Functions

A function is a named block of code that you can run as many times as you like. Functions let you break a program into pieces that each do one thing, give those pieces names, and reuse them.

Defining and calling

def greet():
    print("Hello from a function")

greet()
greet()

def starts the definition, the name is followed by parentheses and a colon, and the body is indented. Defining a function does not run it; calling it with () does.

Parameters

Values passed into a function are called arguments; the names that receive them are parameters.

def greet(name):
    print(f"Hello, {name}")

greet("Sana")
greet("Vikram")

Multiple parameters are separated by commas:

def area(width, height):
    print(width * height)

area(4, 5)

Return values

return sends a value back to the caller and ends the function.

def area(width, height):
    return width * height

room = area(4, 5)
print(room)
print(area(2, 3) + area(1, 1))

A function without a return, or with a bare return, gives back None.

def nothing():
    pass

print(nothing())

Returning several values

Separate them with commas; the caller receives a tuple and can unpack it.

def stats(values):
    return min(values), max(values), sum(values) / len(values)

low, high, mean = stats([4, 8, 15, 16])
print(low, high, mean)

Docstrings

Describe what the function does in a string right after the def line.

def celsius_to_f(c):
    """Convert a temperature from Celsius to Fahrenheit."""
    return c * 9 / 5 + 32

print(celsius_to_f(37))
print(celsius_to_f.__doc__)

Functions calling functions

def square(n):
    return n * n

def sum_of_squares(a, b):
    return square(a) + square(b)

print(sum_of_squares(3, 4))

Functions are values

A function can be stored in a variable, put in a list or passed to another function.

def shout(text):
    return text.upper() + "!"

def whisper(text):
    return text.lower() + "..."

for style in (shout, whisper):
    print(style("Hello"))

Why write functions

• Avoid repetition. Fix a bug once instead of in five copies.
• Name your logic. is_valid_email(x) says more than ten lines of checks.
• Test in isolation. A small function is easy to try in the shell.
• Limit scope. Variables inside a function do not leak out (see the Scope lesson).

Practice

1. Write is_even(n) that returns True or False, and use it in a loop from 1 to 10.
2. Write grade(marks) that returns "A", "B", "C" or "F" and call it with a few values.
3. Write word_count(sentence) that returns the number of words, then use it on two different sentences.