Instance variables, class variables and the self keyword · CAB106

Instance variables are created through self (usually in __init__) and belong to one object. Class variables are defined in the class body and are shared by all instances; read them as ClassName.name or through an instance. Assigning through an instance, as in a.x = 99, creates an instance attribute that shadows the class attribute without changing it.

Kind	Defined	Shared?
Instance variable	`self.x = ...`	One object
Class variable	In class body	All objects

Lookup checks the instance first, then the class, then its parents. self.n += 1 reads the class value but writes a new instance attribute, so the class variable is unchanged; use ClassName.n += 1 to update the shared value. Mutable class variables such as lists are genuinely shared, so append through any object is seen by all. obj.__dict__ lists an object's own attributes. Rebinding self inside a method only changes the local name.

Exam trap: a mutable class attribute used as per-object storage. Create the list in __init__ instead.

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Question 1 · Tier 1, Recall

Which kind of variable stores a single value that all objects built from the same blueprint share?

A class variable
An instance variable
A local variable
A keyword argument

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A. A class variable is defined in the class body and shared by all instances.

Why not B: An instance variable belongs to a single object.

Why not C: A local variable exists only inside one function call.

Why not D: A keyword argument is a way of passing a value in a call.

Key points

Try two questions