How do I create a constant in Python?

In Python, there's no built-in data type for constants like other programming languages. However, Python follows naming conventions and provides several approaches to create constants that signal to other developers that a value shouldn't be changed.

Built-in Constants in Python

Python has six built-in constants: False, True, None, NotImplemented, Ellipsis (...), and __debug__. These cannot be reassigned ?

# Trying to reassign a built-in constant raises SyntaxError
try:
    False = 100
except SyntaxError as e:
    print(f"Error: {e}")

  File "<stdin>", line 2
    False = 100
    ^^^^^
SyntaxError: cannot assign to False

Using Naming Conventions

According to PEP 8, constants should be written in ALL_CAPS with underscores separating words. This signals that the value shouldn't be changed ?

# Mathematical constants
PI = 3.14159
SPEED_OF_LIGHT = 299792458  # meters per second
GOLDEN_RATIO = 1.618

# Application constants
MAX_CONNECTIONS = 100
DEFAULT_TIMEOUT = 30
API_VERSION = "v1.2"

print(f"PI: {PI}")
print(f"Speed of Light: {SPEED_OF_LIGHT} m/s")
print(f"API Version: {API_VERSION}")

PI: 3.14159
Speed of Light: 299792458 m/s
API Version: v1.2

Creating a Constants Module

For larger projects, create a separate module to store all constants. This keeps them organized and easily accessible ?

constants.py

# constants.py - Store all application constants

# Mathematical constants
PI = 3.141592653589793
E = 2.718281828459045

# Physics constants
SPEED_OF_LIGHT_IN_VACUUM = 299792458
GRAVITATIONAL_CONSTANT = 6.67430e-11

# Application settings
MAX_FILE_SIZE = 1024 * 1024  # 1 MB
DEFAULT_PORT = 8080
DATABASE_URL = "localhost:5432"

main.py

import constants

# Calculate circle area
radius = 5
area = constants.PI * radius ** 2
print(f"Circle area: {area}")

# Display physics constant
print(f"Speed of light: {constants.SPEED_OF_LIGHT_IN_VACUUM} m/s")

# Use application settings
print(f"Server running on port: {constants.DEFAULT_PORT}")

Circle area: 78.53981633974483
Speed of light: 299792458 m/s
Server running on port: 8080

Using Enum for Related Constants

For related constants, use the enum module for better organization and type safety ?

from enum import Enum

class Colors(Enum):
    RED = "#FF0000"
    GREEN = "#00FF00"
    BLUE = "#0000FF"

class Status(Enum):
    PENDING = 1
    APPROVED = 2
    REJECTED = 3

# Usage
print(f"Red color code: {Colors.RED.value}")
print(f"Status: {Status.APPROVED.name}")

Red color code: #FF0000
Status: APPROVED

Important Notes

Convention, Not Enforcement: Python's constant naming is purely by convention. Values can still be reassigned, but developers understand they shouldn't be changed.

Immutable Types: Use immutable types like strings, numbers, and tuples for constants. Avoid mutable types like lists or dictionaries.

Conclusion

Python creates constants using ALL_CAPS naming convention and modules for organization. While not enforced by the language, this approach clearly communicates intent to other developers and maintains code readability.