constants.md

Python by Example: Constants

Python does not have built-in constants like some languages. Instead, programmers use a naming convention: UPPER_CASE names mean "treat this as constant—don't change it." The interpreter won't enforce this; it's a convention teams agree on. Constants are useful for configuration values and magic numbers.

What you'll learn:

• The UPPER_CASE convention for constants
• Why Python doesn't enforce immutability

# Convention: UPPER_CASE for "constants"
MAX_SIZE = 100
API_URL = "https://api.example.com"
PI = 3.14159

print(MAX_SIZE)
print(API_URL)
print(PI)

# You can still reassign—Python won't stop you
MAX_SIZE = 200
print(MAX_SIZE)

Nothing prevents reassigning MAX_SIZE—Python allows it. The convention tells other developers (and your future self) not to change it.

To run this program:

$ python source/constants.py
100
https://api.example.com
3.14159
200

Tip: Use constants for values that appear multiple times or that might change later (e.g., API URLs, limits). Centralizing them makes updates easier.

Try it: Add your own constant and use it in a calculation.

Source: constants.py

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