Functions in Python

# Functions in Python

Welcome to Chapter 13! Functions are the building blocks of organized, reusable code. Instead of repeating the same code, you write it once as a function and call it whenever needed.

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1. Learning Objectives #

• Define and call functions.

• Use parameters, arguments, and return values.

• Understand variable scope (local vs global).

• Use default, *args, and **kwargs parameters.

• Write recursive functions.

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2. Defining and Calling Functions #

```python id="py13_ex1" # Defining a function def greet(name): """Greet a user by name.""" print(f"Hello, {name}! Welcome! 👋")

# Calling the function greet("Alice") # Hello, Alice! Welcome! 👋 greet("Bob") # Hello, Bob! Welcome! 👋

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## 3. Parameters and Return Values

python id="py13ex2" def add(a, b): """Return the sum of two numbers.""" return a + b

# Multiple return values def getstats(numbers): return min(numbers), max(numbers), sum(numbers) / len(numbers)

low, high, avg = get_stats([10, 20, 30, 40, 50]) print(f"Min: {low}, Max: {high}, Avg: {avg}")

### Default Parameters

python id="py13_ex3" def power(base, exponent=2): return base ** exponent

print(power(5)) # 25 (default exponent=2) print(power(5, 3)) # 125 print(power(2, 10)) # 1024

### Keyword Arguments

python id="py13ex4" def createprofile(name, age, city="Unknown"): print(f"Name: {name}, Age: {age}, City: {city}")

createprofile("Alice", 25, "NYC") createprofile(age=30, name="Bob") # Keyword args (any order)

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## 4. *args and **kwargs

python id="py13ex5" # *args — accept any number of positional arguments def total(*args): print(f"Args: {args}") # Tuple return sum(args)

# kwargs — accept any number of keyword arguments def printinfo(kwargs): for key, value in kwargs.items(): print(f" {key}: {value}")

print_info(name="Alice", age=25, city="NYC")

# Combined def func(a, b, *args, **kwargs): print(f"a={a}, b={b}, args={args}, kwargs={kwargs}")

func(1, 2, 3, 4, x=5, y=6)

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## 5. Variable Scope

python id="py13_ex6" x = "global" # Global variable

def func(): x = "local" # Local variable print(f"Inside: {x}") # local

func() print(f"Outside: {x}") # global

# Using global keyword counter = 0

def increment(): global counter counter += 1

increment() increment() print(counter) # 2

Scope Diagram: ┌─────────────────────────┐ │ Global Scope │ │ x = "global" │ │ ┌───────────────────┐ │ │ │ Local Scope │ │ │ │ x = "local" │ │ │ │ (shadows global) │ │ │ └───────────────────┘ │ └─────────────────────────┘

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## 6. Recursion

python id="py13_ex7" # Factorial using recursion def factorial(n): if n <= 1: # Base case return 1 return n * factorial(n - 1) # Recursive case

print(factorial(5)) # 120 = 5*4*3*2*1

# Fibonacci def fib(n): if n <= 1: return n return fib(n-1) + fib(n-2)

for i in range(10): print(fib(i), end=" ") # 0 1 1 2 3 5 8 13 21 34

Factorial(4) Call Stack: factorial(4) → 4 * factorial(3) → 3 * factorial(2) → 2 * factorial(1) → 1 (base case) → 2 * 1 = 2 → 3 * 2 = 6 → 4 * 6 = 24

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## 7. Type Hints (Python 3.5+)

python id="py13ex8" def greet(name: str) -> str: return f"Hello, {name}!"

def getinfo(name: str, age: int = 0) -> dict: return {"name": name, "age": age}

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## 8. Practical Examples

### Password Validator

python id="py13ex9" def validatepassword(password: str) -> tuple: errors = [] if len(password) < 8: errors.append("Must be 8+ characters") if not any(c.isupper() for c in password): errors.append("Need uppercase letter") if not any(c.isdigit() for c in password): errors.append("Need a digit") isvalid = len(errors) == 0 return isvalid, errors

valid, errs = validatepassword("hello") print(f"Valid: {valid}, Errors: {errs}")

valid, errs = validatepassword("Hello123!") print(f"Valid: {valid}, Errors: {errs}") ``

1. 1. Mutable default arguments: def f(lst=[]) — use None instead.

1. 2. Forgetting return: Functions return None by default.

1. 3. Modifying global variables without global: Creates local variable instead.

Q2: *args collects arguments into a: A) List B) Tuple C) Dict D) Set Answer: B

Q3: kwargs collects arguments into a: A) List B) Tuple C) Dict D) Set Answer: C

Q8: def f(a, b=5, c) is: A) Valid B) SyntaxError Answer: B — Default params must come after non-default.

Q9: What does global keyword do? A) Creates variable B) Accesses global scope variable C) Deletes variable D) Imports module Answer: B

Q10: factorial(0) should return: A) 0 B) 1 C) Error D) None Answer: B — 0! = 1 by mathematical definition.

1. 5. pass vs return None? pass is a no-op placeholder; return None explicitly exits and returns None.

• *args captures positional args as tuple; kwargs captures keyword args as dict.

In Chapter 14: Lambda Functions and Higher-Order Functions, you'll learn anonymous functions, map(), filter(), and reduce()`! 🚀