Python is an object-oriented programming language, which means it allows developers to build programs using classes and objects. This concept is key to building scalable, reusable, and organized code.
In this guide, we’ll cover:
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What are classes and objects?
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How to define and use classes
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Constructors and the
__init__()method -
Instance and class variables
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Defining methods
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Inheritance basics
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Tips, common pitfalls
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A complete example at the end
What are Classes and Objects?
Class
A class is a blueprint for creating objects. It defines a type of object by bundling data (attributes) and behaviors (methods) together.
Object
An object is an instance of a class. Think of a class as a recipe, and the object as the dish made from it.
Analogy:
-
Class:
Car -
Object: My red Toyota Corolla
Defining a Simple Class
class Person:
pass
Now, let's create an object from this class:
p1 = Person()
print(p1)
Output:
<__main__.Person object at 0x...>
We’ve created an instance (p1) of the class Person.
Adding Attributes with __init__()
The __init__() method is the constructor in Python. It initializes the object when it is created.
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
Creating and using an object:
p1 = Person("Alice", 30)
print(p1.name) # Alice
print(p1.age) # 30
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selfrefers to the instance being created. -
self.nameandself.ageare instance variables.
Defining Methods
You can add functions (called methods) to a class:
class Person:
def __init__(self, name):
self.name = name
def greet(self):
print(f"Hi, I am {self.name}")
Usage:
p = Person("Bob")
p.greet() # Hi, I am Bob
Class Variables vs Instance Variables
Instance Variable:
Defined inside __init__(). Unique to each object.
Class Variable:
Shared among all instances of a class.
class Dog:
species = "Canine" # class variable
def __init__(self, name):
self.name = name # instance variable
d1 = Dog("Max")
d2 = Dog("Buddy")
print(d1.species) # Canine
print(d2.name) # Buddy
Modifying Object State
class Counter:
def __init__(self):
self.count = 0
def increment(self):
self.count += 1
c = Counter()
c.increment()
print(c.count) # 1
Inheritance – Reusing Code
You can create a new class that inherits from another class.
class Animal:
def speak(self):
print("I make a sound")
class Dog(Animal):
def speak(self):
print("Bark!")
d = Dog()
d.speak() # Bark!
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Doginherits fromAnimal. -
You can override or extend base class methods.
Special Methods
Special methods (also known as dunder methods) start and end with double underscores.
__str__() – Object String Representation
class Person:
def __init__(self, name):
self.name = name
def __str__(self):
return f"Person: {self.name}"
p = Person("Eve")
print(p) # Person: Eve
Other common special methods:
-
__len__() -
__eq__() -
__add__()
✅ Complete Example – Bank Account
class BankAccount:
def __init__(self, owner, balance=0):
self.owner = owner
self.balance = balance
def deposit(self, amount):
self.balance += amount
print(f"Deposited ${amount}")
def withdraw(self, amount):
if amount > self.balance:
print("Insufficient funds")
else:
self.balance -= amount
print(f"Withdrew ${amount}")
def __str__(self):
return f"{self.owner}'s balance: ${self.balance}"
Usage:
acc = BankAccount("Alice", 100)
print(acc)
acc.deposit(50)
acc.withdraw(30)
print(acc)
Output:
Alice's balance: $100
Deposited $50
Withdrew $30
Alice's balance: $120
⚠️ Common Pitfalls
| Pitfall | Description | Solution |
|---|---|---|
Forgetting self in methods |
Raises TypeError |
Always include self in instance methods |
| Confusing class/instance vars | Shared class variables when not expected | Define unique data in __init__() |
Reassigning self |
Breaks the object | Never do self = something |
Tips and Best Practices
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✅ Use classes to model real-world objects.
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✅ Keep methods short and focused.
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✅ Use
__str__()for readable print output. -
✅ Prefer composition over inheritance unless necessary.
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✅ Use access control conventions (
_protected,__private) to indicate intent.
What’s Next?
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Inheritance and polymorphism
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Encapsulation with private/protected members
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Static methods and class methods
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Operator overloading
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Advanced OOP design patterns
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
