Instance variable

As explained in Software Design for Python Programmers

An instance variable is state associated with an individual object instance.

An instance variable is state associated with an individual object. A class defines the possible structure, and each object receives its own attribute values at runtime. The current combination of those values characterizes the object’s state; changing one or more values produces a state transition. Methods operate on instance variables and help determine object behavior. section 1.8

Instance variables support several design decisions:

• Encapsulate object state inside a class, exposing public attributes through its interface while restricting private attributes to class methods. section 1.8
• Represent a class’s owned relationships: Catalogue has private _booklist containing zero or more Book objects, whereas method parameters can create only transient dependencies. section 4.3.1
• Translate requirements into object state: Catalogue stores _booklist, Book stores _attributes, Attributes stores _dictionary, and Form stores individual input values. section 3.8.1
• Record the values that drive explicit state machines, such as a ticket machine’s count, card_inserted, and card_validity variables. section 13.1

Private state is not automatically immutable

An Employee object can store _employee_id, _name, and _birthdate as private instance variables. However, assigning the caller’s Date object directly to _birthdate lets the caller change the employee’s state; returning that same object through birthdate creates another mutation path. Copy the Date object when constructing Employee and when returning birthdate to protect the embedded state. section 5.7

Table 3.2 The initial classes of the application

Class	State	Instance variables
Catalogue	List of books	_booklist (list of Book objects)
Book	Book attributes	_attributes (the book’s attributes)
Attributes	Attribute values	_dictionary (dictionary of key–value pairs)
Form	Input values	Individual instance variables for the book attributes

Use the class-state table to compare how instance variables represent a catalogue, book, attribute collection, and form. section 3.8.1

Figure 4.6 Diagrams that show more design detail. Class Catalogue depends on class Book via its private booklist instance variable. The multiplicity indicators show that a single (1) Catalogue object can contain zero or more (*) Book objects.

Use the UML figure to see a private instance variable expressing Catalogue’s dependency on Book and its zero-or-more multiplicity. section 4.3.1

How Software Design for Python Programmers covers it

How deep it goes

Explains state, visibility, dependencies, multiplicity, and defensive copying, but stops short of general language-level attribute mechanics.

Examples

Uses catalogue objects, Employee birthdates, and an automatic ticket machine to demonstrate storage, protection, and state changes.

What it assumes

Assumes basic familiarity with classes and objects while reviewing encapsulation, abstraction, inheritance, and polymorphism.

What you can do after

Evaluate/understand how object state, ownership, visibility, and mutation paths affect class design.

How it connects

Connects instance variables to classes, objects, methods, encapsulation, inheritance, dependency relationships, and state transitions.

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Also explained in

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  The Well-Grounded Rubyist, Fourth EditionAn instance variable is a Ruby variable identified by a single-at-sign prefix and associated with an individual object.
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  100 Java Mistakes and How to Avoid Them
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  liveBookmanning.com
  Secrets of the JavaScript Ninja, Second EditionA property that belongs directly to a particular object instance rather than being inherited from its prototype.

Compare how 4 books explain instance variable →

Related concepts

• static fieldcontrasts with · A named variable whose storage is associated with a class or has static duration, rather than being allocated separately for each object instance or function call.
• scopecontrasts with · Scope is the region of a program in which names are visible and can be resolved according to the language’s lookup rules.
• stateused by · State is information or a configuration describing the condition of a system, entity, or computation at a particular time and potentially influencing subsequent behavior or decisions.
• const referenceconfigured by · A const reference is a C++ reference through which the referred-to object cannot be modified.
• accessor methodused by · An accessor method or function provides access to an object's property or state by retrieving its value, assigning a value, or defining behavior for such access.

This explainer was generated by Manning's knowledge graph from the text of Software Design for Python Programmers. The section links point to the parts of the book it draws on.