How do I create a constant in Python?

Emulating constants in Python

Python doesn't natively support constants, but we can emulate them:

Wield typing.Final

From Python 3.8+, you can use typing.Final to denote that a variable should never be reassigned. This doesn't enforce runtime immutability, but tools like mypy will catch reassignments.

from typing import Final

MY_FINAL: Final = "Constant as a northern star ⭐️"
# Will raise an error if you try to reassign MY_FINAL
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Opt for OOP

Use classes to house your constants. A clever use of properties can prevent modifications.

class MyConfig:
    @property
    def CONSTANT_SIZE(self):
        return "Size doesn't matter 😜"
# Now MyConfig().CONSTANT_SIZE is a constant value
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Explore __slots__

__slots__ prevents the dynamic creation of attributes in a class, providing a way to emulate immutability:

class MyConfigWithSlots:
    __slots__ = ("NO_NEW_ATTRIBUTES",)

    def __init__(self):
        self.NO_NEW_ATTRIBUTES = "Try and create another attribute. I dare you 🙃"
# Any attempts to create new attributes raise AttributeError
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Experiment with immutable collections

Python offers namedtuple and Enums — special types ideal for creating sets of constants:

from enum import Enum

class Planets(Enum):
    MERCURY = (3.303e+23, 2.4397e6)  # Mass and radius constant values
# Planets.MERCURY is now a constant pair that represents Mercury's characteristics
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Constants in disguise

We can disguise variables as constants. Here are some methods:

Sprinkle some decorator magic

A @constant decorator for class properties can make your intent clear and prevent modifications:

def constant_property(func):
    """
    Decorator that transforms a method into a read-only property. It's like putting a do-not-touch sign🚫
    """
    return property(func)
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Enforce in functions

Create constants using functions to prevent them from being redefined within the scope:

def IT_IS_FINAL(): return "Not the final you wished for at the end of class? 😉"
# Attempt to reassign IT_IS_FINAL will yield UnboundLocalError
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Dive into local scope

Locally, read-only variables can be created using the locals() function:

def local_function():
    LOCAL_CONSTANTS = locals()
    LOCAL_CONSTANTS['SECRET_KEY'] = "My precious! 💍"
# Within local_function, SECRET_KEY behaves like a constant
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