On Tue, Mar 17, 2015 at 3:29 PM Zaur Shibzukhov <szport@gmail.com> wrote:
Yes... But I expected that dict constructor will use `__getitem__`  or `items` method of MyDict instance  in order to retrieve items of the MyDict instance during construction of the dict instance... Instead it interpreted MyDict instance as the dict instance during construction of new dict.This exactly caused my confusion.

It's because you subclassed dict. Copying is optimized to skip over using the methods you listed when the object is a dict and so we know the structure of the object at the C level. You can look at https://hg.python.org/cpython/file/22a0c925a7c2/Objects/dictobject.c#l1997 to see the actual code.


Zaur Shibzukhov

2015-03-17 22:12 GMT+03:00 Brett Cannon <brett@python.org>:

On Tue, Mar 17, 2015 at 3:05 PM Zaur Shibzukhov <szport@gmail.com> wrote:

In order to explain, let define subclass of dict:

class Pair:
    def __init__(self, key, val):
        self.key = key
        self.val = val

class MyDict(dict):
    def __init__(self, *args, **kwds):
        if len(args) > 1:
            raise TypeError('Expected at most 1 arguments, but got %d' % len(args))

        for key, val in args[0]:
            self[key] = val
        for key, val in kwds.items():
            self[key] = val

    def __getitem__(self, key):
        pair = dict.__getitem__(key)
        return pair.value

    def __setitem__(self, key, val):
        if key in self:
            pair = dict.__getitem__(key)
            pair.value = value
            pair = Pair(key, val)
            dict.__setitem__(self, key, pair)

    def values(self):
        for key in self:
            p = dict.__getitem__(self, key)
            yield p.value

    def items(self):
        for key, p in dict.__iter__(self):
            yield p.key, p.value

The simple test give me strange result:

>>> d = MyDict([('a', 1), ('b', 2), ('c', 3)])
>>> dict(d)
{'a': <__main__.Pair at 0x104ca9e48>,
 'b': <__main__.Pair at 0x104ca9e80>,
 'c': <__main__.Pair at 0x104ca9eb8>}

instead of {'a':1, 'b':2, 'c':3}.

Is this right behavior of the dict?

Yes because in your __setitem__ call you are storing the value as the Pair. So when dict prints its repr it prints the key and value, and in this case the value is a Pair.