On Mon, May 15, 2017 at 9:50 AM, Brett Cannon <brett@python.org> wrote:

On Mon, 15 May 2017 at 08:30 Guido van Rossum <guido@python.org> wrote:
This should be worked into a PEP, instead of living on as a bunch of python-ideas posts and blogs.

I find the attrs documentation (and Glyph's blog post about it) almost unreadable because of the exalted language -- half the doc seems to be *selling* the library more than *explaining* it. If this style were to become common I would find it a disturbing trend.

But having something alongside NamedTuple that helps you declare classes with mutable attributes using the new PEP 526 syntax (and maybe a few variants) would definitely be useful. Will someone please write a PEP? Very few of the specifics of attrs need be retained (its punny naming choices are too much for the stdlib).

In case someone decides to take this on, I wrote a blog post back in March that shows how to use __init_subclass__() to do a rough approximation of what Guido is suggesting: https://snarky.ca/customizing-class-creation-in-python/ .

Based on my thinking on the topic while writing my blog post, the tricky bit is going to be deciding how to handle default values (i.e. if you set a default value like `attr: int = 42` on the class definition then you have `cls.attr` exist which might not be what you want if you would rather have the default value explicitly set on every instance but not fall through to the class (e.g. `del ins.attr; ins.attr` raises an AttributeError instead of falling through to `cls.attr`). You could remove the default from the class in your __init_subclass__(), but then you have to decide if that's too unexpected/magical for someone looking at the code.

I would personally prefer the initializer to stay in the class in cases like this. If the initializer needs to be a default instance of a mutable class (e.g. an empty list or dict) there could be a special marker to indicate that, e.g.

  attacks: List[int] = MAKE_NEW  # Creates a new [] for each instance

while if the default needs to be something more custom it could be a similar marker with a callable argument, e.g.

  fleet: Dict[str, str] = MAKE_NEW(lambda: {'flagship': 'Enterprise'})

I would prefer not to have cleverness like initialization with a callable automatically does something different.
 
And I too would be interested in seeing something like this, if for any other reason than to help people not to misuse NamedTuple for quick-and-dirty data objects in new APIs (NamedTuple is meant to help move old-style tuple-based APIs to a class-based one).

Not sure I agree that is its only purpose.

--
--Guido van Rossum (python.org/~guido)