How do classes and objects work in Python?

Question

Accepted Answer

A **class** is a blueprint for creating **objects** (instances) that bundle data (attributes) and behavior (methods). Python's OOP is flexible and has some distinctive conventions, notably the explicit `self`.

## Defining a class

```python
class Dog:
    species = "Canis familiaris"     # CLASS attribute (shared by all instances)

def __init__(self, name, age):   # the constructor — runs on creation
        self.name = name              # INSTANCE attributes (per object)
        self.age = age

def bark(self):                   # a method — `self` is the instance
        return f"{self.name} says woof"

d = Dog("Rex", 3)        # create an instance (calls __init__)
d.name                   # "Rex"
d.bark()                 # "Rex says woof"
```

## The explicit self

```python
def bark(self):          # `self` is ALWAYS the first parameter of a method
    return self.name      # access instance data through self
```

Unlike many languages with an implicit `this`, Python requires **`self`** as the explicit first parameter of every instance method — it refers to the instance the method is called on. You don't pass it manually; `d.bark()` passes `d` as `self` automatically.

## Class vs instance attributes

```python
Dog.species          # "Canis familiaris" — shared by all dogs
d.species            # also accessible from an instance
d.name               # "Rex" — unique to this instance
```

Class attributes are shared across all instances; instance attributes (set via `self.x =`) are per-object.

## Method types

```python
class Circle:
    def area(self): ...                    # instance method — needs self

@classmethod
    def from_diameter(cls, d):             # class method — receives the CLASS (cls)
        return cls(d / 2)                   # often used as alternative constructors

@staticmethod
    def pi(): return 3.14159               # static — no self/cls, just namespaced
```

## Encapsulation conventions

```python
self.public                # public
self._protected            # "internal use" by convention (not enforced)
self.__private             # name-mangled to _ClassName__private (stronger hiding)
```

Python has no true private members — leading underscores are conventions signaling intent ("don't touch this"), not hard enforcement.

## Why it matters

Classes are how you model entities and organize code in object-oriented Python — bundling related data and behavior, used heavily in frameworks, domain modeling, and large applications.

Understanding the distinctive Python conventions — the explicit `self`, `__init__` constructor, class vs instance attributes, the three method types (`instance`/`@classmethod`/`@staticmethod`), and the underscore privacy conventions — is essential for writing and reading object-oriented Python.

These differ enough from other languages (explicit self, convention-based privacy) that knowing them well prevents common confusion.