Python Functions

Posted On February 1, 2023

In Python, a function is a block of code that can be reused multiple times in a program. Functions are defined using the def keyword, followed by the function name and a set of parentheses. Any input values, or arguments, are placed within the parentheses. The block of code that makes up the function is indented underneath the function definition.

For example:

def say_hello(name):
    print("Hello, " + name + "!")

say_hello("John") 
# Output: Hello, John!

Test Case
def print_args(*args):
    for arg in args:
        print(arg)

print_args("Hello", "world!", 1, [1,2,3])
# Output:
# Hello
# world!
# 1
# [1, 2, 3]

Functions can also return a value using the return keyword.

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

result = add_numbers(1, 2)
print(result) #Output: 3

Test cases:

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

# Test Case 1
# Check that the function returns the expected output for basic input
def test_add_numbers():
    expected_output = 3
    result = add_numbers(1, 2)
    assert result == expected_output
    
# Test Case 2
# Check that the function returns the expected output for negative input
def test_add_numbers_negative():
    expected_output = -3
    result = add_numbers(-1, -2)
    assert result == expected_output

Functions can also have default values for arguments, and can be called with keyword arguments, which allow the caller to specify the argument by name

def repeat_message(message, times=2):
    for i in range(times):
        print(message)

repeat_message("Hello", times=3)
# Output:
# Hello
# Hello
# Hello

Test case:

def repeat_message(message, times=2):
    for i in range(times):
        print(message)

# Test Case 1
# Check that the function prints the expected number of messages
def test_repeat_message():
    expected_output = "Hello\nHello"
    repeat_message("Hello")
    # check that function output matches expected output
    assert actual_output == expected_output
    
# Test Case 2
# Check that the function prints the expected number of messages when passing the times argument
def test_repeat_message_times_arg():
    expected_output = "Hello\nHello\nHello"
    repeat_message("Hello", times=3)
    # check that function output matches expected output
    assert actual_output == expected_output

Functions can also be called with a varying number of arguments using the * and ** syntaxes.

def print_args(*args):
    for arg in args:
        print(arg)

print_args("Hello", "world!", 1, [1,2,3])
# Output:
# Hello
# world!
# 1
# [1, 2, 3]

Test case:

def print_args(*args):
    for arg in args:
        print(arg)
        
# Test Case 1
# Check that the function prints all the arguments
def test_print_args():
    expected_output = "Hello\nworld!\n1\n[1, 2, 3]"
    print_args("Hello", "world!", 1, [1,2,3])
    # check that function output matches expected output
    assert actual_output == expected_output
    
# Test Case 2
# Check that the function prints no arguments when no arguments are passed
def test_print_args_none():
    expected_output = ""
    print_args()
    # check that function output matches expected output
    assert actual_output == expected_output