Artur Siekielski a écrit :
> Hi.
>
> I would like to have declarative properties in Python, ie. something
> like slots definitions in defclass in Common Lisp. It seems that even
> Java will have it, using a library ( https://bean-properties.dev.java.net/
> ).
>
> I know about 'property' function
Actually, it's a class.
> in Python, but it's normal usage
> isn't declarative, because I have to code imperatively getters and
> setters:
Indeed. What would be the use of a property (AKA computed attribute) if
you don't need to do something specific ?
> class Person(object):
> def __init__(self, name):
> self._name = name
> def _get_name(self):
> return self._name
> def _set_name(self, new_name):
> self._name = new_name
> name = property(_get_name, _set_name)
While it's often seen as demonstration code for a property, it's
actually totally useless. If all you want to do is to get and set an
attribute (I mean, without any computation anywhere), then just use an
attribute - you will always be able to turn it into a property if and
whene the need arises.
> I would like to have something like that:
>
> class Person(object):
> name = property('_name')
>
> I assume that this causes "generation" of instance field '_name' and
> default getters and setters.
So far, it's a waste of time. That's exactly what you'd get with a plain
attribute, with far less complexity and overhead.
> And if I would like to customize (g|
> s)etters, I would write them by hand,
Which bring us back to how properties actually work !-)
> or, better, use more declarative
> approach (like @NotNull annotations in Java version).
> I could probably code a solution to my problem with help of wonderful
> introspection functionalities. But I'm looking for a standard and/or
> proven way of doing this. Maybe some PEP is being prepared for this?
I guess you want to have a look at the descriptor protocol - it's what
makes properties work, and it allow you to write your own custom 'smart
attributes'. If you want automatic validation/conversion, you could
write custom descriptors working with the FormEncode package (I started
such a thing for Elixir, but had no time to finish it so far). It would
then looks like:
class Person(object):
name = StringField(empty=False)
// etc
HTH
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