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Sun Oct 29 13:04:30 EDT 2017 · https://mail.python.org/pipermail/python-dev/2017-October/150032.html

It's somewhat problematic. If I subclass dict with a different constructor,
but I don't overload copy(), how can the dict.copy() method construct a
correct instance of the subclass? Even if the constructor signatures match,
how can dict.copy() make sure it copies all attributes properly? Without an
answer to these questions I think it's better to admit defeat and return a
dict instance -- classes that want to do better should overload copy().

I notice that Counter.copy() has all the problems I indicate here -- it
works as long as you don't add attributes or change the constructor
signature. I bet this isn't documented anywhere.

On Sun, Oct 29, 2017 at 9:40 AM, Brett Cannon <brett at python.org> wrote:

> It probably should be more consistent and I have a vague recollection that
> this has been brought up before.
>
> On Sun, Oct 29, 2017, 08:21 Serhiy Storchaka, <storchaka at gmail.com> wrote:
>
>> The copy() methods of list, dict, bytearray, set, frozenset,
>> WeakValueDictionary, WeakKeyDictionary return an instance of the base
>> type containing the content of the original collection.
>>
>> The copy() methods of deque, defaultdict, OrderedDict, Counter,
>> ChainMap, UserDict, UserList, WeakSet, ElementTree.Element return an
>> instance of the same type as the original collection.
>>
>> The copy() method of mappingproxy returns a copy of the underlying
>> mapping (using its copy() method).
>>
>> os.environ.copy() returns a dict.
>>
>> Shouldn't it be more consistent?
>>
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>
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-- 
--Guido van Rossum (python.org/~guido)
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[Python-Dev] The type of the result of the copy() method