在Python中，ABC（抽象基类）是一个提供接口定义和部分实现的类。它们不能直接实例化，而是用作其他类的基类。在ABC集合中，def(_f)和del(_f)分别代表“定义”和“删除”方法。

下面是一个示例代码：

from collections import ABC, MutableMapping

class MyDict(MutableMapping):
    # 继承MutableMapping基类

    def __init__(self, **kwargs):
        self.store = dict()
        self.update(kwargs)

    def __getitem__(self, key):
        return self.store[self._keytransform(key)]

    def __setitem__(self, key, value):
        self.store[self._keytransform(key)] = value

    def __delitem__(self, key):
        del self.store[self._keytransform(key)]

    def __iter__(self):
        return iter(self.store)

    def __len__(self):
        return len(self.store)

    def _keytransform(self, key):
        return key

class ReadOnlyMapping(ABC):
    # 继承ABC基类

    @def(MutableMapping)
    def __getitem__(self, key):
        return self.store[self._keytransform(key)]

    @def(MutableMapping)
    def __iter__(self):
        return iter(self.store)

    @def(MutableMapping)
    def __len__(self):
        return len(self.store)

    @del(MutableMapping)
    def __setitem__(self, key, value):
        raise NotImplementedError()

    @del(MutableMapping)
    def __delitem__(self, key):
        raise NotImplementedError()

class MyReadOnlyDict(ReadOnlyMapping):
    # 继承ReadOnlyMapping基类

    def __init__(self, **kwargs):
        self.store = dict()
        self.update(kwargs)

    def __setitem__(self, key, value):
        self.store[self._keytransform(key)] = value

    def __delitem__(self, key):
        del self.store[self._keytransform(key)]

    def _keytransform(self, key):
        return key.upper()

d = MyDict()
print(d) # {}

d['a'] =