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PEP 591 -- Adding a final qualifier to typing

PEP:591
Title:Adding a final qualifier to typing
Author:Michael J. Sullivan <sully at msully.net>, Ivan Levkivskyi <levkivskyi at gmail.com>
BDFL-Delegate:Guido van Rossum <guido at python.org>
Discussions-To:typing-sig at python.org
Status:Draft
Type:Standards Track
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 [3]).

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[<type>]. 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 [4] type checker supports Final and final. A reference implementation of the runtime component is provided in the typing_extensions [5] 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.

It would be possible to have the @final decorator on classes dynamically prevent subclassing at runtime. Nothing else in typing does any runtime enforcement, though, so final will not either. A workaround for when both runtime enforcement and static checking is desired is to use this idiom (possibly in a support module):

if typing.TYPE_CHECKING:
    from typing import final
else:
    from runtime_final import final

References

[1]PEP 484, Type Hints, van Rossum, Lehtosalo, Langa (http://www.python.org/dev/peps/pep-0484)
[2]PEP 526, Syntax for Variable Annotations, Gonzalez, House, Levkivskyi, Roach, van Rossum (http://www.python.org/dev/peps/pep-0526)
[3]PEP 486, Literal Types, Lee, Levkivskyi, Lehtosalo (http://www.python.org/dev/peps/pep-0586)
[4]http://www.mypy-lang.org/
[5]https://github.com/python/typing/typing_extensions
Source: https://github.com/python/peps/blob/master/pep-0591.rst