On Sun, 04 May 2014 20:03:35 +1200, Gregory Ewing wrote:
> Steven D'Aprano wrote:
>> If it were a class method, you would call it by MyBaseClass.__new__()
>> rather than explicitly providing the cls argument.
>
> But that wouldn't be any good, because the base __new__ needs to receive
> the actual class being instantiated, not the class that the __new__
> method belongs to.
Which is exactly what method descriptors -- whether instance methods or
class descriptors -- can do. Here's an example, using Python 2.7:
class MyDict(dict):
@classmethod
def fromkeys(cls, *args, **kwargs):
print "Called from", cls
return super(MyDict, cls).fromkeys(*args, **kwargs)
class AnotherDict(MyDict):
pass
And in use:
py> MyDict.fromkeys('abc')
Called from <class '__main__.MyDict'>
{'a': None, 'c': None, 'b': None}
py> AnotherDict().fromkeys('xyz')
Called from <class '__main__.AnotherDict'>
{'y': None, 'x': None, 'z': None}
In both cases, MyDict's __new__ method receives the class doing the
calling, not the class where the method is defined.
Whatever the difficulty is with __new__, it isn't something obvious.
--
Steven D'Aprano
http://import-that.dreamwidth.org/
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