[Python-ideas] Add __parent__ to all classes, functions, and modules

[Guido van Rossum]

This is a clever idea but I doubt it's worth adding.

The pattern proposed in the example in the first post doesn't strike me as
very readable -- though you don't give any examples of *using* the
decorator from the example, so it's a little difficult to imagine what the
use case would be.

Adding __parent__ would have to happen as an extra pass once the class is
defined -- the decorator (at least its outer part) runs at the time that
the method is being defined as a plain function object, and at that time
the class object hasn't been created yet. This also means that some
decorators can't use the __parent__ attribute (because the outer function
of the decorator runs before the object to be stored in __parent__ has been
created).

If you really want to explore this idea further, you can probably define a
metaclass that adds a __parent__ attribute to the function objects in the
class __dict__ when the class is being created. Then you can experiment
with using the pattern in some real code. Please report back here with some
examples where the presence of __parent__ lets you write cleaner code.

An alternative that wouldn't even require a metaclass would be to write a
helper function that looks the class object up by name after parsing
__qualname__. There are some limitations to this, e.g. classes defined
inside functions -- but that's already a suspect pattern anyway.


On Sun, Oct 5, 2014 at 2:18 PM, Neil Girdhar <mistersheik at gmail.com> wrote:

>
>
> On Sun, Oct 5, 2014 at 5:11 PM, Benjamin Peterson <benjamin at python.org>
> wrote:
>
>> Neil Girdhar <mistersheik at ...> writes:
>>
>> >
>> >
>> >
>> > On Sun, Oct 5, 2014 at 2:09 PM, Benjamin Peterson <benjamin <at>
>> python.org> wrote:
>> > Neil Girdhar <mistersheik <at> ...> writes:
>> > >
>> > > Many classes, functions, and modules are defined within the context of
>> > another class, function, or module thereby forming a mathematical
>> forest of
>> > declarations.  It is possible to walk the descendants using __dict__
>> (for
>> > classes and modules), but not the ancestors.  I propose adding
>> __parent__
>> > that would be filled at the same time that __qualname__ is filled
>> in.This
>> is unlikely to work.
>> > 1) It turns basically everything into a cycle.
>> >
>> >
>> > Why a cycle?
>>
>> Because, for example, the class would reference methods, which would
>> reference the class.
>>
>
> Right.  I don't see what's wrong with that.  This already happens with the
> __module__ member and the module's immediate children.
>
>>
>> >
>> >
>> > 2) __qualname__ is determined strictly from syntax, whereas __parent__
>> could
>> > not be. For example, what happens if I take a method from one class and
>> set
>> > it on another? __parent__ would not be well-defined.
>> >
>> >
>> > I'm suggesting that parent be determined purely from declaration.  If
>> you
>> copy something, neither qualname nor parent would change unless you change
>> them.
>>
>> It would mean your trick with super would only work for some methods.
>>
>
> It would only work for that are decorated in the usual way using @blah on
> methods defined in the class.  If someone wants to copy that method
> somewhere else then they'll have to update __parent__ themselves.
>
>>
>>
>>
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-- 
--Guido van Rossum (python.org/~guido)
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