Functions

Why Functions?

• Break big problems into smaller steps
• Reduce repetition (DRY)
• Improve readability and testing

Defining a Function

• def introduces a function definition
• Parameters are inputs; body runs when called
• Use return to send a value back

def add(a, b):
    total = a + b
    return total

x = add(3, 4)
print(x)  # 7

return vs print

• return gives data where the function was called
• print shows text on screen for humans
• Rule of thumb: compute with returns, communicate with prints

def add(a, b):
    total = a + b
    return total
                
x = add(3, 4)  # Nothing is printed; the return value is stored in x
print(x)       # 7 is printed

Scope: Where Names Live

• Names inside a function are local to that function
• Reading a global variable is allowed, but assigning creates a local variable
• Prefer passing values in and returning values out

x = 10  # global variable (can be accessed anywhere)

def bump(x):
    x = x + 1  # local variable (only exists inside the function)
    return x

print(bump(10))  # 11 (the local variable is returned)
print(x)         # 10 (the global variable is unchanged)