Session 5 · Functions#

📖 Based on the blog posts Python Functions and Tasks: Python Function.

The big idea#

A function is a recipe: write the steps once, give them a name, and use them again whenever you need them, with different ingredients each time.

Everyday example: you don’t reinvent dosa every morning. You follow the same recipe; only the batter amount changes. Functions give you:

Reuse: write once, use many times.
Organisation: like sections in a recipe book.
No repetition: one “chop vegetables” recipe, used by many dishes.
Smaller problems: a big dinner becomes chutney + tiffin + sweet.

Before and after#

Without a function, the same formula appears three times:

celsius1 = 25
print(f"{celsius1}°C is {(celsius1 * 9/5) + 32}°F")
celsius2 = 30
print(f"{celsius2}°C is {(celsius2 * 9/5) + 32}°F")

25°C is 77.0°F
30°C is 86.0°F

With a function, the formula lives in one place:

def celsius_to_fahrenheit(celsius):
    return (celsius * 9/5) + 32

for c in [25, 30, 15]:
    print(f"{c}°C is {celsius_to_fahrenheit(c)}°F")

25°C is 77.0°F
30°C is 86.0°F
15°C is 59.0°F

If the formula ever has a bug, you fix it once.

Anatomy of a function#

def gives the function a name and parameters. Calling it passes arguments in; return sends a value back to the caller.

Part	Meaning
`def`	“I’m defining a function”
`celsius_to_fahrenheit`	its name (same rules as variables)
`(celsius)`	parameters: the inputs it expects
indented body	the steps; runs only when called
`return`	sends a value back and ends the function
`celsius_to_fahrenheit(25)`	a call; `25` is the argument

print vs return#

def greet(name):
    print(f"Hello, {name}!")       # shows text, returns None

def add(a, b):
    return a + b                   # gives a value back

greet("Alice")
result = add(5, 3)
print(f"The sum is: {result}")
print(greet("Bob"))

Hello, Alice!
The sum is: 8
Hello, Bob!
None

print is for people to read. return is for the program to keep using the value. A function without return gives back None.

More examples#

import math

def is_even(number):
    return number % 2 == 0

def max_of_three(a, b, c):
    largest = a if a > b else b
    return largest if largest > c else c

def factorial(n):
    if n == 0:
        return 1
    return n * factorial(n - 1)      # a function can call itself (recursion)

def area_of_circle(radius):
    return math.pi * radius ** 2

print(is_even(4), is_even(7))
print(max_of_three(3, 9, 5))
print(factorial(5))
print(round(area_of_circle(5), 2))

True False
9
120
78.54

Default values and keyword arguments#

A parameter can have a default, used when the caller leaves it out:

def power(number, exponent=2):
    return number ** exponent

print(power(5))               # exponent defaults to 2
print(power(2, 10))
print(power(exponent=3, number=2))   # keyword arguments: any order

25
1024
8

Common mistakes#

Forgetting the brackets: greet (no ()) refers to the function, it doesn’t call it.
Printing instead of returning: total = add_and_print(2, 3) is None if the function only prints.
Code after return never runs.
Calling before defining: Python runs top to bottom, so def must come before the first call.
Naming a variable max: it hides the built-in max() function (the blog’s version did this; this page uses largest).

Hands-on exercises#

The seven tasks from Tasks: Python Function.

Task 1. Write greet(name) that prints a greeting.

Solution

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

greet("Priya")

Task 2. Write sum_two(a, b) that returns the sum.

Solution

def sum_two(a, b):
    return a + b

print(sum_two(10, 20))     # 30

Task 3. Write is_even(n) that returns True for even numbers and False for odd ones.

Solution

def is_even(n):
    return n % 2 == 0

print(is_even(10), is_even(7))     # True False

n % 2 == 0 is already True or False, so there’s no need for if … return True else return False.

Task 4. Write find_max(a, b) that returns the larger number.

Solution

def find_max(a, b):
    if a > b:
        return a
    return b

print(find_max(4, 9))     # 9

Task 5. Write multiplication_table(n) that prints the table of n from 1 to 10.

Solution

def multiplication_table(n):
    for i in range(1, 11):
        print(f"{n} x {i} = {n * i}")

multiplication_table(7)

Task 6. Write celsius_to_fahrenheit(c) that returns the temperature in Fahrenheit.

Solution

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

print(celsius_to_fahrenheit(37))    # 98.6

Task 7. Write power(number, exponent) where exponent defaults to 2.

Solution

def power(number, exponent=2):
    return number ** exponent

print(power(9))       # 81
print(power(2, 5))    # 32

Bonus · Bill splitter. Write split_bill(total, people, tip_percent=10) that returns how much each person pays, rounded to 2 decimals.

Solution

def split_bill(total, people, tip_percent=10):
    with_tip = total * (1 + tip_percent / 100)
    return round(with_tip / people, 2)

print(split_bill(1200, 4))         # 330.0
print(split_bill(1200, 4, 0))      # 300.0