Class attributes

• When defining a class, you can create a variable inside a class. This becomes a class attribute.

  >>> class A:
  ...    foo = 1
  

• A class attributed can be accessed and assigned to just like an attribute of an object. (A class is an object too!)

  >>> A.foo
  1
  >>> A.foo = 10
  >>> A.foo
  10
  

• A class attribute can be useful when you want to have one variable that all the instances of the class can access.

  For example, the following class has attribute count, which is equal to the number of instances created so far. Each instance use count to get its order of creation.

  >>> class B:
  ...    count = 0
  ...    def __init__(self):
  ...      self.__class__.count += 1
  ...      self.order = self.__class__.count 
  ...
  >>> class B:
  ...   count = 0
  ...   def __init__(self):
  ...     self.__class__.count += 1
  ...     self.order = self.__class__.count
  ... 
  >>> foo = B()
  >>> foo.order
  1
  >>> B.count
  1
  >>> bar = B()
  >>> bar.order
  2
  >>> B.count
  2
  >>> B()
  <__main__.B instance at 0x403490>
  >>> B.count
  3
  

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