[Python-3000] Nix dict.copy()
"Martin v. Löwis"
martin at v.loewis.de
Sun Feb 10 22:51:44 CET 2008
>>>> I'm not fond of this idea. dict.copy() is polymorphic -- but dict(d)
>>> Can't say dict.copy() is really polymorphic if only one other class
>>> defines the method.
>> Why do you say it's only one? I found atleast UserDict.copy,
>> os._Environ.copy, WeakValueDictionary.copy, WeakKeyDictionary.copy,
> It would have been better for me to say that copy() seems to appear only
> in variants of dicts, but __copy__() is a long-standing defined protocol
> that is supported across non-mapping types as well.
But that's Phillip's point: dict(d) is not polymorphic, yet d.copy()
is. That's important in applications where you want to copy a dict-like
object polymorphically. So indeed, your claim that .copy "is [not]
really polymorphic" is quite puzzling - we now seem to agree that it
So going back to your original message, you should remove dict(d)
from the list of alternatives for dict.copy() (or remark that this
will be a copy-to-basetype, so not quite the same thing).
> Had copy() been a builtin, we probably wouldn't be having this discussion.
Interestingly enough, dict.copy was added *after* copy.copy (in r8135),
which supported __copy__ right from the beginning (since r4555). Even
more interesting, it was added along with dict.absorb, which then
got renamed dict.update a week later. No rationale is given in the
checkin message on the purpose of the copy method.
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