Constants

Constants complement variables by giving fixed values (such as configuration options, limits, or domain-specific numbers) a stable, descriptive name.

When working with constants, the following best practices can improve clarity and maintainability:

• Use UPPER_SNAKE_CASE for module-level constants. Python does not enforce constants at runtime, so following the PEP 8 naming convention signals intent.
• Replace magic numbers with named constants. Replace unexplained literal values like 0.9, 3.14, or 42 with descriptive constants such as DISCOUNT_RATE, PI, or ULTIMATE_ANSWER. This removes magic numbers and gives you a single source of truth when a value is reused.
• Prefer immutable values for shared data. Use immutable types such as numbers, strings, tuples, or frozen data structures to reduce accidental modification.

To see some of these ideas in action, consider the following code example:

🔴 Avoid this:

def compute_net_salary(hours):
    return hours * 35 * (1 - (0.04 + 0.10))

This function hides meaning behind numeric literals, forcing readers to infer intent from the numbers alone.

✅ Favor this:

HOURLY_SALARY = 35
SOCIAL_SECURITY_TAX_RATE = 0.04
FEDERAL_TAX_RATE = 0.10

def compute_net_salary(hours):
    return (
        hours
        * HOURLY_SALARY
        * (1 - (SOCIAL_SECURITY_TAX_RATE + FEDERAL_TAX_RATE))
    )

In this version, HOURLY_SALARY, SOCIAL_SECURITY_TAX_RATE, and FEDERAL_TAX_RATE are defined as module-level constants, making the intent explicit and keeping the values in one place.

Related Resources

Tutorial

Python Constants: Improve Your Code's Maintainability

In this tutorial, you'll learn how to properly define constants in Python. By coding a bunch of practical example, you'll also learn how Python constants can improve your code's readability, reusability, and maintainability.

intermediate python

For additional information on related topics, take a look at the following resources:

• Build Enumerations of Constants With Python's Enum (Tutorial)
• Python's Mutable vs Immutable Types: What's the Difference? (Tutorial)
• Defining Python Constants for Code Maintainability (Course)
• Building Enumerations With Python's enum (Course)
• Differences Between Python's Mutable and Immutable Types (Course)