[Python-ideas] JavaScript-Style Object Creation in Python (using a constructor function instead of a class to create objects)

Michel Desmoulin desmoulinmichel at gmail.com
Wed May 17 04:15:17 EDT 2017

Le 17/05/2017 à 07:22, Guido van Rossum a écrit :
> On Tue, May 16, 2017 at 8:14 PM, Juancarlo Añez <apalala at gmail.com
> <mailto:apalala at gmail.com>> wrote:
>     What I like about attrs is:
>       * The class level declaration of instance attributes
>       * That the reasonable *init*, *repr*, and *eq* are generated
> OK, the former should be doable using PEP 526 (the type is stored in
> __annotations__ and the default in the class dict given to the
> metaclass), while the latter should be doable using a standard metaclass
> -- assuming we can agree on what the "reasonable" __init__, __repr__ and
> __eq__ should do.
>     I don’t like the excessive wordiness in attrs,
> Really? @attr.s is wordy? :-) I think it's deadly cute. (The only
> library I've ever seen that did something worse was "monocle" which used
> @_o.)

>>> import attr
>>> @attr.s
... class Point:
...     x = attr.ib(default=42)
...     y = attr.ib(default=attr.Factory(list))

Is pretty wordy compared to something like another language would do
such as:

class Point:
   int x = 42
   list y = []

Now I get that:

- Python already has a similar syntax creating class attributes.
- You can't put mutable objects here.
- attr does more since it generates dunder methods.

But having an import, a decorator and verbose calls to attr.ib does not
feel like idiomatic Python at all. Python is an elegant and expressive
language. This is none of the above. Also Python is beginner friendly.
Now OPP is already hard to teach to my students, but if I have to add
this to the mix, I will just have to tell them to copy / paste it
blindly for a long time before I can get to the point they can
understand what it does.

We should be able to find a middle ground.

First, if we have something LIKE attr, should it need an import? Basic
data structures may not require an import to work. async/await are way
better than import @asyncio.coroutine.

Secondly, while I really, really, really want this feature, I think we
should not rush it.

Some ideas have to be explored.


Adding keywords for it ? I know adding a keyword is the worst thing one
can suggest on this list ever. But it has to be mentioned because most
other languages do it this way.

class Point:
    instancevar x = 42
    instancevar y = lazy [] # we add a debate about this a few months ago

Adding a special syntax for it ? ruby has something similar.

class Point:
    @x = 42
    @@y = list

Upgrading the class constructor? It does not address the mutablility
issue though.

class Point(x=42, y=[])

Mixing concepts?

class Point(metaclass=autoclass(x=42, y=lazy [])):

class Point:

class Point:

Just adding attrs, which is a workaround to a missing feature in Python,
as a boiler plate and calling it a day seems unwise.

Don't get me wrong, I like attrs, I like asyncio and I like the whole
battery included concept. But we lived without it until now, so let's
not go too fast on this.

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