Class.attr 共享变量
=========================

Class.attr 与 obj.attr 的区别


```python
class Employee:
    'Common base class'
    empCount = 0                # 此变量会被共享

    def __init__(self, name, salary):
        self.name = name
        self.salary = salary        
        
        
        Employee.empCount +=1       # 每创建1个对象时,  empCount值加1   
        # self.empCount +=1    
        
        '''
        #NOTE
        
        Employee.empCount  与 self.empCount 不是同一个东西:
        
            - Employee.empCount 变量会在新创建的 object 中重用, 
              相当于可以在 object 中传递信息
        
            - 而如果是 self.empCount, 则每个对象都拥有自己独立的 
              empCount 变量. 
        
        '''
        

    def displayCount(self):
        print ("Total Employee: %d" % Employee.empCount)

    def displayEmployee(self):
        print ("Name: ", self.name)
        print ("Salary: ", self.salary)
        
        
E1 = Employee('Bob',1234)       # 两个参数会被 __init__ 用来初始化, 必须有.

E2 = Employee('Tom',2345)

E2.displayCount()

# result: 2
```