Pure and Impure Functions

Pure Functions​

A Pure Function is predictable: given the same input, it always returns the same output and has no side effects (it doesn't change anything in the outside world).

# Pure: returns a result without changing anything outside
def pure_chai(cups):
    return cups

Impure Functions​

An Impure Function, however, modifies external state or relies on global variables. These are generally harder to test and maintain because their results can change depending on the state of the program.

total_chai = 10

# Impure: modifies a global variable
def impure_chai(cups: int) -> None:
    global total_chai
    total_chai += cups # Side effect: changing global state