[Python-Dev] PEP487: Simpler customization of class creation

Sylvain Corlay sylvain.corlay at gmail.com
Wed Jul 20 13:40:19 EDT 2016

Hi Nick,

Thank you for your reply. I understand your argument about using protocol
method names that are
very specific to a particular intended use case.

Interestingly, the one example that is provided in the PEP is that of a
"trait" which is pretty much the same as traitlets. (traitlets started as a
pure python implementation of Enthought's traits library).

My point is that in any real-world implementation of traits, __set_name__
will do a lot more than setting the name, which makes the name misleading.



On Wed, Jul 20, 2016 at 6:22 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> Hi Sylvain,
> Thanks for getting in touch! The traitlets library sounds interesting,
> and provides good additional evidence that this is a capability that
> folks are interested in having available.
> On 20 July 2016 at 15:26, Sylvain Corlay <sylvain.corlay at gmail.com> wrote:
> > My understanding is that the proposed __set_name__ in PEP487 exactly
> > corresponds to our class_init, although interestingly we often do much
> more
> > in class_init than setting the name of the descriptor, such as setting a
> > this_class attribute or calling class_init on contained descriptors.
> > Therefore I do not think that the names __set_name__ or __set_owner__ are
> > appropriate for this use case.
> >
> > In a way, the long-form explicit names for our class_init and
> instance_init
> > methods would be something like __init_fom_owner_class__, and
> > __touch_instance__.
> It's certainly a reasonable question/concern, but we've learned from
> experience that we're better off using protocol method names that are
> very specific to a particular intended use case, even if they can be
> adapted for other purposes. The trick is that we want educators
> teaching Python to be able to very easily answer the question of "What
> is this special method for?" (even if they later go on to say "And
> it's also used for these other things...")
> One previous example of that is the __index__ protocol, where the
> actual semantics are "instances of this type can be losslessly
> converted to integers", but the protocol is named for the particular
> use case "instances of this type can be used as sequence indices".
> For PEP 487, the two operations guiding the naming of the methods are
> "notify a base class when a new subclass is defined" and "notify a
> descriptor of its attribute name when assigned to a class". The
> precise verbs then mirror those already used in other parts of the
> related protocols (with __init__ leading to __init_subclass__, and
> __set__ leading to __set_name__).
> The main capability that __set_name__ provides that was previously
> difficult is letting a descriptor know its own name in the class
> namespace. The fact a descriptor implementor can do anything else they
> want as a side-effect of that new method being called isn't
> substantially different from the ability to add side-effects to the
> existing __get__, __set__ and __delete__ protocol methods.
> Cheers,
> Nick.
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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