Python Functions

🐍 Python Functions — Full Tutorial

A function in Python is a block of reusable code that performs a specific task. Functions help reduce code repetition and make programs modular.

 1. Creating a Function

Use the def keyword:

def greet():
print("Hello, Welcome!")

Calling the function:

greet()

✅ Output:

Hello, Welcome!

 2. Function with Parameters

Parameters allow you to pass data to a function:

def greet(name):
print(f"Hello, {name}!")greet(“Vipul”)

✅ Output:

Hello, Vipul!

 3. Function with Return Value

Use return to send data back:

def add(a, b):
return a + bresult = add(5, 3)
print(result) # 8

 4. Default Parameters

Provide a default value if argument is not passed:

def greet(name="Guest"):
print(f"Hello, {name}!")greet() # Hello, Guest!
greet(“Amit”) # Hello, Amit!

 5. Keyword Arguments

You can pass arguments by name, order doesn’t matter:

def info(name, age):
print(f"Name: {name}, Age: {age}")info(age=25, name=“Vipul”)

 6. Variable-length Arguments

6.1 *args → Non-keyword variable arguments

def add(*numbers):
total = 0
for num in numbers:
total += num
return totalprint(add(1, 2, 3, 4)) # 10

6.2 **kwargs → Keyword variable arguments

def info(**details):
for key, value in details.items():
print(f"{key}: {value}")info(name=“Vipul”, age=25, city=“Surat”)

 7. Lambda (Anonymous) Functions

A lambda function is a small one-line function:

square = lambda x: x ** 2
print(square(5)) # 25add = lambda a, b: a + b
print(add(10, 5)) # 15

 8. Scope of Variables

8.1 Local Variables

Defined inside a function, accessible only inside.

def func():
x = 10
print(x)func()
# print(x) # Error ❌

8.2 Global Variables

Defined outside a function, accessible anywhere.

x = 50

def func():
print(x)

func()
print(x)

8.3 Using global Keyword

Modify global variable inside a function:

x = 10

def change():
global x
x = 20

change()
print(x) # 20

 9. Nested Functions (Inner Functions)

def outer():
def inner():
print("Hello from inner function")
inner()
print("Hello from outer function")outer()

 10. Recursion (Function Calling Itself)

def factorial(n):
if n == 0:
return 1
else:
return n * factorial(n-1)print(factorial(5)) # 120

  • Important: Always define a base case to avoid infinite recursion.

 11. Docstrings (Documentation Strings)

def greet(name):
"""This function greets a person by name."""
print(f"Hello, {name}")print(greet.__doc__)

 12. Higher-Order Functions

Functions that take other functions as arguments:

def square(x):
return x ** 2def apply(func, value):
return func(value)print(apply(square, 5)) # 25

Python built-ins like map(), filter(), and reduce() also use higher-order functions.

 13. Practical Examples

13.1 Calculator Function


 

13.2 Fibonacci Series Using Function


 

 14. Summary Table

Feature Example
Function Definition def func():
Parameters def greet(name):
Default Parameters def greet(name="Guest"):
Variable Arguments *args, **kwargs
Return Value return value
Lambda Function lambda x: x**2
Docstring """This is a function"""
Scope Local vs Global

🧠 Practice Exercises

  1. Create a function to check if a number is prime.

  2. Write a function that returns factorial of a number.

  3. Create a function that counts vowels in a string.

  4. Write a function that checks if a string is a palindrome.

  5. Use *args to sum all numbers in a function call.

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