EasyExperience level: 0-2 years · 3-5 years1 min read

What are __init__ and self in a Python class?

Reviewed by Gurusankar M.

Asked inInfosysTCSAccentureDeloitte
#oop#class#init#self#instance
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Short Answer

`__init__` is the initializer that runs when you create an object — it sets up instance attributes. `self` is the reference to the current instance, passed automatically as the first parameter of every instance method, so methods can read/write that object's own data.

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Concept Made Simple

Coffee Chat Question

“What are __init__ and self in a Python class?”

Remember It Faster

Mind Map Answer

class
blueprint for objects
__init__
runs on creation, sets attributes
self
the current instance
instance vs class attr
per-object vs shared

Learn by Doing

Hands-on Keyboard

python
class Account:
    def __init__(self, owner, balance=0):
        self.owner = owner        # instance attribute
        self.balance = balance

    def deposit(self, amount):
        self.balance += amount
        return self.balance

a = Account("Guru", 100)
print(a.deposit(50))
Output
150

Think Beyond the Expected

What If?

What happens if you forget `self` as the first method parameter?

Calling the method raises a TypeError about arguments, because Python passes the instance as the first positional argument automatically — the method needs a parameter (conventionally `self`) to receive it. Without it, the counts don't match. `self` isn't a keyword, just a strong convention.

Real World

Classes model real entities — User, Order, Account — bundling their data and behavior; `__init__` + `self` are the first things every Python developer uses when structuring an app's domain objects.

Interviewer's Expectation

Keywords they're listening for:

  • __init__ initializes
  • self = current instance
  • instance attributes via self
  • auto-passed first arg
  • class vs instance attribute

Common Mistakes

  • Forgetting self in method signatures
  • Using a mutable class attribute as if it were per-instance
  • Doing heavy work in __init__

Best Practices

  • Set all instance state in __init__
  • Use self for per-object data; class attrs for shared constants
  • Add __repr__ for debuggable objects

Follow-up Questions

  1. Instance vs class attribute — where does each live?
  2. What is __str__ / __repr__ for?
  3. What are @classmethod and @staticmethod?

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: OOP (Python)
Interview question: "What are __init__ and self in a Python class?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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