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In Python, a function is like a “recipe” that tells the computer what steps to perform when you “call” it. Functions allow us to:

  • Group related lines of code together.
  • Reuse code easily.
  • Make programs organized and easier to read.

Creating a Function

To create a function in Python, you use the def keyword. Here’s the basic structure:

def function_name():
    # Code inside the function
    print("Hello from the function!")

Explanation:

  • def tells Python that you’re defining a function.
  • function_name is the name of your function (you can pick any name, but no spaces!).
  • () follows the name and can hold “inputs” (explained later).
  • Inside the function, you write the steps (or instructions) for the computer.

Example 1: A Simple Function

Let’s write a simple function to print “Welcome to Python!”.

def greet():
    print("Welcome to Python!")

How to Use the Function

You need to call the function to make it run:

greet()

Output:

Welcome to Python!

Functions with Parameters (Inputs)

Parameters let you give extra information to a function. Think of it like adding ingredients to a recipe.

Example 2: A Function with One Parameter

Let’s make a function that greets a person by name.

def greet_person(name):
    print(f"Hello, {name}! How are you?")

How to Call the Function:

You provide the name when calling the function:

greet_person("Alice")
greet_person("Bob")

Output:

Hello, Alice! How are you?
Hello, Bob! How are you?

Explanation:

  • name is a parameter (a placeholder for input).
  • f"..." is used to insert the value of name into the string.

Functions with Multiple Parameters

You can add more parameters if needed.

Example 3: A Function with Two Parameters

Let’s create a function to calculate the total price after adding tax.

def calculate_price(price, tax_rate):
    total = price + (price * tax_rate)
    print(f"The total price is {total:.2f}")

How to Call the Function:

calculate_price(100, 0.15)
calculate_price(200, 0.20)

Output:

The total price is 115.00
The total price is 240.00

Explanation:

  • price and tax_rate are inputs.
  • The function calculates the total price and shows it formatted to 2 decimal places (:.2f).

Returning a Value from a Function

Sometimes, you don’t want to just print() the result but want to return it for use later.

Example 4: A Function that Returns a Value

Let’s return the total price instead of printing it.

def calculate_price(price, tax_rate):
    total = price + (price * tax_rate)
    return total

How to Use the Returned Value:

result = calculate_price(100, 0.15)
print(f"The total price is {result:.2f}")

Output:

The total price is 115.00

Default Parameters

You can set default values for parameters. If no value is provided, the function uses the default.

Example 5: A Function with Default Parameters

Let’s assume the default tax rate is 10%.

def calculate_price(price, tax_rate=0.10):
    total = price + (price * tax_rate)
    return total

How to Call the Function:

print(calculate_price(100))        # Uses default tax rate of 10%
print(calculate_price(100, 0.20)) # Uses provided tax rate of 20%

Output:

110.0
120.0

Functions with Loops

Functions can have loops to repeat actions.

Example 6: A Function to Print a Message Multiple Times

def repeat_message(message, times):
    for _ in range(times):
        print(message)

How to Call the Function:

repeat_message("Hello!", 3)

Output:

Hello!
Hello!
Hello!

Combining Functions

Functions can call other functions!

Example 7: Combining Two Functions

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

def farewell(name):
    return f"Goodbye, {name}!"

def conversation(name):
    print(greet(name))
    print("It's nice to meet you.")
    print(farewell(name))

How to Call the Combined Function:

conversation("Alice")

Output:

Hello, Alice!
It's nice to meet you.
Goodbye, Alice!

Practical Example: A Simple Calculator

Let’s create a calculator that can add, subtract, multiply, or divide two numbers.

def calculator(a, b, operation):
    if operation == "add":
        return a + b
    elif operation == "subtract":
        return a - b
    elif operation == "multiply":
        return a * b
    elif operation == "divide":
        return a / b
    else:
        return "Invalid operation"

How to Use the Calculator:

print(calculator(10, 5, "add"))
print(calculator(10, 5, "subtract"))
print(calculator(10, 5, "multiply"))
print(calculator(10, 5, "divide"))
print(calculator(10, 5, "unknown"))

Output:

15
5
50
2.0
Invalid operation
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