6 - Final qualifier

The Final type annotation helps in documenting constants in a type checker (e.g. mypy) friendly way. The final decorator can be used to restrict the use of inheritance (also for whole classes) and method overrides.

Similar features are available in many other programming languages but I feel that these are not widely known among Python peeps. Available in Python 3.8+.

Read more

• typing.Final: https://docs.python.org/3/library/typing.html#typing.Final
• @typing.final: https://docs.python.org/3/library/typing.html#typing.final

The code

from typing import Final, final

MY_STR: Final = "Can't change me :)"
MY_INT: Final = 13

# error: Cannot assign to final name "MY_STR"
MY_STR = "Something else"
# error: Cannot assign to final name "MY_STR"
MY_STR += "Add this to end"
# error: Cannot assign to final name "MY_INT"
MY_INT = 0


class MyClass:
    CLASS_VARIABLE: Final = "foo"

    @final
    def method(self) -> None:
        ...


class MyChildClass(MyClass):
    # error: Cannot assign to final name "CLASS_VARIABLE"
    CLASS_VARIABLE = "bar"

    # error: Cannot override final attribute "method"
    # (previously declared in base class "MyClass")
    def method(self) -> None:
        ...