What are bound and unbound methods in Python? | Interview Question

Q: What are bound and unbound methods in Python?

**Target Audience:** Developers who can write a class and want to understand what `self` really is — beginners welcome, intermediate developers will find the CPython detail. **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11 and 3.13 (identical output; bytecode shown for 3.11 and 3.12); 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 A coffee shop has one recipe card for "make a latte". The card itself is general: it says "take *the customer's* cup, add espresso, add milk". On its own the card cannot do anything, because it does not know which customer. When Ann orders, th

The problem

Question presented to candidate: "If I write obj.method without calling it, what do I actually get back? What is a bound method, what did 'unbound method' mean, and how does self get passed in?"

What a strong answer should cover:

A bound method is an object that pairs a function with an instance: obj.method gives a method object with __self__ (the instance) and __func__ (the function); calling it calls __func__(__self__, *args).
In Python 3, Class.method is just the plain function; the separate "unbound method" type of Python 2 (which type-checked its first argument) was removed in 3.0.
Binding is done by the descriptor protocol: functions define __get__, so attribute lookup through an instance calls function.__get__(obj, type(obj)).
Every access creates a new bound method object (obj.m is obj.m is False, but == is True); CPython avoids that allocation for direct calls with LOAD_METHOD (3.11) / LOAD_ATTR method form (3.12+).
Bound methods are first-class callables (callbacks, map), and they hold a strong reference to the instance; weakref.WeakMethod avoids keeping it alive.
Functions stored on an instance are not bound; types.MethodType binds one by hand.

Code / implementation expected: Optional — show type(C.f) vs type(c.f), __self__/__func__, and C.f(c) being the same as c.f().

methods
binding

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Solution

Target Audience: Developers who can write a class and want to understand what self really is — beginners welcome, intermediate developers will find the CPython detail. Difficulty: Medium Python version: Behaviour checked on CPython 3.11 and 3.13 (identical output; bytecode shown for 3.11 and 3.12); 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

A coffee shop has one recipe card for "make a latte". The card itself is general: it says "take the customer's cup, add espresso, add milk". O

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11 min read · 11 sections

Run the code

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

PythonFunction vs bound method, __self__/__func__, and methods as callbacks

Practice

PythonBinding by hand: __get__, types.MethodType, built-in methods, WeakMethod

Practice

Discussion

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