Itse1359-1410-overview of python classes  (Page 5/9)

Persistence

The ability of your car radio to remember your list of favorite stations is often referred to as persistence. An object that has the ability to store and remember values is often said to have persistence.

State

It is often said that the state of an object at a particular point in time is determined by the values stored in the object. In our analogy, even if we own identical radios, unless the two of us have the same list of favorite radio stations, associated with the same combination of buttons, the state of your radio object at any particular point in time will be different from the state of my radio object.

It is perfectly OK for the two of us to own identical radios and to cause the two radio objects to contain the same list of frequencies. Even if two objects have the same state at the same time, they are still separate and distinct objects. While this is obvious in the real world of car radios, it may not be quite as obvious in the virtual world of computer programming.

Sending a message

A person who speaks in OOP-speak might say that pressing one of the station-selector buttons on the front of the radio sends a message to the radio object, asking it to perform an action (tune to a particular station) . That person might also say that storing a new frequency that corresponds to a particular button entails sending a message to the radio object asking it to change its state.

Calling a method

Python-speak is a little more specific than general OOP-speak. In Python-speak, we might say that pressing one of the selector buttons on the front of the radio calls a method on the radio object. The behavior of the method is to cause the object to perform an action.

As a practical matter, the physical manifestation of sending a message to an object in Python is to cause that object to execute one of its methods.

Setter methods and getter methods

Similarly, we might say that storing a new frequency that corresponds to a particular button calls a setter method on the radio object. (In an earlier paragraph, I said that I could follow a specific procedure to set the frequency value associated with a button to correspond to one of the radio stations in Dallas. Note the use of the words set and setter in this jargon.)

Behavior of an object

In addition to state, objects are often also said to have behavior. The overall behavior of an object is determined by the combined behaviors of its individual methods. For example, one of the behaviors exhibited by our radio object is the ability to play the radio station at a particular frequency. When a frequency is selected by pressing a selector button, the radio knows how to translate the radio waves at that frequency into audio waves compatible with our range of hearing, and to send those audio waves out through the speakers.Thus, the radio object behaves in a specific way in response to a message asking it to tune to a particular frequency.

Where do objects come from ?

In order to mass-produce car radios, someone must first create a set of manufacturing plans (drawings, or blueprints) for the radio. Once the plans are available, the manufacturing people can produce millions of nearly identical radios.