Question

我试图学习python类，我试图了解类变量的工作原理。当我执行下面的代码时，我看到了这个错误。我只是想了解为什么python抱怨名字＆＃34;员工＆＃34;。当我将语句移到Employee.employeecount = Employee.employeecount + 1 __init__方法内时，我看到它没有抱怨。有人可以帮我理解究竟发生了什么。

非工作代码：

class Employee(object):

    employeecount = 0
    Employee.employeecount = Employee.employeecount + 1

    def __init__(self,name,salary):
        self.name = name
        self.salary = salary

    def getdata(self):
        self.salary += 1
        print "Employee name: %s \nEmployee Salary: %d" % (self.name,self.salary)

    def getname(self):
        return self.name

    def getsalary(self):
        return self.salary

emp1 = Employee('Pradeep',8000)
emp1.getdata()

emp2 = Employee('Maddy',9000)
emp2.getdata()

print "Employee Count is %d" % Employee.employeecount
print "Employee Count is %d" % Employee.employeecount

name = emp1.getname()
salary = emp1.getsalary()

print "Name is %s and salary is %d" % (name,salary)

python classes.py

Traceback (most recent call last):
  File "classes.py", line 4, in <module>
    class Employee(object):
  File "classes.py", line 6, in Employee
    Employee.employeecount = Employee.employeecount + 1
NameError: name 'Employee' is not defined

Answer 1

问题是您在类定义中使用了Employee.employeecount = Employee.employeecount + 1行;这将在您的程序启动的那一刻开始执行，而不是在创建新的Employee时执行。

在那个时间点，Employee课程将会看到Employee.employeecount，并尝试寻找一些已经制作的Employee课程 - 它不会意识到你想要它指自己。在那个时间点，Employee尚未正确定义，因为您正在定义它。另一方面，如果将它放在__init__函数中，您会发现没有问题 - 因为当您实际创建Employee时，该类已被正确定义。

那时你要做的只是employeecount = employeecount + 1。这将摆脱错误，但这可能不是你想要做的。如果您想在每次创建新员工时添加计数，请将Employee.employeecount = Employee.employeecount + 1行添加到__init__函数中。这应该可以解决问题。

Answer 2

您希望更改代码，以便在init中增加计数器，以便每次实例化Employee时，计数器都会增加。

class Employee(object):
    employeecount = 0 # Start of program this is set to 0

    def __init__(self,name,salary):
        self.name = name
        self.salary = salary
        Employee.employeecount += 1 # Add 1 each time you create an Employee

您没有在employeecount下方Employee引用employeecount = 0，因为您已经在班级。

class Employee(object):
    # This is at class level - you don't use "ClassName.variableName"
    # You just use "variableName"

这就是employeecount = 0没有给你错误而Employee.employeecount = Employee.employeecount + 1没有错误的原因。

您的代码在init中工作，因为您必须在变量名之前使用类名来访问类变量。

def __init__(self,name,salary):
    # This is at the instance level
    # To access a class variable you have to put the class name first

Answer 3

当您将Employee.employeecount = Employee.employeecount + 1移动到__init__时，您每次向新对象发送内容时都会增加此employeecount变量。创建对象时会调用类的__init__方法，这就是错误消失的原因。

Python中类变量的用法

3 个答案: