Python 3.8+Annotations & typingterm

Final

constants

Marks a variable as a constant — it must not be reassigned after initialization. For static analysis only.

Syntax

from typing import Final
X: Final = value
X: Final[type] = value

Final forbids rebinding the name, not mutating the object: a list marked Final can still be appended to and sorted in place. Only a static checker enforces it — the interpreter happily performs a reassignment, with no runtime error at all. Do not confuse it with the @final decorator, which forbids subclassing a class or overriding a method.

Examples

from typing import Final
MAX_SIZE: Final = 100
PI: Final[float] = 3.14159
print(MAX_SIZE)  # → 100
print(PI)  # → 3.14159

from typing import Final
class Config:
    HOST: Final = 'localhost'
    PORT: Final[int] = 8080
print(Config.HOST)  # → localhost
print(Config.PORT)  # → 8080

See also

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