influence how
type checkers perceive the mutability of the assigned object. Immutable :term:`ABCs <abstract base class>`
and :mod:`containers <collections.abc>` may be used in combination with ``ReadOnly``
to forbid mutation of such values at a type checker level:

. code-block:: python

from collections import abc
from dataclasses import dataclass
from typing import Protocol, ReadOnly

@dataclass
class Game:
name: str

class HasGames[T: abc.Collection[Game]](Protocol):
games: ReadOnly[T]

def add_games(shelf: HasGames[list[Game]]) -> None:
shelf.games.append(Game("Half-Life")) # ok: list is mutable
shelf.games[-1].name = "Black Mesa" # ok: "name" is not read-only
shelf.games = [] # error: "games" is read-only
del shelf.games # error: "games" is read-only and cannot be deleted

def read_games(shelf: HasGames[abc.Sequence[Game]]) -> None:
shelf.games.append(...) # error: "Sequence" has no attribute "append"
shelf.games[0].name = "Blue Shift" # ok: "name" is not read-only
shelf.games = [] # error: "games" is read-only

All instance attributes of frozen dataclasses and ``NamedTuple`` should be
implied to be read-only. Type checkers may inform that annotating such attributes
with ``ReadOnly`` is redundant, but it should not be seen as an error:

. code-block:: python

from dataclasses import dataclass
from typing import NewType, ReadOnly

@dataclass(frozen=True)
class Point:
x: int # implicit read-only
y: ReadOnly[int] # ok, redundant

uint = NewType("uint", int)

@dataclass(frozen=True)
class UnsignedPoint(Point):
x: ReadOnly[uint] # ok, redundant; narrower type
y: Final[uint] # not redundant, Final imposes extra restrictions; narrower type

. _init:

Initialization
--------------

Assignment to a read-only attribute can only occur in the class declaring the attribute.
There is no restriction to how many times the attribute can be assigned to.
The assignment must be allowed in the following contexts:

* In ``__init__``, on the instance received as the first parameter (likely, ``self``).
* In ``__new__``, on instances of the declaring class created via a call
to a super-class' ``__new__`` method.
* At declaration in the body of the class.

Additionally, a type checker may choose to allow the assignment:

* In ``__new__``, on instances of the declaring class, without regard
to the origin of the instance.
(This choice trades soundness, as the instance may already be initialized,
for the simplicity of implementation.)
* In ``@classmethod``\ s, on instances of the declaring class created via
a call to the class' or super-class' ``__new__`` method.

Note that a child class cannot assign to any read-only attributes of a parent class
in any of the aforementioned contexts, unless the attribute is redeclared.

. code-block:: python

from collections import abc
from typing import ReadOnly

class Band:
name: str
songs: ReadOnly[list[str]]

def __init__(self, name: str, songs: abc.Iterable[str] | None = None) -> None:
self.name = name
self.songs = []

if songs is not None:
self.songs = list(songs) # multiple assignments are fine

def clear(self) -> None:
# error: assignment to read-only "songs" outside initialization
self.songs = []

band = Band(name="Bôa", songs=["Duvet"])
band.name = "Python" # ok: "name" is not read-only
band.songs = [] # error: "songs" is read-only
band.songs.append("Twilight") # ok: list is mutable

class SubBand(Band):
def __init__(self) -> None:
self.songs = [] # error: cannot assign to a read-only attribute of a base class

. code-block:: python

# a simplified immutable Fraction class
class Fraction:
numerator: ReadOnly[int]
denominator: ReadOnly[int]

def __new__(
cls,
numerator: str | int | float | Decimal | Rational = 0,
denominator: int | Rational | None = None
) -> Self:
self = super().__new__(cls)

if denominator is None:
if type(numerator) is int:
self.numerator = numerator
self.denominator = 1
return self

elif isinstance(numerator, Rational): ...

else: ...

@classmethod
def from_float(cls, f: float, /) -> Self:
self = super().__new__(cls)
self.numerator, self.denominator = f.as_integer_ratio()
return self

When a class-level declaration has an initializing value, it can serve as a `flyweight <https://en.wikipedia.org/wiki/Flyweight_pattern>`_
default for instances:

. code-block:: python

class Patient:
number: ReadOnly[int] = 0

def __init__(self, number: int | None = None) -> None:
if number is not None:
self.number = number

. note::
This feature conflicts with :data:`~object.__slots__`. An attribute with
a class-level value cannot be included in slots, effectively making it a class variable.

Type checkers may choose to warn on read-only attributes which could be left uninitialized
after an instance is created (except in :external+typing:term:`stubs <stub>`,
protocols or ABCs)::

class Patient:
id: ReadOnly[int] # error: "id" is not initialized on all code paths
name: ReadOnly[str] # error: "name" is never initialized

def __init__(self) -> None:
if random.random() > 0.5:
self.id = 123

class HasName(Protocol):
name: ReadOnly[str] # ok

Subtyping
---------

Read-only attributes are covariant. This has a few subtyping implications.
Borrowing from :pep:`705#inheritance`:

* Read-only attributes can be redeclared as writable attributes, descriptors
or class variables::

@dataclass
class HasTitle:
title: ReadOnly[str]

@dataclass
class Game(HasTitle):
title: str
year: int

game = Game(title="DOOM", year=1993)
game.year = 1994
game.title = "DOOM II" # ok: attribute is not read-only

class TitleProxy(HasTitle):
@functools.cached_property
def title(self) -> str: ...

class SharedTitle(HasTitle):
title: ClassVar[str] = "Still Grey"

* If a read-only attribute is not redeclared, it remains read-only::

class Game(HasTitle):
year: int

def __init__(self, title: str, year: int) -> None:
super().__init__(title)
self.title = title # error: cannot assign to a read-only attribute of base class
self.year = year

game = Game(title="Robot Wants Kitty", year=2010)
game.title = "Robot Wants Puppy" # error: "title" is read-only

* Subtypes can :external+typing:term:`narrow` the type of read-only attributes::

class GameCollection(Protocol):
games: ReadOnly[abc.Collection[Game]]

@dataclass
class GameSeries(GameCollection):
name: str
games: ReadOnly[list[Game]] # ok: list[Game] is assignable to Collection[Game]

* Nominal subclasses of protocols and ABCs should redeclare read-only attributes
in order to implement them, unless the base class initializes them in some way::

class MyBase(abc.ABC):
foo: ReadOnly[int]
bar: ReadOnly[str] = "abc"
baz: ReadOnly[float]

def __init__(self, baz: float) -> None:
self.baz = baz

@abstractmethod
def pprint(self) -> None: ...

@final
class MySubclass(MyBase):
# error: MySubclass does not override "foo"

def pprint(self) -> None:
print(self.foo, self.bar, self.baz)

* In a protocol attribute declaration, ``name: ReadOnly[T]`` indicates that a structural
subtype must support ``.name`` access, and the returned value is assignable to ``T``::

class HasName(Protocol):
name: ReadOnly[str]

class NamedAttr:
name: str

class NamedProp:
@property
def name(self) -> str: ...

class NamedClassVar:
name: ClassVar[str]

class NamedDescriptor:
@cached_property
def name(self) -> str: ...

# all of the following are ok
has_name: HasName
has_name = NamedAttr()
has_name = NamedProp()
has_name = NamedClassVar
has_name = NamedClassVar()
has_name = NamedDescriptor()

Interaction with Other Type Qualifiers
--------------------------------------

``ReadOnly`` can be used with ``ClassVar`` and ``Annotated`` in any nesting order:

. code-block:: python

class Foo:
foo: ClassVar[ReadOnly[str]] = "foo"
bar: Annotated[ReadOnly[int], Gt(0)]

. code-block:: python

class Foo:
foo: ReadOnly[ClassVar[str]] = "foo"
bar: ReadOnly[Annotated[int, Gt(0)]]

This is consistent with the interaction of ``ReadOnly`` and :class:`typing.TypedDict`
defined in :pep:`705`.

An attribute annotated as both ``ReadOnly`` and ``ClassVar`` can only be assigned to
at declaration in the class body.

An attribute cannot be annotated as both ``ReadOnly`` and ``Final``, as the two
qualifiers differ in semantics, and ``Final`` is generally more restrictive.
``Final`` remains allowed as an annotation of attributes that are only implied
to be read-only. It can be also used to redeclare a ``ReadOnly`` attribute of a base class.

Backwards Compatibility
=======================

This PEP introduces new contexts where ``ReadOnly`` is valid. Programs inspecting
those places will have to change to support it. This is expected to mainly affect type checkers.

However, caution is advised while using the backported ``typing_extensions.ReadOnly``
in older versions of Python. Mechanisms inspecting annotations may behave incorrectly
when encountering ``ReadOnly``; in particular, the ``@dataclass`` decorator
which `looks for <https://docs.python.org/3/library/dataclasses.html#class-variables>`_
``ClassVar`` may mistakenly treat ``ReadOnly[ClassVar[...]]`` as an instance attribute.

To avoid issues with introspection, use ``ClassVar[ReadOnly[...]]`` instead of ``ReadOnly[ClassVar[...]]``.

Security Implications
=====================

There are no known security consequences arising from this PEP.

How to Teach This
=================

Suggested changes to the :mod:`typing` module documentation,
following the footsteps of :pep:`705#how-to-teach-this`:

* Add this PEP to the others listed.
* Link :external+py3.13:data:`typing.ReadOnly` to this PEP.
* Update the description of ``typing.ReadOnly``:

A special typing construct to mark an attribute of a class or an item of
a ``TypedDict`` as read-only.

* Add a standalone entry for ``ReadOnly`` under the
`type qualifiers <https://typing.readthedocs.io/en/latest/spec/qualifiers.html>`_ section:

The ``ReadOnly`` type qualifier in class attribute annotations indicates
that the attribute of the class may be read, but not reassigned or ``del``\ eted.
For usage in ``TypedDict``, see `ReadOnly <https://typing.readthedocs.io/en/latest/spec/typeddict.html#typing-readonly-type-qualifier>`_.

Rejected Ideas
==============

Clarifying Interaction of ``@property`` and Protocols
-----------------------------------------------------

The :ref:`protocols` section mentions an inconsistency between type checkers in
the interpretation of properties in protocols. The problem could be fixed
by amending the typing specification, clarifying what implements the read-only
quality of such properties.

This PEP makes ``ReadOnly`` a better alternative for defining read-only attributes
in protocols, superseding the use of properties for this purpose.

Assignment Only in ``__init__`` and Class Body
----------------------------------------------

An earlier version of this PEP proposed that read-only attributes could only be
assigned to in ``__init__`` and the class' body. A later discussion revealed that
this restriction would severely limit the usability of ``ReadOnly`` within
immutable classes, which typically do not define ``__init__``.

:class:`fractions.Fraction` is one example of an immutable class, where the
initialization of its attributes happens within ``__new__`` and classmethods.
However, unlike in ``__init__``, the assignment in ``__new__`` and classmethods
is potentially unsound, as the instance they work on can be sourced from
an arbitrary place, including an already finalized instance.

We find it imperative that this type checking feature is useful to the foremost
use site of read-only attributes - immutable classes. Thus, the PEP has changed
since to allow assignment in ``__new__`` and classmethods under a set of rules
described in the :ref:`init` section.

Open Issues
===========

Extending Initialization
------------------------

Mechanisms such as :func:`dataclasses.__post_init__` or attrs' `initialization hooks <https://www.attrs.org/en/stable/init.html#hooking-yourself-into-initialization>`_
augment object creation by providing a set of special hooks which are called
during initialization.

The current initialization rules defined in this PEP disallow assignment to
read-only attributes in such methods. It is unclear whether the rules could be
satisfyingly shaped in a way that is inclusive of those 3rd party hooks, while
upkeeping the invariants associated with the read-only-ness of those attributes.

The Python type system has a long and detailed `specification <https://typing.readthedocs.io/en/latest/spec/constructors.html>`_
regarding the behavior of ``__new__`` and ``__init__``. It is rather unfeasible
to expect the same level of detail from 3rd party hooks.

A potential solution would involve type checkers providing configuration in this
regard, requiring end users to manually specify a set of methods they wish
to allow initialization in. This however could easily result in users mistakenly
or purposefully breaking the aforementioned invariants. It is also a fairly
big ask for a relatively niche feature.

``ReadOnly[ClassVar[...]]`` and ``__init_subclass__``
-----------------------------------------------------

Should read-only class variables be assignable to within the declaring class'
``__init_subclass__``?

. code-block:: python

class URI:
protocol: ReadOnly[ClassVar[str]] = ""

def __init_subclass__(cls, protocol: str = "") -> None:
cls.foo = protocol

class File(URI, protocol="file"): ...

Footnotes
=========

. [#overriding_property]
Pyright in strict mode disallows non-property overrides.
Mypy does not impose this restriction and allows an override with a plain attribute.
`[Pyright playground] <https://pyright-play.net/?strict=true&code=MYGwhgzhAEBiD28BcBYAUNT0D6A7ArgLYBGApgE5LQCWuALuultACakBmO2t1d22ACgikQ7ADTQCJClVp0AlNAC0APmgA5eLlKoMzLMNEA6PETLloAXklmKjPZgACAB3LxnFOgE8mWNpylzIRF2RVUael19LHJSOnxyXGhDdhNAuzR7UEgYACEwcgEEeHkorHTKCIY0IA>`_
`[mypy playground] <https://mypy-play.net/?mypy=latest&python=3.12&flags=strict&gist=6f860a865c5d13cce07d6cbb08b9fb85>`_

. [#runtime]
This PEP focuses solely on the type-checking behavior. Nevertheless, it should
be desirable the name is read-only at runtime.

. [#property_in_protocol]
Pyright disallows class variable and non-property descriptor overrides.
`[Pyright] <https://pyright-play.net/?pyrightVersion=1.1.389&pythonVersion=3.13&strict=true&code=GYJw9gtgBAhgRgYygSwgBzCALrOBnLEGBLCAUywAswATAKFEimAFcA7EsMAGzxXUw4ExSmRoB9NODRlsATwbhoWOWmRsA5vwzYoAYW4w8eAGowQAGigAFcFjAIeV4Opjc6HhIeNQAEkYAxLgAKWzB7R24ASgAuOigEqAABKTAZeXjEpPgCIhJyKlpMhJoyYGYQvDJuYCioAFoAPhQ2LBioADoujzoAYlhjZA02eGRuZBUeryM%2BILAAQSxCZDgWLDI4xIqwdvUsT29ZrjD0lU2s1NOFLdLy4Erq2obmvfaQChYQNigABgOZqBzAwzMwgc4Je47fSHUEAbT2AF0oABeX7-HxzAAiZDwCBAyDQ9jBxWSwgQogkl1kkxuZW2wSqNTqTRabSg7ywn2%2Bfzo0wxx2k1LkejAADdzMgYK1wckqRlaXcHkznlA4FxuG8Pl9AW4quijmAAJJscXjGgyyHtXL6qAAOTAcxlcHMVsIPTAcAAVu1-Hg5nQPZ6UYCuItlqt1sE6lB%2BmAANYBr3BuYnIVRxKxhOB5NcYHGUFbGNQeOJoOooEw8zphKZ0s5sDY3H4wmYdO17PlgVpIUi8X4qVYNvFrNJztGk1uZA0as1qCyEB11H2uYzwtF%2BeLu1gNhkNdr-objwHgAeaHGCAm2ncNrmYfxEbIhvQ3GCvrmsRJltZN67VyfZ9fQIuA-LYUkFeVEluelGSeFlXnZLVuR-MA81Mcx-xfN9gItLh2lQuFEWDHk%2BQNRs8QJIkMMAv1sJJJIyQpSRwJpSC6UhBlHmZF5pQQzltWIw4QzAVN5F7CUByorCwBAi5mOuVjFTADjlRZNUeE1PjvgCXUyGQ41TSnSSgOknCoWtOgkhcEZ3BIrc5iMmiTJJZ0wSga0gA>`_
`[mypy] <https://mypy-play.net/?mypy=1.13.0&python=3.12&flags=strict&gist=12d556bb6ef4a9a49ff4ed4776604750>`_
`[Pyre] <https://pyre-check.org/play/?input=%23%20pyre-strict%0Afrom%20abc%20import%20abstractmethod%0Afrom%20functools%20import%20cached_property%0Afrom%20typing%20import%20ClassVar%2C%20Protocol%2C%20final%0A%0A%0Aclass%20HasFoo(Protocol)%3A%0A%20%20%20%20%40property%0A%20%20%20%20%40abstractmethod%0A%20%20%20%20def%20foo(self)%20-%3E%20int%3A%20...%0A%0A%0A%23%20assignability%0A%0A%0Aclass%20FooAttribute%3A%0A%20%20%20%20foo%3A%20int%0A%0Aclass%20FooProperty%3A%0A%20%20%20%20%40property%0A%20%20%20%20def%20foo(self)%20-%3E%20int%3A%20return%200%0A%0Aclass%20FooClassVar%3A%0A%20%20%20%20foo%3A%20ClassVar%5Bint%5D%20%3D%200%0A%0Aclass%20FooDescriptor%3A%0A%20%20%20%20%40cached_property%0A%20%20%20%20def%20foo(self)%20-%3E%20int%3A%20return%200%0A%0Aclass%20FooPropertyCovariant%3A%0A%20%20%20%20%40property%0A%20%20%20%20def%20foo(self)%20-%3E%20bool%3A%20return%20False%0A%0Aclass%20FooInvalid%3A%0A%20%20%20%20foo%3A%20str%0A%0Aclass%20NoFoo%3A%0A%20%20%20%20bar%3A%20str%0A%0A%0Aobj%3A%20HasFoo%0Aobj%20%3D%20FooAttribute()%20%20%23%20ok%0Aobj%20%3D%20FooProperty()%20%20%20%23%20ok%0Aobj%20%3D%20FooClassVar%20%20%20%20%20%23%20ok%0Aobj%20%3D%20FooClassVar()%20%20%20%23%20ok%0Aobj%20%3D%20FooDescriptor()%20%23%20ok%0Aobj%20%3D%20FooPropertyCovariant()%20%23%20ok%0Aobj%20%3D%20FooInvalid()%20%20%20%20%23%20err%0Aobj%20%3D%20NoFoo()%20%20%20%20%20%20%20%20%20%23%20err%0Aobj%20%3D%20None%20%20%20%20%20%20%20%20%20%20%20%20%23%20err%0A%0A%0A%23%20explicit%20impl%0A%0A%0Aclass%20FooAttributeImpl(HasFoo)%3A%0A%20%20%20%20foo%3A%20int%0A%0Aclass%20FooPropertyImpl(HasFoo)%3A%0A%20%20%20%20%40property%0A%20%20%20%20def%20foo(self)%20-%3E%20int%3A%20return%200%0A%0Aclass%20FooClassVarImpl(HasFoo)%3A%0A%20%20%20%20foo%3A%20ClassVar%5Bint%5D%20%3D%200%0A%0Aclass%20FooDescriptorImpl(HasFoo)%3A%0A%20%20%20%20%40cached_property%0A%20%20%20%20def%20foo(self)%20-%3E%20int%3A%20return%200%0A%0Aclass%20FooPropertyCovariantImpl(HasFoo)%3A%0A%20%20%20%20%40property%0A%20%20%20%20def%20foo(self)%20-%3E%20bool%3A%20return%20False%0A%0Aclass%20FooInvalidImpl(HasFoo)%3A%0A%20%20%20%20foo%3A%20str%0A%0A%40final%0Aclass%20NoFooImpl(HasFoo)%3A%0A%20%20%20%20bar%3A%20str%0A>`_

. [#final_mutability]
As noted above the second-to-last code example of https://typing.readthedocs.io/en/latest/spec/qualifiers.html#semantics-and-examples

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