On 20 May 2017 at 10:19, Steven D'Aprano email@example.com wrote:
On Fri, May 19, 2017 at 11:24:53AM -0700, Guido van Rossum wrote:
Easily allow to specify a conversion function. For example I have some code like below: note that I can store a numpy array while keeping hashability and I can make it convert to a numpy array in the constructor.
@attr.s(cmp=False, hash=False) class SvgTransform(SvgPicture): child = attr.ib() matrix = attr.ib(convert=numpy.asarray)
I find that completely enigmatic, there's far too much implicit behaviour going on behind the scenes. I couldn't even begin to guess what SvgTransform as a class does, or what SvgTransform.child and SvgTransform.matrix are.
I suppose that's okay for experts to whom the attrs module is second nature, but I think this approach is far too "magical" for my tastes.
Some of the key problems I personally see are that attrs reuses a general noun (attributes) rather than using other words that are more evocative of the "data record" use case, and many of the parameter names are about "How attrs work" and "How Python magic methods work" rather than "Behaviours I would like this class to have".
That's fine for someone that's already comfortable writing those behaviours by hand and just wants to automate the boilerplate away (which is exactly the problem that attrs was written to solve), but it's significantly more problematic once we assume people will be using a feature like this before learning how to write out all the corresponding boilerplate themselves (which is the key additional complication that a language level version of this will have to account for).
However, consider instead the following API sketch:
from autoclass import data_record, data_field @data_record(orderable=False, hashable=False) class SvgTransform(SvgPicture): child = data_field() matrix = data_field(setter=numpy.asarray)
Here, the core concepts to be learned would be:
In this particular API sketch, data_record is just a class decorator factory, and data_field is a declarative helper type for use with that factory, so if you wanted to factor out particular combinations, you'd just write ordinary helper functions.
Instead of trying to cover every possible use-case from a single decorator with a multitude of keyword arguments, I think covering the simple cases is enough. Explicitly overriding methods is not a bad thing! It is much more comprehensible to see an explicit class with methods than a decorator with multiple keyword arguments and callbacks.
This isn't the case for folks that have to actually read dunder methods to find out what a class does, thought. Reading an imperatively defined class only works that way once you're able to mentally pattern match "Oh, that's a conventional __init__, that's a conventional __repr__, that's a conventional __hash__, that's a conventional __eq__, that's a conventional __lt__ implementation, etc, etc".
Right now, telling Python "I want to do the same stock-standard things that everyone always does" means writing a lot of repetitive logic (or, more likely, copying the logic from an existing class that you or someone else wrote, and editing it to fit).
The idea behind offering some form of declarative class definitions is to build out a vocabulary of conventional class behaviours, and make that vocabulary executable such that folks can use it to write applications even if they haven't learned how it works under the hood yet. As with descriptors before it, that vocabulary may also take advantage of the fact that Python offers first class functions to allow callbacks and transformation functions to be injected at various steps in the process without requiring you to also spell out all the other steps in the process that you don't want to alter.
I like the namedtuple approach: I think it hits the sweet spot between "having to do everything by hand" and "everything is magical".
It's certainly a lot better than nothing at all, but it brings a lot of baggage with it due to the fact that it is a tuple. Declarative class definitions aim to offer the convenience of namedtuple definitions, without the complications that arise from the "it's a tuple with some additional metadata and behaviours" aspects.
Database object-relational-mapping layers like those in SQL Alchemy and Django would be the most famous precursors for this, but there are also things like Django Form definitions, and APIs like JSL (which uses Python classes to declaratively define JSON Schema documents).
For folks already familiar with ORMs, declarative classes are just a matter of making in memory data structures as easy to work with as database backed ones. For folks that aren't familiar with ORMs yet, then declarative classes provide a potentially smoother learning curve, since the "declarative class" aspects can be better separated from the "object-relational mapping" aspects.
-- Nick Coghlan | firstname.lastname@example.org | Brisbane, Australia