[Python-Dev] Possible wrong behavior of the dict?

[Zaur Shibzukhov]

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.

---
*Zaur Shibzukhov*


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

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