修改类属性

例子一：

一下是修改类属性，

class counter:
    count = 0  
    def __init__(self):
        self.__class__.count+=1 #此时类属性被类和所有类实例共享

print counter.count # 0
c=counter()
print c.count # 1

d=counter() 
print d.count #2

print counter.count #2

例子二：

class counter(object):
    count=0
    class_co=0
    def __init__(self):
        self.count+=1  #实例所拥有
        counter.class_co+=1

print counter.count,counter.class_co # 0 0

c=counter()
print c.count,c.class_co # 1 1

a=counter()
print a.count,c.class_co # 1 2