What is a constant in Python?

In Python, a constant is essentially a variable whose value is intended to remain unchanged throughout the program's execution. Unlike some other programming languages, Python doesn't have a built-in keyword or mechanism to enforce immutability for variables declared as constants. Instead, the convention is to use naming to indicate that a variable is intended to be treated as a constant.

Here's a breakdown:

1. Naming Convention:

- Constants in Python are conventionally named using all uppercase letters with words separated by underscores (e.g., MAX_VALUE, PI, DEFAULT_NAME). This naming convention signals to other programmers that the variable should be treated as a constant and not modified.

2. No Enforcement of Immutability:

- While the naming convention strongly suggests a variable is a constant, Python does not prevent you from reassigning a value to that variable. The immutability is purely by convention, relying on the programmer's adherence to established best practices.

3. Example:

PI = 3.14159
GRAVITY = 9.81
MAX_USERS = 1000

- In this example, PI, GRAVITY, and MAX_USERS are intended to be constants. A programmer reading the code should avoid modifying these variables.

4. Why use constants?

- Constants make code more readable and maintainable. Using constants instead of magic numbers (hardcoded literal values) makes the code easier to understand and modify. If a value needs to be changed later, you only need to change it in one place - the constant definition - rather than scattered throughout your code.

5. Immutability through Data Types (indirect):

- While variables can always be reassigned, you can get immutability indirectly. Data types like strings, tuples, and frozensets are immutable, which means their contents cannot be changed after they're created. While you can't change the contents of an existing string, tuple, or frozenset assigned to a constant, you can reassign a new object to that constant. The immutability here is regarding the data, not the variable as a constant, which is still just a reference.

- For example: CONSTANT_TUPLE = (1, 2, 3), CONSTANT_STRING = "hello". Attempting to modify either the tuple or the string will raise an error.

Key Takeaway:

- In Python, a constant is a variable that is intended not to change after it is defined, and that convention is expressed through naming. The language does not enforce immutability, and thus, you need to use constants wisely for more structured and understandable code.

Learn more about this