Functions Defining Functions, Parameters, return, Lambda Functions

Python Basics: Functions

Learn to create reusable code blocks with functions – your coding superpower!

What Are Functions?

A function is like a lunchbox maker – you define it once, and it can create lunches (perform tasks) whenever you need!

Why Use Functions?

• Reuse code: Write once, use many times.
• Organize code: Break complex tasks into smaller steps.
• Simplify debugging: Fix issues in one place.

1. Defining Functions

Use def to create a function.

Syntax:

def function_name():  
    # Code to run  

            

Example: A greeting robot!

def greet():  
    print("Hello! 🌟")  
    print("How are you?")  

# Call the function  
greet()  

            

Output:

Hello! 🌟  
How are you?  

            

2. Parameters & Arguments: Custom Inputs

Parameter: Variable in the function’s definition.

Argument: Actual value passed to the function.

Example: Pizza Maker

def make_pizza(topping):  # Parameter: topping  
    print(f"🍕 with {topping}")  

make_pizza("cheese")      # Argument: "cheese"  
make_pizza("pineapple")   # Argument: "pineapple"  

            

Output:

🍕 with cheese  
🍕 with pineapple  

            

Multiple Parameters:

def calc_area(length, width):  
    area = length * width  
    print(f"Area: {area} sq units")  

calc_area(5, 4)  # Output: Area: 20 sq units  

            

3. return: Get Results Back

The return statement sends a result out of the function.

Example: Math Helper

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

total = add_numbers(3, 7)  
print(total)  # Output: 10  

            

Key Notes:

• return exits the function immediately.
• If no return, the function gives None.

Advanced Example:

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

print(is_even(4))  # Output: True  

            

4. Lambda Functions: Quick & Simple

A lambda is a tiny, one-line function without a name.

Syntax:

lambda arguments: expression  

            

Example: Doubler

double = lambda x: x * 2  
print(double(5))  # Output: 10  

            

Real-World Use:

# Sort a list of tuples by the second item  
points = [(1, 5), (3, 2), (2, 8)]  
points.sort(key=lambda point: point[1])  
print(points)  # Output: [(3, 2), (1, 5), (2, 8)]  

            

Real-World Project: Guessing Game

import random  

def generate_number():  
    return random.randint(1, 10)  

def check_guess(secret, guess):  
    if guess == secret:  
        return "🎉 Correct!"  
    elif guess < secret:  
        return "📉 Too low!"  
    else:  
        return "📈 Too high!"  

secret_num = generate_number()  
guess = int(input("Guess a number (1-10): "))  
print(check_guess(secret_num, guess))  

            

Common Mistakes

• ❌ Forgetting () when calling a function: greet vs greet().
• ❌ Missing : after def or lambda.
• ❌ Using print() instead of return to output a result.

Function Types at a Glance

Type	Syntax	Use Case
Regular	def func(): ...	Reusable code blocks.
Lambda	lambda x: x + 1	Quick, simple operations.
Built-in	print(), len(), input()	Predefined by Python.

Practice Quiz

What’s wrong with this code?

def say_hello  
    print("Hello")  

            

Answer: Missing () after say_hello.

What does this return?

multiply = lambda a, b: a * b  
print(multiply(3, 4))  

            

Answer: 12.

Fun Activity: Mad Libs Generator

def mad_libs(noun, verb):  
    return f"The {noun} loves to {verb}!"  

user_noun = input("Enter a noun: ")  
user_verb = input("Enter a verb: ")  
print(mad_libs(user_noun, user_verb))  

            

Example Output:

The cat loves to dance!  

            

Key Takeaways

• ✅ Functions are reusable code blocks defined with def.
• ✅ Parameters are inputs; arguments are actual values.
• ✅ Use return to send back results.
• ✅ Lambda functions are quick one-liners for simple tasks.

What’s Next?

Learn about file handling to read/write data from files!

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