Python Functions Tutorial - def, args, kwargs, return Examples

# Define a function
def greet():
    print("Hello, World!")

# Call the function
greet()   # Hello, World!

# Function with parameters
def greet_user(name):
    print(f"Hello, {name}!")

greet_user("Alice")   # Hello, Alice!
greet_user("Bob")     # Hello, Bob!

# Function with return value
def add(a, b):
    return a + b

result = add(3, 5)
print(result)   # 8

# Default parameters
def greet(name, greeting="Hello"):
    print(f"{greeting}, {name}!")

greet("Alice")              # Hello, Alice!
greet("Bob", "Hi")          # Hi, Bob!
greet("Charlie", greeting="Hey")  # Hey, Charlie!

# Keyword arguments - order doesn't matter
def describe(name, age, city):
    print(f"{name}, {age}, from {city}")

describe(age=25, city="London", name="Alice")

# *args - variable number of positional arguments
def total(*numbers):
    return sum(numbers)

print(total(1, 2, 3))        # 6
print(total(10, 20, 30, 40)) # 100

# **kwargs - variable number of keyword arguments
def show_info(**details):
    for key, value in details.items():
        print(f"{key}: {value}")

show_info(name="Alice", age=25, city="London")

# Combining all parameter types
def full_example(pos, /, normal, *, kw_only, **kwargs):
    print(pos, normal, kw_only, kwargs)

# Return multiple values (as a tuple)
def min_max(numbers):
    return min(numbers), max(numbers)

low, high = min_max([3, 1, 4, 1, 5, 9])
print(low, high)   # 1 9

# Early return
def is_even(n):
    if n % 2 == 0:
        return True
    return False

# Functions without return return None
def say_hi():
    print("Hi!")

result = say_hi()
print(result)   # None

# Return a function (higher-order function)
def multiplier(factor):
    def multiply(x):
        return x * factor
    return multiply

double = multiplier(2)
triple = multiplier(3)
print(double(5))   # 10
print(triple(5))   # 15

def add(a: int, b: int) -> int:
    return a + b

def greet(name: str, times: int = 1) -> str:
    return (f"Hello, {name}! " * times).strip()

def process(items: list[int]) -> dict[str, int]:
    return {"sum": sum(items), "count": len(items)}

# Optional type (can be None)
from typing import Optional

def find_user(user_id: int) -> Optional[str]:
    users = {1: "Alice", 2: "Bob"}
    return users.get(user_id)   # returns str or None

print(find_user(1))   # Alice
print(find_user(99))  # None

def calculate_area(radius: float) -> float:
    """
    Calculate the area of a circle.

    Args:
        radius (float): The radius of the circle.

    Returns:
        float: The area of the circle.

    Raises:
        ValueError: If radius is negative.

    Example:
        >>> calculate_area(5)
        78.53981633974483
    """
    import math
    if radius < 0:
        raise ValueError("Radius cannot be negative")
    return math.pi * radius ** 2

# Access the docstring
print(calculate_area.__doc__)
help(calculate_area)

# Factorial: n! = n * (n-1) * ... * 1
def factorial(n: int) -> int:
    if n <= 1:       # base case
        return 1
    return n * factorial(n - 1)  # recursive call

print(factorial(5))   # 120
print(factorial(10))  # 3628800

# Fibonacci sequence
def fibonacci(n: int) -> int:
    if n <= 1:
        return n
    return fibonacci(n - 1) + fibonacci(n - 2)

print([fibonacci(i) for i in range(10)])
# [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]