On Tue, May 16, 2017 at 8:14 PM, Juancarlo Añez <apalala@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.)

and I don’t need “the kitchen sink” be available to have instance attributes declared at the class level. A solution based on the typing module would be much better.

That's what I am hoping, yes.

Basically, Python is lacking a way to declare instance fields with default values away of the initializer. Several of the mainstream OO languages (Java, Swift, Go) provide for that.

Hm, there are some issues here of course -- while it's simple to set the default to e.g. 0, (1, 2, 3) or '<string>', it's not so easy to set a default to [] or {'foo': 'bar'} unless you just state "do whatever copy.copy() does".

I haven’t thought much about this, except about if there’s indeed a need (and there is), but I can’t know if the solution if through decorators, or inheritance.

I suppose we could do it using either a class decorator or a metaclass -- we'll have to compare the pros and cons and specific use cases to choose. (Example: https://github.com/python/typing/issues/427.)

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