4.9 Constants

Concept of Constants

What Is It?

A value that’s meant to never change after it’s set.

Why Doesn’t Python Enforce It?

Python has no true language-enforced constant — unlike const in other languages, nothing stops you from reassigning an UPPER_CASE name. “Constants” in Python are a naming convention, not a compiler guarantee.

Naming Convention

How Is It Used?

Constants are written in UPPER_SNAKE_CASE to visually signal “don’t reassign this” to anyone reading the code:

MAX_RETRIES = 3
DEFAULT_TIMEOUT = 30
API_BASE_URL = "https://api.example.com"

Best Practices

• Define constants at the top of the module, right after imports
• Use UPPER_SNAKE_CASE consistently so violations of the convention stand out
• For values that truly must be immutable, use a tuple or Enum rather than relying on naming alone

Quick Interview Answer

“Python has no const keyword — a ‘constant’ is purely a naming convention: UPPER_SNAKE_CASE, defined at the top of a module, meant to signal ‘don’t reassign this.’ Nothing at the language level actually prevents reassignment. If true immutability matters, use a tuple or an Enum instead of relying on the naming convention alone.”

Common Mistakes

• Believing UPPER_CASE = value is protected from reassignment the way const is in JavaScript or Java — it is not; it’s purely convention.
• Defining constants scattered throughout a module instead of grouped at the top, making them harder for a reader to find.