return, Precisely

return is not print

Mixing up return and print is the most common design bug in beginner functions, and the difference has teeth: a returned value can be stored, compared, tested, and fed into the next function, while a printed one is gone the moment it hits the screen. Getting return exactly right is what makes functions combine into bigger programs.

print shows a value to a human and gives nothing back. return hands a value to the calling code, silently. A function that computes should return, so callers can keep working with the result: store it, compare it, pass it onward.

Two more rules with teeth:

1. return ends the function instantly. Any code after the executed return never runs.
2. No return means None. Fall off the end of a function and the caller receives None.

Rule 1 enables the tidy early return style, no else needed:

def classify(n):
    if n < 0:
        return "negative"
    if n == 0:
        return "zero"
    return "positive"

Reaching the last line already proves both ifs failed, the same top-to-bottom logic as your elif chains in lesson 4-1.