What is a metaclass? | Interview Question

Q: What is a metaclass?

**Target Audience:** Python developers who are comfortable with classes and want to know what sits one level above them **Difficulty:** Hard **Python version:** Checked on CPython 3.11, 3.12 and 3.13 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); the hook order is identical on all of them > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. Every output block in the **Verified** section was printed by actually running the examples, not typed from memory. ## 1. Why This Even Matters — A Story First Picture a toy factory. A **mould** stamps out toy robots: every robot that comes out of it has the same shape. That mould is a *class*, and each robot is an *instance*. But somebody had to make the mould. Moulds come from a **mould-making machine**. If you want every mould in the factory to carry a serial number, or to be rejected when it is missing a part,

The problem

Question presented to candidate: "In Python everything is an object, including classes. So what creates a class? Explain what a metaclass is, walk me through what happens when Python runs a class statement, and tell me when you would actually write one, and when you would not."

What a strong answer should cover:

A metaclass is the class of a class: classes are instances of type by default, so type(int) and type(MyClass) are both type, and type(type) is type.
The class statement is sugar for metaclass(name, bases, namespace): Python resolves the metaclass, calls __prepare__ to get the namespace, runs the class body in it, then calls the metaclass __new__ and __init__.
Instance creation goes through the metaclass __call__ (type.__call__ calls __new__ then __init__), which is why a metaclass can implement singletons or caching.
Real uses: registries, validation at definition time, ORMs and enums (ABCMeta, EnumType in the standard library); metaclasses are inherited, and unrelated metaclasses in multiple bases raise a "metaclass conflict" TypeError.
The modern advice: prefer class decorators or __init_subclass__ / __set_name__ (PEP 487, Python 3.6) for most of those jobs; reach for a metaclass only when you must control namespace creation or instantiation.

Code / implementation expected: Optional — a tiny logging metaclass that shows the hook order, or a registry written both with a metaclass and with __init_subclass__.

metaclass

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Solution

Target Audience: Python developers who are comfortable with classes and want to know what sits one level above them Difficulty: Hard Python version: Checked on CPython 3.11, 3.12 and 3.13 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); the hook order is identical on all of them

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. Every output block in the Verified section was printed by actually running the examples, not typed from memory.

1. Why This Even Matters — A Story First

Picture a toy factory. A mould stamps out toy robots: every robot that comes out of it has the same shape. That mould is a class, and each robot is an instance.

But somebody had to make the mould. Moulds come from a mould-making machine. If you wa

The solution is hidden so you can try first

14 min read · 12 sections

Run the code

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

PythonThe hook order: type(), __prepare__, __new__, __init__ and __call__

Practice

PythonRegistry, validation, __prepare__, singleton and the metaclass conflict

Practice

Discussion

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