Final names, methods and classes

This section introduces these related features:

  1. Final names are variables or attributes that should not reassigned after initialization. They are useful for declaring constants.
  2. Final methods should not be overridden in a subclass.
  3. Final classes should not be subclassed.

All of these are only enforced by mypy, and only in annotated code. They is no runtime enforcement by the Python runtime.

Note

These are experimental features. They might change in later versions of mypy. The final qualifiers are available through the typing_extensions package on PyPI.

Final names

You can use the typing_extensions.Final qualifier to indicate that a name or attribute should not be reassigned, redefined, or overridden. This is often useful for module and class level constants as a way to prevent unintended modification. Mypy will prevent further assignments to final names in type-checked code:

from typing_extensions 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

Another use case for final attributes is to protect certain attributes from being overridden in a subclass:

from typing_extensions import Final

class Window:
    BORDER_WIDTH: Final = 2.5
    ...

class ListView(Window):
    BORDER_WIDTH = 3  # Error: can't override a final attribute

You can use @property to make an attribute read-only, but unlike Final, it doesn’t work with module attributes, and it doesn’t prevent overriding in subclasses.

Syntax variants

You can use Final in one of these forms:

  • You can provide an explicit type using the syntax Final[<type>]. Example:

    ID: Final[float] = 1
    
  • You can omit the type:

    ID: Final = 1
    

    Here mypy will infer type int for ID. 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].

  • Finally, you can write 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.

Details of using Final

These are the two main rules for defining a final name:

  • 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.

A final attribute declared in a class body without an initializer must be initialized in the __init__ method (you can skip the initializer 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

Final can 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!
    ...

Final and ClassVar should not be used together. Mypy will infer the scope of a final declaration automatically depending on whether it was initialized in the class body or in __init__.

A final attribute can’t be overridden by a subclass (even with another explicit final declaration). Note however that a final attribute can override a read-only property:

class Base:
    @property
    def ID(self) -> int: ...

class Derived(Base):
    ID: Final = 1  # OK

Declaring a name as final only guarantees that the name wll not be re-bound to another value. It doesn’t make the value immutable. You can use immutable ABCs and containers to prevent mutating such values:

x: Final = ['a', 'b']
x.append('c')  # OK

y: Final[Sequence[str]] = ['a', 'b']
y.append('x')  # Error: Sequence is immutable
z: Final = ('a', 'b')  # Also an option

Final methods

Like with attributes, sometimes it is useful to protect a method from overriding. You can use the typing_extensions.final decorator for this purpose:

from typing_extensions import final

class Base:
    @final
    def common_name(self) -> None:
        ...

class Derived(Base):
    def common_name(self) -> None:  # Error: cannot override a final method
        ...

This @final decorator can be used with instance methods, class methods, static methods, and properties.

For overloaded methods you should add @final on the implementation to make it final (or on the first overload in 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):
        ...

Final classes

You can apply the typing_extensions.final decorator to a class to indicate to mypy that it should not be subclassed:

from typing_extensions import final

@final
class Leaf:
    ...

class MyLeaf(Leaf):  # Error: Leaf can't be subclassed
    ...

The decorator acts as a declaration for mypy (and as documentation for humans), but it doesn’t actually prevent subclassing at runtime.

Here are some situations where using a final class may be useful:

  • A class wasn’t designed to be subclassed. Perhaps subclassing would not work as expected, or subclassing would be error-prone.
  • Subclassing 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.