wen Python编程整理：通过修改实例类变量改变类的类变量的问题（成因：类变量为可变变量）
先上代码，
class C1(object):
class_variable = 1
class C2(object):
class_variable = []
object1 = C1()
object2 = C2()
print(C1.class_variable)
# 1
print(object1.class_variable)
# 1
object1.class_variable = 20
print(object1.class_variable)
print(C1.class_variable)
# 20
# 1
