if and else

if condition: runs the indented block underneath when the condition is true. else: runs a different block otherwise. The colon and the indentation are the syntax: Python uses the indentation (four spaces by convention) to know which lines belong to the block, so keep it consistent.

A function that decides something usually returns from inside the branches. Once return runs, the function is over, so the code after the if only executes when the condition was false; that is why a trailing return often needs no else.

def sign(n):
    if n < 0:
        return "negative"
    return "not negative"

print(sign(-4))

Your task

Write describe_temp(c) that returns "cold" when the temperature is below 10, "mild" when it is below 25, and "hot" otherwise.

def describe_temp(c):
    if c < 10:
        return "cold"
    if c < 25:
        return "mild"
    return "hot"