调用类方法会在Python中引发TypeError

Question

我不明白如何使用类.当我尝试使用该类时,以下代码给出了一个错误.

class MyStuff:
    def average(a, b, c): # Get the average of three numbers
        result = a + b + c
        result = result / 3
        return result

# Now use the function `average` from the `MyStuff` class
print(MyStuff.average(9, 18, 27))

错误:

File "class.py", line 7, in <module>
    print(MyStuff.average(9, 18, 27))
TypeError: unbound method average() must be called with MyStuff instance as first argument (got int instance instead)

怎么了？

Answer 1

您可以通过声明变量并将类调用为实例来实例化该类:

x = mystuff()
print x.average(9,18,27)

但是,这不适用于您提供给我们的代码.在给定对象(x)上调用类方法时,它总是在调用函数时将指向对象的指针作为第一个参数传递.因此,如果您现在运行代码,您将看到以下错误消息:

TypeError: average() takes exactly 3 arguments (4 given)

要解决此问题,您需要修改平均方法的定义以获取四个参数.第一个参数是对象引用,其余3个参数是3个数字.

Answer 2

从您的示例中,我觉得您想要使用静态方法.

class mystuff:
  @staticmethod
  def average(a,b,c): #get the average of three numbers
    result=a+b+c
    result=result/3
    return result

print mystuff.average(9,18,27)

请注意,在python中大量使用静态方法通常是一些难闻气味的症状 - 如果你真的需要函数,那么直接在模块级别声明它们.

Answer 3

要最低限度地修改您的示例,您可以将代码修改为:

class myclass(object):
        def __init__(self): # this method creates the class object.
                pass

        def average(self,a,b,c): #get the average of three numbers
                result=a+b+c
                result=result/3
                return result


mystuff=myclass()  # by default the __init__ method is then called.      
print mystuff.average(a,b,c)

或者更全面地扩展它,允许您添加其他方法.

class myclass(object):
        def __init__(self,a,b,c):
                self.a=a
                self.b=b
                self.c=c
        def average(self): #get the average of three numbers
                result=self.a+self.b+self.c
                result=result/3
                return result

a=9
b=18
c=27
mystuff=myclass(a, b, c)        
print mystuff.average()