Книга: Fedora™ Unleashed, 2008 edition

Class and Object Variables

Class and Object Variables

Each object has its own set of functions and variables, and you can manipulate those variables independent of objects of the same type. Additionally, some class variables are set to a default value for all classes and can also be manipulated globally.

This script demonstrates two objects of the dog class being created, each with its own name:

class dog(object):
 name = "Lassie"
 def bark(self):
  print self.name + " says 'Woof!'"
 def setName(self, name):
  self.name = name
fluffy = dog()
fluffy.bark()
poppy = dog()
poppy.setName("Poppy")
poppy.bark()

That outputs the following:

Lassie says 'Woof!'
Poppy says 'Woof!'

Each dog starts with the name Lassie, but it gets customized. Keep in mind that Python assigns by reference by default, meaning each object has a reference to the class's name variable, and as you assign that with the setName() method, that reference is lost. What this means is that any references you do not change can be manipulated globally. Thus, if you change a class's variable, it also changes in all instances of that class that have not set their own value for that variable. For example:

class dog(object):
 name = "Lassie"
 color = "brown"
fluffy = dog()
poppy = dog()
print fluffy.color
dog.color = "black"
print poppy.color
poppy.color = "yellow"
print fluffy.color
print poppy.color

So, the default color of dogs is brown — both the fluffy and poppy dog objects start off as brown. Then, with dog.color, the default color is set to black, and because neither of the two objects has set its own color value, they are updated to be black. The third to last line uses poppy.color to set a custom color value for the poppy object — poppy becomes yellow, but fluffy and the dog class in general remain black.

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