What is the purpose of self in instance methods, and why must it be explicitly declared in Python?
self is the explicit first parameter; binding happens through the descriptor protocol, not a keyword
self is just a conventional name for the first parameter of an instance method. It is not a keyword. When you write obj.method(x), Python first looks up method on the class, finds a function object, and because functions implement get, it returns a bound method that already has obj as the first argument. So obj.method(x) is effectively Class.method(obj, x). Python requires the first parameter to be explicit for readability and because functions are plain objects; there is no hidden this. This also means you can call the underlying function directly with an explicit instance, which is what super() and unbound-method patterns exploit.
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self is not special to the parser; it is special only by convention. Naming it cls in a classmethod is also convention.
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Binding happens via functions being descriptors: function.get(instance, owner) returns a method object.
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Trade-off: explicitness makes it clear what is bound, but it adds visual noise and is the most common source of TypeError: missing 1 required positional argument.
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Common mistake: thinking you must name it self. You can name it anything, but doing so breaks readability and tooling assumptions.
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Common mistake: calling Class.method(x) on an instance method and forgetting that you must pass the instance explicitly.
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Version note: unbound method objects from Python 2 were removed in Python 3; functions are simply descriptors now.
Scenario Questions
0-2 years experience
- 1You call Class.method(arg) and get a missing positional argument error. What happened?
- 2Is self a reserved keyword in Python?
2-5 years experience
- 1You store a method in a variable and call it later. Does binding still happen?
- 2You need to call a parent method explicitly without using super(). How do you pass the instance?
5-8 years experience
- 1You attach a plain function as an instance attribute and expect binding. Why does it fail and how do you emulate binding?
- 2How does the descriptor protocol decide between a function, a property, and a classmethod when accessed on an instance?
8+ years experience
- 1Design a method decorator that preserves binding semantics and remains inspectable under mypy and IDEs.
- 2Explain how CPython's method object caches __self__ and __func__, and how that affects performance and pickling.
Follow-up Questions
- Why can you call Counter.incr(c) directly and what does that reveal about binding?
- What does super() return and how does it use the instance?