Python - Class Variables

Class Variables
	Chapter 22. Advanced Class Definition

Class Variables

The notion of object depends on having instance variables (or “attributes”) which have unique values for each object. We can extend this concept to include variables that are not unique to each instance, but shared by every instance of the class. Class level variables are created in the class definition itself; instance variables are created in the individual class method functions (usually __init__).

Class level variables are usually "variables" with values that don't change; these are sometimes called manifest constants or named constants. In Python, there's no formal declaration for a named constant.

A class level variable that changes will be altered for all instances of the class. This use of class-level variables is often confusing to readers of your program. Class-level variables with state changes need a complete explanation.

This is an example of the more usual approach with class-level constants. These are variables whose values don't vary; instead, they exist to clarify and name certain values or codes.

Example 22.5. wheel.py

import random
class Wheel( object ):
    """Simulate a roulette wheel."""
    green, red, black= 0, 1, 2
    redSet= [1,3,5,7,9,12,14,16,18,19,21,23,25,27,30,32, 34,36]
    def __init__( self ):
        self.lastSpin= ( None, None )
    def spin( self ):
        """spin() -> ( number, color )

        Spin a roulette wheel, return the number and color."""
        n= random.randrange(38)
        if n in [ 0, 37 ]: n, color= 0, Wheel.green
        elif n in Wheel.redSet: color= Wheel.red
        else: color= Wheel.black
        self.lastSpin= ( n, color )
        return self.lastSpin

	Part of definition of the class `Wheel` includes some class variables. These variables are used by all instances of the class. By defining three variables, `green`, `red` and `black`, we can make our programs somewhat more clear. Other parts of our program that use the `Wheel` class can then reference the colors by name, instead of by an obscure numeric code. A program would use `Wheel.green` to refer to the code for green within the `Wheel` class.
	The `Wheel` class also creates a class-level variable called `redSet`. This is the set of red positions on the Roulette wheel. This is defined at the class level because it does not change, and there is no benefit to having a unique copy within each instance of `Wheel`.
	The `__init__` method creates an instance variable called `lastSpin`. If we had multiple wheel objects, each would have a unique value for `lastSpin`. They all would all, however, share a common definition of `green`, `red`, `black` and `redSet`.
	The `spin` method updates the state of the wheel. Notice that the class level variables are referenced with the class name: `Wheel.green`. The instance level variables are referenced with the instance parameter: `self.lastSpin`. The class level variables are also available using the instance parameter, `Wheel.green` is the same object as `self.green`. The `spin` method determines a random number between 0 and 37. The numbers 0 and 37 are treated as 0 and 00, with a color of green; a number in the `Wheel.redSet` is red, and any other number is black. We also update the state of the `Wheel` by setting `self.lastSpin`. Finally, the `spin` method returns a `tuple` with the number and the code for the color. Note that we can't easily tell 0 from 00 with this particular class definition.

The following program uses this Wheel class definition. It uses the class-level variables red and black to clarify the color code that is returned by spin.

w= Wheel()
n,c= w.spin()
if c == Wheel.red: print n, "red"
elif c == Wheel.black: print n, "black"
else: print n

Our sample program creates an instance of Wheel, called w. The program calls the spin method of Wheel, which updates w.lastSpin and returns the tuple that contains the number and color. We use multiple assignment to separate the two parts of the tuple. We can then use the class-level variables to decode the color. If the color is Wheel.red, we can print "red".


Initializer Techniques		Static Methods and Class Method