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_() 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()

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'