Functions are fundamental to writing clean, efficient, and reusable code in Python. They allow you to group code into logical blocks that can be called multiple times, with or without inputs, and can return outputs.
This tutorial includes:
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What functions are and why they’re useful
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Defining and calling functions
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Parameters vs arguments
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Return values
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Default, keyword, and variable-length arguments
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Scope and lifetime
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Lambda functions
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Common pitfalls and tips
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A complete example at the end
✅ What is a Function?
A function is a block of code that only runs when called. It can take inputs (parameters) and return outputs.
Benefits of Using Functions
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Avoid repetition (DRY principle)
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Improve readability and structure
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Easy debugging and testing
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Promote code reusability
Defining a Function
def greet():
print("Hello, welcome to Python!")
Calling the Function
greet()
Output:
Hello, welcome to Python!
Function with Parameters
def greet_user(name):
print(f"Hello, {name}!")
greet_user("Alice")
Output:
Hello, Alice!
Returning Values with return
def square(number):
return number * number
result = square(4)
print(result)
Output:
16
Multiple Parameters
def add(a, b):
return a + b
print(add(5, 3))
⚙️ Default Parameters
def greet(name="Guest"):
print(f"Hello, {name}!")
greet() # Hello, Guest!
greet("John") # Hello, John!
Keyword Arguments
def describe_pet(animal, name):
print(f"{name} is a {animal}.")
describe_pet(animal="dog", name="Buddy")
Variable-Length Arguments
✳️ *args – Multiple Positional Arguments
def add_all(*numbers):
return sum(numbers)
print(add_all(1, 2, 3, 4)) # 10
✳️ **kwargs – Multiple Keyword Arguments
def print_user_info(**info):
for key, value in info.items():
print(f"{key}: {value}")
print_user_info(name="Alice", age=30)
Scope: Local vs Global
x = 10 # Global variable
def show():
x = 5 # Local variable
print("Inside:", x)
show()
print("Outside:", x)
Output:
Inside: 5
Outside: 10
Lambda Functions – Anonymous One-Liners
square = lambda x: x * x
print(square(5)) # 25
Another Example
add = lambda a, b: a + b
print(add(3, 4)) # 7
Use lambdas for small, throwaway functions — not complex logic.
Complete Example: Calculator
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":
if b == 0:
return "Cannot divide by zero"
return a / b
else:
return "Unknown operation"
print(calculator(10, 5, "add")) # 15
print(calculator(10, 0, "divide")) # Cannot divide by zero
Common Pitfalls
| Mistake | Description | Fix |
|---|---|---|
| Forgetting to call function | def func(): does nothing unless called |
Use func() to call it |
| Confusing parameters vs args | Parameters are declared, arguments are passed | Understand function signatures |
| Modifying global variables | Can cause bugs | Use global keyword carefully |
No return where expected |
Function returns None |
Add a return statement |
Tips and Best Practices
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✅ Use functions to break large tasks into small steps
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✅ Use meaningful names for functions and parameters
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✅ Add docstrings for documentation
def greet(name):
"""Prints a greeting to the user."""
print(f"Hello, {name}")
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✅ Keep functions short and focused on a single task
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✅ Use type hints (Python 3.5+):
def add(a: int, b: int) -> int:
return a + b
Summary Table
| Concept | Example |
|---|---|
| Basic function | def func(): |
| With parameters | def func(x): |
| Return value | return x * x |
| Default values | def greet(name="Guest") |
| Variable args | *args, **kwargs |
| Lambda function | lambda x: x + 1 |
| Docstring | """Function doc.""" |
What’s Next?
After mastering functions, explore:
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Recursion (functions calling themselves)
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Function decorators
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First-class functions & closures
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Generators &
yield
Tips and Tricks
What is pass in Python?
Python | Pass Statement
The pass statement is used as a placeholder for future code. It represents a null operation in Python. It is generally used for the purpose of filling up empty blocks of code which may execute during runtime but has yet to be written.
def myfunction():
pass
How can you generate random numbers?
Python | Generate random numbers
Python provides a module called random using which we can generate random numbers. e.g: print(random.random())
We have to import a random module and call the random() method as shown below:
import random
print(random.random())
The random() method generates float values lying between 0 and 1 randomly.
To generate customized random numbers between specified ranges, we can use the randrange() method
Syntax: randrange(beginning, end, step)
import random
print(random.randrange(5,100,2))
What is lambda in Python?
Python | Lambda function
A lambda function is a small anonymous function. This function can have any number of parameters but, can have just one statement.
Syntex:
lambda arguments : expression
a = lambda x,y : x+y
print(a(5, 6))
It also provides a nice way to write closures. With that power, you can do things like this.
def adder(x):
return lambda y: x + y
add5 = adder(5)
add5(1) #6
As you can see from the snippet of Python, the function adder takes in an argument x and returns an anonymous function, or lambda, that takes another argument y. That anonymous function allows you to create functions from functions. This is a simple example, but it should convey the power lambdas and closures have.
What is swapcase() function in the Python?
Python | swapcase() Function
It is a string's function that converts all uppercase characters into lowercase and vice versa. It automatically ignores all the non-alphabetic characters.
string = "IT IS IN LOWERCASE."
print(string.swapcase())
How to remove whitespaces from a string in Python?
Python | strip() Function | Remove whitespaces from a string
To remove the whitespaces and trailing spaces from the string, Python provides a strip([str]) built-in function. This function returns a copy of the string after removing whitespaces if present. Otherwise returns the original string.
string = " Python "
print(string.strip())
What is the usage of enumerate() function in Python?
Python | enumerate() Function
The enumerate() function is used to iterate through the sequence and retrieve the index position and its corresponding value at the same time.
lst = ["A","B","C"]
print (list(enumerate(lst)))
#[(0, 'A'), (1, 'B'), (2, 'C')]
Can you explain the filter(), map(), and reduce() functions?
Python | filter(), map(), and reduce() Functions
- filter() function accepts two arguments, a function and an iterable, where each element of the iterable is filtered through the function to test if the item is accepted or not.
>>> set(filter(lambda x:x>4, range(7))) # {5, 6}
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map() function calls the specified function for each item of an iterable and returns a list of result
>>> set(map(lambda x:x**3, range(7))) # {0, 1, 64, 8, 216, 27, 125}
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reduce() function reduces a sequence pair-wise, repeatedly until we arrive at a single value..
>>> reduce(lambda x,y:y-x, [1,2,3,4,5]) # 3Let’s understand this:
2-1=1
3-1=2
4-2=2
5-2=3Hence, 3.
What is a namedtuple?
Python | namedtuple
A namedtuple will let us access a tuple’s elements using a name/label. We use the function namedtuple() for this, and import it from collections.
>>> from collections import namedtuple
#format
>>> result=namedtuple('result','Physics Chemistry Maths')
#declaring the tuple
>>> Chris=result(Physics=86,Chemistry=92,Maths=80)
>>> Chris.Chemistry
# 92
Write a code to add the values of same keys in two different dictionaries and return a new dictionary.
We can use the Counter method from the collections module
from collections import Counter
dict1 = {'a': 5, 'b': 3, 'c': 2}
dict2 = {'a': 2, 'b': 4, 'c': 3}
new_dict = Counter(dict1) + Counter(dict2)
print(new_dict)
# Print: Counter({'a': 7, 'b': 7, 'c': 5})
Python In-place swapping of two numbers
Python | In-place swapping of two numbers
>>> a, b = 10, 20
>>> print(a, b)
10 20
>>> a, b = b, a
>>> print(a, b)
20 10
Reversing a String in Python
Python | Reversing a String
>>> x = 'PythonWorld'
>>> print(x[: : -1])
dlroWnohtyP
Python join all items of a list to convert into a single string
Python | Join all items of a list to convert into a single string
>>> x = ["Python", "Online", "Training"]
>>> print(" ".join(x))
Python Online Training
python return multiple values from functions
Python | Return multiple values from functions
>>> def A():
return 2, 3, 4
>>> a, b, c = A()
>>> print(a, b, c)
2 3 4
Python Print String N times
Python | Print String N times
>>> s = 'Python'
>>> n = 5
>>> print(s * n)
PythonPythonPythonPythonPython
Python check the memory usage of an object
Python | Check the memory usage of an object
>>> import sys
>>> x = 100
>>> print(sys.getsizeof(x))
28
