What is the difference between class variables and instance variables?

Q: What is the difference between class variables and instance variables?

**Target Audience:** Beginners who have written a class with `__init__`, up to intermediate developers revising Python's object model for interviews. **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11 and 3.13 (identical output); every example also runs in the page on Pyodide 0.25.1 (CPython 3.11.3). > **How to read this doc:** Each idea is explained in plain language first. When a word appears that an interviewer expects you to know, it is tagged with `📌 Interview term:` and defined right there. All of them are collected in the Quick Glossary at the end. Every output shown was produced by actually running the code. ## 1. Why This Even Matters — A Story First In a school classroom there is one whiteboard at the front, and every pupil has their own notebook. When the teacher writes "Homework: page 42" on the whiteboard, every pupil sees the same message — change it once and everyone sees the change. What a pupil writes in their own notebook only exists in t

The problem

Question presented to candidate: "In a Python class you can put a variable directly in the class body or set it on self inside __init__. What is the difference between class variables and instance variables, how does Python decide which one you get when you read obj.x, and what bug do people hit with them?"

What a strong answer should cover:

A class variable is defined in the class body and stored once in the class __dict__, shared by every instance; an instance variable is set on self and stored in that object's own __dict__.
Lookup order for obj.x: data descriptors on the type, then the instance __dict__, then the class and its bases along the MRO; so an instance variable shadows a class variable of the same name.
Assignment obj.x = ... always writes to the instance (it creates a shadow); to change the shared value, assign through the class (Cls.x = ... or type(self).x = ...).
The classic bug: a mutable class variable (members = []) mutated through self is shared by all instances; create it in __init__ instead.
self.count += 1 on a class variable reads the class value and writes a new instance variable; subclasses inherit class variables until they assign their own.
Typical uses: constants, defaults, counters and caches shared by all instances; typing.ClassVar marks them for type checkers and dataclasses.

Clarifying questions expected:

Should the value be shared by all instances or be per object?
Will subclasses need their own copy of the shared state?

Code / implementation expected: Yes — a class with one of each, showing __dict__, shadowing, and the shared-list bug with its fix.

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class-vars

Try it yourself

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A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners who have written a class with __init__, up to intermediate developers revising Python's object model for interviews. Difficulty: Medium Python version: Behaviour checked on CPython 3.11 and 3.13 (identical output); every example also runs in the page on Pyodide 0.25.1 (CPython 3.11.3).

How to read this doc: Each idea is explained in plain language first. When a word appears that an interviewer expects you to know, it is tagged with 📌 Interview term: and defined right there. All of them are collected in the Quick Glossary at the end. Every output shown was produced by actually running the code.

1. Why This Even Matters — A Story First

In a school classroom there is one whiteboard at the front, and every pupil has their own notebook. When the teacher writes "Homework: page 42" on the whiteboard, every pupil sees the same message — cha

The solution is hidden so you can try first

10 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonShared vs per-object state, __dict__, shadowing and the shared list bug

Practice

Pythonself.x += 1, inheritance, type(self) counters and dataclass ClassVar

Practice

Discussion

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