Data Classification

Is there a place for non-@dataclass classes in Python any more?

I have previously — and somewhat famously — written favorably about @dataclass’s venerable progenitor, attrs, and how you should use it for pretty much everything.

At the time, attrs was an additional dependency, a piece of technology that you could bolt on to your Python stack to make your particular code better. While I advocated for it strongly, there are all the usual implicit reasons against using a new thing. It was an additional dependency, it might not interoperate with other convenience mechanisms for type declarations that you were already using (i.e. NamedTuple), it might look weird to other Python programmers familiar with existing tools, and so on. I don’t think that any of these were good counterpoints, but there was nevertheless a robust discussion to be had in addressing them all.

But for many years now, dataclasses have been — and currently are — built in to the language. They are increasingly integrated to the toolchain at a deep level that is difficult for application code — or even other specialized tools — to replicate. Everybody knows what they are. Few or none of those reasons apply any longer.

For example, classes defined with @dataclass are now optimized as a C structure might be when you compile them with mypyc, a trick that is extremely useful in some circumstances, which even attrs itself now has trouble keeping up with.

This all raises the question for me: beyond backwards compatibility, is there any point to having non-@dataclass classes any more? Is there any remaining justification for writing them in new code?

Consider my original example, translated from attrs to dataclasses. First, the non-dataclass version:

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class Point3D:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

And now the dataclass one:

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from dataclasses import dataclass

@dataclass
class Point3D:
    x: int
    y: int
    z: int

Many of my original points still stand. It’s still less repetitive. In fewer characters, we’ve expressed considerably more information, and we get more functionality (repr, sorting, hashing, etc). There doesn’t seem to be much of a downside besides the strictness of the types, and if typing.Any were a builtin, x: any would be fine for those who don’t want to unduly constrain their code.

The one real downside of the latter over the former right now is the need for an import. Which, at this point, just seems… confusing? Wouldn’t it be nicer to be able to just write this:

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class Point3D:
    x: int
    y: int
    z: int

and not need to faff around with decorator semantics and fudging the difference between Mypy (or Pyright or Pyre) type-check-time and Mypyc or Cython compile time? Or even better, to not need to explain the complexity of all these weird little distinctions to new learners of Python, and to have to cover import before class?

These tools all already treat the @dataclass decorator as a totally special language construct, not really like a decorator at all, so to really explore it you have to explain a special case and then a special case of a special case. The extension hook for this special case of the special case notwithstanding.

If we didn’t want any new syntax, we would need a from __future__ import dataclassification or some such for a while, but this doesn’t seem like an impossible bar to clear.

There are still some folks who don’t like type annotations at all, and there’s still the possibility of awkward implicit changes in meaning when transplanting code from a place with dataclassification enabled to one without, so perhaps an entirely new unambiguous syntax could be provided. One that more closely mirrors the meaning of parentheses in def, moving inheritance (a feature which, whether you like it or not, is clearly far less central to class definitions than ‘what fields do I have’) off to its own part of the syntax:

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data Point3D(x: int, y: int, z: int) from Vector:
    def method(self):
        ...

which, for the “I don’t like types” contingent, could reduce to this in the minimal case:

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data Point3D(x, y, z):
    pass

Just thinking pedagogically, I find it super compelling to imagine moving from teaching def foo(x, y, z):... to data Foo(x, y, z):... as opposed to @dataclass class Foo: x: int....

I don’t have any desire for semantic changes to accompany this, just to make it possible for newcomers to ignore the circuitous historical route of the @dataclass syntax and get straight into defining their own types with legible reprs from the very beginning of their Python journey.

(And make it possible for me to skip a couple of lines of boilerplate in short examples, as a bonus.)

I’m curious to know what y’all think, though. Shoot me an email or a toot and let me know.

In particular:

1. Do you think there’s some reason I’m missing why Python’s current method for defining classes via a bunch of dunder methods is still better than dataclasses, or should stick around into the future for reasons beyond “compatibility”?
2. Do you think “compatibility” is sufficient reason to keep the syntax the way it is forever, and I’m underestimating the cost of adding a keyword like this?
3. If you do think that a change should be made, would you prefer:
   1. changing the meaning of class itself via a __future__ import,
   2. a new data keyword like the one I’ve proposed,
   3. a new keyword that functions exactly like the one I have proposed but really want to bikeshed the word data a bunch,
   4. something more incremental like just putting dataclass and field in builtins,
   5. or an option I haven’t even contemplated here?

If I find I’m not alone in this perhaps I will wander over to the Python discussion boards to have a more substantive conversation...

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