Serhiy Storchaka <storchaka+cpyt...@gmail.com> added the comment: > I'm not sure that this should be in the stdlib. The three-line function can > be enough for your simple case, and it is too simple for including it in the > stdlib. But for general stdlib quality solution it lacks many details. > > 1. It doesn't work with classes that implement the constructor as __new__, > but not __init__. It may need of using metaclasses with overridden __call__. > But how then handle classes with custom metaclasses? > > Can you illustrate how you would do it for these kinds of classes?
I wouldn't try to implement the generic partialclass() at first place. But if I should, the possible code: class PartialMeta(type(cls)): __call__ = partialmethod(type(cls).__call__, *args, **kwargs) class PartialClass(cls, metaclass=PartialMeta): pass return PartialClass Not sure it works. Another solution -- add a wrapped __new__, but __new__ and __init__ should be added only when they where defined in the original class or its parenthesis. There may be caveats with inheriting from some builtin classes. > Anyway, I think using __new__ on user classes is extremely rare. This is not good reason of not supporting them. The library function should support all corner cases. > 2. inspect.signature() doesn't give the correct signature as for partial(). > This requires changing the inspect module. > > Good point. I can look into this. > > 3. pickling and copying of instances are broken in many cases. Pickling a > class is always broken. I afraid that this can't be solved without > significant reworking of the pickle and the copy modules. > > Okay, interesting, but this doesn't seem relevant to partialclass. If you generate a class, it should behave correctly. Otherwise you could just use partial(). > 4. It adds instance attributes __dict__ and __weakref__ even if they were > disabled by using __slots__. This increases the size of instances and breaks > some properties. > > Can we just copy the parent class' __slots__ to the partialclass return value? This is not so easy. You should add an empty __slots__ if the original class doesn't have __dict__ in instances. And testing that may be non-trivial task. > 6. Many alternate constructors and copying methods are broken. For example > the copy() method of partialclass(defaultdict, list) in your example. There > is no general solution of this, it should be solved individually for every > class. > > Ultimately, if pickling works, then copying should work too. The usual way I > do it is implementing __getnewargs__, etc. This should work fine? I meant the defaultdict.copy() method, not the copy module, > Should we discuss this on github? The bug tracker is the place for the design discussion. GitHub is a place for discussing implementation details. It looks to me that this should first be discussed on Python-ideas. You should convince core developers that it is worth to add this feature, and provide a plan of solving all implementation problems mentioned above. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue33419> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
