Constants

Constants

In most programming languages there’s a convention for naming constants. Python is no different—and the convention is quite similar to many other languages.

In Python, we use ALL_CAPS for constant names, with underscores to separate words if necessary. Here are some examples:

# Physical constants
C = 299792458  # speed of light: meters / second ** -1
MASS_ELECTRON = 9.1093837015 * 10 ** -31  # mass in kg

# Mathematical constants
PI = 3.1415926535  # pi
PHI = 1.6180339887  # phi (golden ratio)

# Unit conversions
FEET_PER_METER = 3.280839895
KM_PER_NAUTICAL_MILES = 1.852

# Other constants
EGGS_PER_CARTON = 12

Unlike Java, there is no final keyword, which tells the compiler that a constant must not be changed (same for other languages like C++ or Rust which have a const keyword).

What prevents a user from changing a constant? In Python, nothing. All the more reason to make it immediately clear—visually—that we’re dealing with a constant.

So the rule in Python is to use ALL_CAPS for constants and nothing else. Then it’s up to you, the programmer, to ensure these remain unchanged.

Copyright © 2023–2025 Clayton Cafiero

No generative AI was used in producing this material. This was written the old-fashioned way.