Defining and calling functions

Why functions?

You have already used functions that Python gives you: print(), len() and sum(). A function is a named block of code that does one job. You can also make your own. Then you write the steps once and use them again and again just by calling the name.

Defining a function

Defining a function means writing it. Start with the word def, then the function name, round brackets and a colon. The lines below it, indented by 4 spaces, are the body: the steps the function runs. Defining a function does not run it. Calling a function means running it: you write its name with brackets.

def greet():
    print("Namaste!")
    print("Welcome to the class.")

greet()
greet()

Inputs: parameters

def greet(name):
    print("Namaste,", name)

greet("Asha")
greet("Ravi")

return: giving a value back

return ends the function and hands a value back to the code that called it. You can store that value in a variable, print it, or use it in a calculation.

def with_gst(price):
    return price + price * 18 / 100

bill = with_gst(200) + with_gst(100)
print(bill)

return versus print

def show_total(a, b):
    print(a + b)

result = show_total(2, 3)
print(result)

return also stops the function at once. Any lines after a return that runs are skipped.

def result(marks):
    if marks >= 40:
        return "Pass"
    return "Fail"

print(result(75))
print(result(30))

Docstrings: explaining your function

def area(length, width):
    """Return the area of a rectangle in square metres."""
    return length * width

print(area(5, 4))
print(area.__doc__)