什么内部函数决定价值和项目使用，我如何覆盖它们？

Question

I'm trying to write a class that inherits from dictionary and overwrote __setitem__ and __getitem__ and everything was going so well until I noticed that items and values do not use __getitem__ . Anyone know how to overwrite their look up functions?

Example implementation below:

class ReturnStringsDict(dict):

    def __getitem__(self, key):
        """Return value only."""
        return str(super().__getitem__(key))


test = ReturnStringsDict()
test['a'] = 1
test['b'] = 2

# Throws assertion error.
for value in test.values():
    assert isinstance(value, str)

# Throws assertion error.
for key, value in test.items():
    assert test[key] == value

Answer 1

According to the documentation on emulating container types ( https://docs.python.org/3.5/reference/datamodel.html#emulating-container-types ), you should implement items() and values() (and a few others) directly.

It might help to implement __iter__ , because that's what a lot of those functions do, but you should really check the docs here.

Answer 2

Well you can overwrite the values() and items() functions:

class ReturnStringsDict(dict):

    # ...

    def values(self):
        for v in super().values():
            yield str(v)

    def items(self):
        for k,v in super().items():
            yield k,str(v)

As far as I know dictionaries are not implemented in Python itself. So they do not use Python code to obtain a key or value. Simply overwriting the __getitem__ will thus not work since values() are implemented at the interpreter level. This is for instance the source code of a Python dictionary of Python 2.6.6 .