What is the difference between a class and an object?

Question

Accepted Answer

A **class** is a *blueprint* or template that defines structure (fields) and behavior (methods). An **object** (or *instance*) is a concrete thing built from that blueprint, with its own values in memory.

## Blueprint vs. instance

```text
class Car          ← ONE definition (the blueprint)
   │  fields: color, speed
   │  methods: accelerate()
   ▼
new Car("red")     ← many OBJECTS (instances), each with its own state
new Car("blue")
```

## In code

```python
class Car:
    def __init__(self, color):
        self.color = color      # per-instance state
        self.speed = 0
    def accelerate(self):
        self.speed += 10

a = Car("red")                  # object #1
b = Car("blue")                 # object #2 — independent state
a.accelerate()
print(a.speed, b.speed)         # 10 0  → separate instances
print(a.color is b.color)       # False → different objects
```

The class exists once; you can create many objects from it, each holding independent data.

## Common confusion

- A class itself uses no per-instance memory until you instantiate it.
- "Class variables" (shared across instances) differ from "instance variables" (per object) — mixing them up causes shared-state bugs.

## Why it matters

This is the most fundamental distinction in OOP: every other concept builds on knowing that one class produces many independent objects.

Getting it wrong leads to accidental shared state — e.g. a mutable default or class-level list that every instance unexpectedly shares.