Backport python 3.7 dataclass

[dileep-kishore]

A new dataclass decorator was recently accepted into python 3.7 (https://www.python.org/dev/peps/pep-0557/).

The implementation seems similar to coconut's implementation of the data type (subclassing the namedtuple) but they differ in terms of their syntax. I was just wondering whether coconut would keep the current syntax (for backward compatibility) or would it be changing to the new syntax (to avoid feature duplication). I just asked this question because I'm curious.

[evhub]

@dileep-kishore The problem with dataclass from a functional standpoint is that data classes are mutable. Coconut's data types, on the other hand, are immutable. That's very important if you're doing functional programming, so dataclass won't be replacing Coconut's data types.

[dileep-kishore]

@evhub you're right. I assumed they inherited from namedtuples but it looks like they just create normal classes.

[benji-york]

The dataclass decorator accepts a "freeze" argument which makes the instances read-only (well, as read-only as anything gets in Python).

On Fri, Dec 8, 2017 at 3:27 PM, Dileep Kishore [email protected] wrote:

@evhub https://github.com/evhub you're right. I assumed they inherited from namedtuples but it looks like they just create normal classes.

— You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub https://github.com/evhub/coconut/issues/376#issuecomment-350376198, or mute the thread https://github.com/notifications/unsubscribe-auth/AAAkrw92l2B-HryokOOuaZNCODXzhnbfks5s-amkgaJpZM4Q7lbY .

-- Benji York

[evhub]

@benji-york Really? I didn't know that. I don't see it in the PEP, though--where's the source for that?

[evhub]

Though dataclass won't be replacing data, I think it would be really nice to have Coconut port dataclass to work on any Python version, so I'm going to reopen this issue with that in mind.

[dileep-kishore]

@evhub This is what it says:

"frozen: If true (the default is False), assigning to fields will generate an exception. This emulates read-only frozen instances."

[evhub]

@dileep-kishore Ah, I see. Yeah, I'd definitely like to use Coconut to port dataclass to older Python versions, though I still won't be removing data, since there are still things like data Tuple(*args) that can't be done with dataclass.

[Hong-Xiang]

For older versions of Python, we can get same functionality of dataclasses via attrs, as this says, it support PyPi, Python 2.7, 3.4+.

It supports

• frozen,
• __slot__,
• typed __eq__

import attr

@attr.s(auto_attribs=True)
class vector20:
    x: int
    y: int

@attr.s(auto_attribs=True)
class vector21:
    x: int
    y: int


print(vector20(0, 1) == vector20(0, 1)) # return True
print(vector20(0, 1) == vector21(0, 1)) # return False

and lots of other features.

I'm wondering if it is allowed to add this package as dependency of coconut?

If it is allowed, I'm wondering how to learn about the compiling process of data. I've found data_handle in L1202 of compiler/compiler.py, and it seems that for basic data support, only block after L1331 elif saw_defaults: should be modified, but there I'm confused of matching simple, docstring, e.t.c in stmts. How to learn this block of code? I can't add break point to compiler, and adding print statement seems not work.

On the other hand, May data support annotations from [PEP 526]? Thus like

data vector2:
    x: int = 0
    y: int = 0

[majidaldo]

Q: is it possible to have different compilation 'backends' for 'data'? like dataclass(frozen=True) or attr.s(frozen=True)? They have a good amount of ecosystem support and utilities.

[evhub]

@majidaldo What sort of ecosystem support are you referring to? Can you provide an example of a situation where an attr/dataclass-based data would be able to do something that a namedtuple-based data wouldn't?

[majidaldo]

@majidaldo What sort of ecosystem support are you referring to? Can you provide an example of a situation where an attr/dataclass-based data would be able to do something that a namedtuple-based data wouldn't?

auto serialization, evolution, conversion.

A user could probably implement these in coconut code. But, it would be nice to be able to (declaratively) create a 'schema' in Python which can serve as an interop b/w coconut and python. coconut would just need to add fmap (?).

[evhub]

serialization, evolution, conversion

@majidaldo .asdict() is already available as ._asdict(). Field transformation is possible via pattern-matching (see below) or writing a __new__ and updating is possible via ._replace. cattrs support is the one thing there I'm not sure if data types can do, since it seems like cattrs might not support namedtuples—if there's something Coconut can do to give data types cattrs support, I'd definitely be willing to look into it. My guess is that's probably more on the cattrs end, though—all they should need to do is support namedtuples, though.

Here's field transformation via pattern-matching, btw:

>>> data lenient_2vec(int -> x, int -> y)
>>> lenient_2vec("1", 2.5)
lenient_2vec(x=1, y=2)

[majidaldo]

serialization, evolution, conversion

@majidaldo .asdict() is already available as ._asdict(). Field transformation is possible via pattern-matching (see below) or writing a __new__ and updating is possible via ._replace. cattrs support is the one thing there I'm not sure if data types can do, since it seems like cattrs might not support namedtuples—if there's something Coconut can do to give data types cattrs support, I'd definitely be willing to look into it. My guess is that's probably more on the cattrs end, though—all they should need to do is support namedtuples, though.

Here's field transformation via pattern-matching, btw:

>>> data lenient_2vec(int -> x, int -> y)
>>> lenient_2vec("1", 2.5)
lenient_2vec(x=1, y=2)

._asdict() isn't recursive though. an effect that i've been able to achieve with attrs is to fully define a data structure for my programs that represent state that can be programmatically 'lifted' and 'exported' to whatever...structure intact.

[majidaldo]

more comprehensively https://github.com/python-attrs/attrs/wiki/Extensions-to-attrs. on another note, attrs just completely solves what you might want out of dataclasses.

[evhub]

on another note, attrs just completely solves what you might want out of dataclasses.

@majidaldo Perhaps the better question, then, since you can just import attrs in Coconut, is what features from data types you would like to be available when using attrs.