How to Define and Protect Constants in Python?

Declaring Constants in Python

In contrast to Java, Python lacks a built-in mechanism for defining constants.

Convention for Indicating Constants

To convey the immutability of a variable, Python programmers typically assign it a name in all uppercase letters:

CONST_NAME = "Name"

Preventing Constant Modification

Although convention suggests that constants should remain unchanged, there is no inherent protection against their alteration. However, external libraries offer solutions for raising exceptions upon constant modification. Alex Martelli's "Constants in Python" article outlines one such approach, though its use is uncommon.

typing.Final Annotation (Python 3.8 )

In Python 3.8, the typing.Final annotation aids static type checkers (e.g., mypy) in recognizing variables that should not be reassigned. Despite its intent, this annotation does not prevent reassignment in runtime:

from typing import Final

a: Final[int] = 1

# Executes without error, but mypy will report an error if run:
a = 2

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