PEP 591 – Adding a final qualifier to typing
- Adding a final qualifier to typing
- Michael J. Sullivan <sully at msully.net>, Ivan Levkivskyi <levkivskyi at gmail.com>
- Guido van Rossum <guido at python.org>
- Standards Track
This PEP proposes a “final” qualifier to be added to the
module—in the form of a
final decorator and a
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
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,
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.
typing module lacks a way to indicate that a variable
will not be assigned to. This is a useful feature in several
- 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
@propertycan 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
Literaltype (PEP 586)).
typing.final decorator is used to restrict the use of
inheritance and overriding.
A type checker should prohibit any class decorated with
from being subclassed and any method decorated with
being overridden in a subclass. The method decorator version may be
used with all of instance methods, class methods, static methods, and properties.
from typing import final @final class Base: ... class Derived(Base): # Error: Cannot inherit from final class "Base" ...
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.
typing.Final type qualifier is used to indicate that a
variable or attribute should not be reassigned, redefined, or overridden.
Final may be used in one of several forms:
- With an explicit type, using the syntax
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
int). Note that unlike for generic classes this is not the same as
- 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
- 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
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
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
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)])
The mypy  type checker supports
reference implementation of the runtime component is provided in the
typing_extensions  module.
Const was also considered as the name for the
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 revisited in the future.
It would be possible to have the
@final decorator on classes
dynamically prevent subclassing at runtime. Nothing else in
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
This document has been placed in the public domain.
Last modified: 2022-01-21 11:03:51 GMT