Python Essential Reference: Types and Objects

• By David M. Beazley
• Jul 9, 2009
• Sample Chapter is provided courtesy of Addison-Wesley Professional

This chapter is from the book
Python Essential Reference, 4th Edition

All the data stored in a Python program is built around the concept of an object. Objects include fundamental data types such as numbers, strings, lists, and dictionaries. However, it’s also possible to create user-defined objects in the form of classes. In addition, most objects related to program structure and the internal operation of the interpreter are also exposed. This chapter describes the inner workings of the Python object model and provides an overview of the built-in data types. Chapter 4, “Operators and Expressions,” further describes operators and expressions. Chapter 7, “Classes and Object-Oriented Programming,” describes how to create user-defined objects.

Terminology

Every piece of data stored in a program is an object. Each object has an identity, a type (which is also known as its class), and a value. For example, when you write a = 42, an integer object is created with the value of 42. You can view the identity of an object as a pointer to its location in memory. a is a name that refers to this specific location.

The type of an object, also known as the object’s class, describes the internal representation of the object as well as the methods and operations that it supports. When an object of a particular type is created, that object is sometimes called an instance of that type. After an instance is created, its identity and type cannot be changed. If an object’s value can be modified, the object is said to be mutable. If the value cannot be modified, the object is said to be immutable. An object that contains references to other objects is said to be a container or collection.

Most objects are characterized by a number of data attributes and methods. An attribute is a value associated with an object. A method is a function that performs some sort of operation on an object when the method is invoked as a function. Attributes and methods are accessed using the dot (.) operator, as shown in the following example:

a = 3 + 4j          # Create a complex number
r = a.real          # Get the real part (an attribute)

b = [1, 2, 3]       # Create a list
b.append(7)         # Add a new element using the append method