Python使用metaclass不允许类方法同名

我这里大部分代码出自The Python Master一书

1. 在python的class里面,允许同名的方法存在。但是后定义的方法会覆盖掉先定义的方法.
   例如

    class Dod1:  
        def method1(self):
            return "first definition"
           
        '''The second definition takes precedence because
        it overwrites the first entry in the namespace 
        dictionary as the class definition is processed
        '''
        def method1(self):
            return "second definition"
           
    def test_():
           
        d1 = Dod1()
        print(d1.method1())
   
    test_()
   

   test_()该函数会返回second definition
   又如

    class T:
           
        def test1(self, second:str):
            print("I am test1, and second is ", second)
           
        def test1(self):
            print("I am test1")
   
    def test_():
           
        t = T()
        a = range(1,5)
           
        try:
            t.test1(a)
        except TypeError as e:
            print(e)
           
        t.test1()
           
           
   
    test_()
   

   这一段代码的运行结果是

    test1() takes 1 positional argument but 2 were given
    I am test1
   

2. 但是可以使用metaclass 避免这个情况，即不允许class里面有同名的method.
   we can write metaclass which detects and prevents this.
   rather than using a regular dictionary as the namespace object used during class construction
   we need a dictionary which raises an error when we try to assign to an existing key.
   代码如下:

    class OneShotDict(dict):
           
        def __init__(self, existing=None):
            super().__init__()
            if existing is not None:
                for k, v in existing:
                    self[k] = v
               
        def __setitem__(self, key, value):
            if key in self:
                raise KeyError("Cannot assign existing key {!r} " 
                "in {!r}".format(key, type(self).__name__))
           
            super().__setitem__(key, value)
           
   
    class ProhibitDuplicateMeta(type):
           
        @classmethod
        def __prepare__(mcs, name, bases):
            return OneShotDict()
   
   
    if __name__ == "__main__":
        '''Can not define a class with duplicate methods using this
        metaclass''' 
        class Dod2(metaclass=ProhibitDuplicateMeta):
            def method1(self): 
                return "first definition"
           
            def method1(self): 
                return "second definition"
   

   但是出错信息不够清晰如下：
   KeyError: "Cannot assign existing key 'method1' in 'OneShotDict'"
   改进以后则如下:

    class OneShotClassNamespace(dict):
           
        def __init__(self, name, existing=None):
            super().__init__()
            self._name = name
            if existing is not None:
                for k,v in existing:
                    self[k] = v
           
        def __setitem__(self, key, value):
            if key in self:
                raise TypeError("Can not reassign existing class "
                "attribute {!r} of {!r}".format(key, self._name))
           
            super().__setitem__(key, value)
   
    class ProhibitDupInClass(type):
           
        @classmethod
        def __prepare__(mcs, name, bases):
            return OneShotClassNamespace(name)
   
    if __name__ == "__main__":
        '''Can not define a class with duplicate methods using this metaclass'''
        class Dod2(metaclass=ProhibitDupInClass):
               
            def method1(self):
                return "first definition"
               
            def method1(self):
                return "second definition"
   

   这时候出错信息如下:
   TypeError: Can not reassign existing class attribute 'method1' of 'Dod2'

    if __name__ == "__main__":
        '''Can not define a class with duplicate methods using this metaclass'''
        class T2(metaclass=ProhibitDupInClass):
            def test1(self, second:str):
                print("I am test1, and second is ", second)
               
            def test1(self):
                print("I am test1")
   
    #出错信息
    TypeError: Can not reassign existing class attribute 'test1' of 'T2'