[Python-Dev] The type of the result of the copy() method (original) (raw)
Guido van Rossum guido at python.org
Sun Oct 29 13:04:30 EDT 2017
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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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