PEP: 591 Title: Adding a final qualifier to typing Author: Michael J. Sullivan , Ivan Levkivskyi BDFL-Delegate: Guido van Rossum Discussions-To: typing-sig@python.org Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 15-Mar-2019 Post-History: Abstract ======== This PEP proposes a "final" qualifier to be added to the ``typing`` module---in the form of a ``final`` decorator and a ``Final`` type annotation---to serve three related purposes: * Declaring that a method should not be overridden * Declaring that a class should not be subclassed * Declaring that a variable or attribute should not be reassigned Motivation ========== The ``final`` decorator ----------------------- The current ``typing`` module lacks a way to restrict the use of inheritance or overriding at a typechecker level. This is a common feature in other object-oriented languages (such as Java), and is useful for reducing the potential space of behaviors of a class, easing reasoning. Some situations where a final class or method may be useful include: * A class wasn’t designed to be subclassed or a method wasn't designed to be overridden. Perhaps it would not work as expected, or be error-prone. * Subclassing or overriding would make code harder to understand or maintain. For example, you may want to prevent unnecessarily tight coupling between base classes and subclasses. * You want to retain the freedom to arbitrarily change the class implementation in the future, and these changes might break subclasses. The ``Final`` annotation ------------------------ The current ``typing`` module lacks a way to indicate that a variable will not be assigned to. This is a useful feature in several situations: * Preventing unintended modification of module and class level constants and documenting them as constants in a checkable way. * Creating a read-only attribute that may not be overridden by subclasses. (``@property`` can make an attribute read-only but does not prevent overriding) * Allowing a name to be used in situations where ordinarily a literal is expected (for example as a field name for ``NamedTuple``, a tuple of types passed to ``isinstance``, or an argument to a function with arguments of ``Literal`` type [#PEP-586]_). Specification ============= The ``final`` decorator ----------------------- The ``typing.final`` decorator is used to restrict the use of inheritance and overriding. A type checker should prohibit any class decorated with ``@final`` from being subclassed and any method decorated with ``@final`` from being overridden in a subclass. The method decorator version may be used with all of instance methods, class methods, static methods, and properties. For example:: from typing import final @final class Base: ... class Derived(Base): # Error: Cannot inherit from final class "Base" ... and:: from typing import final class Base: @final def foo(self) -> None: ... class Derived(Base): def foo(self) -> None: # Error: Cannot override final attribute "foo" # (previously declared in base class "Base") ... For overloaded methods, ``@final`` should be placed on the implementation (or on the first overload, for stubs):: from typing import Any, overload class Base: @overload def method(self) -> None: ... @overload def method(self, arg: int) -> int: ... @final def method(self, x=None): ... It is an error to use ``@final`` on a non-method function. The ``Final`` annotation ------------------------ The ``typing.Final`` type qualifier is used to indicate that a variable or attribute should not be reassigned, redefined, or overridden. Syntax ~~~~~~ ``Final`` may be used in in one of several forms: * With an explicit type, using the syntax ``Final[]``. Example:: ID: Final[float] = 1 * With no type annotation. Example:: ID: Final = 1 The typechecker should apply its usual type inference mechanisms to determine the type of ``ID`` (here, likely, ``int``). Note that unlike for generic classes this is *not* the same as ``Final[Any]``. * In class bodies and stub files you can omit the right hand side and just write ``ID: Final[float]``. If the right hand side is omitted, there must be an explicit type argument to ``Final``. * Finally, as ``self.id: Final = 1`` (also optionally with a type in square brackets). This is allowed *only* in ``__init__`` methods, so that the final instance attribute is assigned only once when an instance is created. Semantics and examples ~~~~~~~~~~~~~~~~~~~~~~ The two main rules for defining a final name are: * There can be *at most one* final declaration per module or class for a given attribute. There can't be separate class-level and instance-level constants with the same name. * There must be *exactly one* assignment to a final name. This means a type checker should prevent further assignments to final names in type-checked code:: from typing import Final RATE: Final = 3000 class Base: DEFAULT_ID: Final = 0 RATE = 300 # Error: can't assign to final attribute Base.DEFAULT_ID = 1 # Error: can't override a final attribute Note that a type checker need not allow ``Final`` declarations inside loops since the runtime will see multiple assignments to the same variable in subsequent iterations. Additionally, a type checker should prevent final attributes from being overridden in a subclass:: from typing import Final class Window: BORDER_WIDTH: Final = 2.5 ... class ListView(Window): BORDER_WIDTH = 3 # Error: can't override a final attribute A final attribute declared in a class body without an initializer must be initialized in the ``__init__`` method (except in stub files):: class ImmutablePoint: x: Final[int] y: Final[int] # Error: final attribute without an initializer def __init__(self) -> None: self.x = 1 # Good Type checkers should infer a final attribute that is initialized in a class body as being a class variable. Variables should not be annotated with both ``ClassVar`` and ``Final``. ``Final`` may only be used as the outermost type in assignments or variable annotations. Using it in any other position is an error. In particular, ``Final`` can't be used in annotations for function arguments:: x: List[Final[int]] = [] # Error! def fun(x: Final[List[int]]) -> None: # Error! ... Note that declaring a name as final only guarantees that the name will not be re-bound to another value, but does not make the value immutable. Immutable ABCs and containers may be used in combination with ``Final`` to prevent mutating such values:: x: Final = ['a', 'b'] x.append('c') # OK y: Final[Sequence[str]] = ['a', 'b'] y.append('x') # Error: "Sequence[str]" has no attribute "append" z: Final = ('a', 'b') # Also works Type checkers should treat uses of a final name that was initialized with a literal as if it was replaced by the literal. For example, the following should be allowed:: from typing import NamedTuple, Final X: Final = "x" Y: Final = "y" N = NamedTuple("N", [(X, int), (Y, int)]) Reference Implementation ======================== The mypy [#mypy]_ type checker supports `Final` and `final`. A reference implementation of the runtime component is provided in the ``typing_extensions`` [#typing_extensions]_ module. Rejected/deferred Ideas ======================= The name ``Const`` was also considered as the name for the ``Final`` type annotation. The name ``Final`` was chosen instead because the concepts are related and it seemed best to be consistent between them. We considered using a single name ``Final`` instead of introducing ``final`` as well, but ``@Final`` just looked too weird to us. A related feature to final classes would be Scala-style sealed classes, where a class is allowed to be inherited only by classes defined in the same module. Sealed classes seem most useful in combination with pattern matching, so it does not seem to justify the complexity in our case. This could be revisisted in the future. References ========== .. [#PEP-484] PEP 484, Type Hints, van Rossum, Lehtosalo, Langa (http://www.python.org/dev/peps/pep-0484) .. [#PEP-526] PEP 526, Syntax for Variable Annotations, Gonzalez, House, Levkivskyi, Roach, van Rossum (http://www.python.org/dev/peps/pep-0526) .. [#PEP-586] PEP 486, Literal Types, Lee, Levkivskyi, Lehtosalo (http://www.python.org/dev/peps/pep-0586) .. [#mypy] http://www.mypy-lang.org/ .. [#typing_extensions] https://github.com/python/typing/typing_extensions Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: