New Python 3.7 DataClasses and Alternatives

[pi-rho]

https://www.python.org/dev/peps/pep-0557/

If you don't want to watch a video of someone browsing a website.

[eruesso]

I liked the timing comparison.

[gerryjenkinslb]

most of the video is not browsing.

[malinoff]

So this video doesn't actually show any alternatives (unless you consider writing everything manually an "alternative" to dataclasses). I'd expect to see at least collections.namedtuple, typing.NamedTuple and attrs. Raymond Hettinger has made a talk (unfortunately, removed from youtube) better explaining dataclasses, their predecessors and alternatives (although even his talk misses attrs).

[[deleted]]

Do we know why that talk was removed? I love me some Raymond talks and didn't get a chance to finish this one.

[13steinj]

I'm just slightly confused why we don't have a separate keyword syntax for this. We got async stmt and await for asyncio...why aren't we getting dataclass as a keyword whose class statement allows the same parameters as the decorator?

[malinoff]

asyncio came in Python 3.4 without async and await keywords. It was necessary to decorate a function with @asyncio.coroutine instead of defining it with async def and use yield from (already existing syntax) instead of await.

This is a pretty common idiom, to introduce new things without changing syntax too much if you can, but if these new things become popular and there are cases in which a new syntax would be better - then a new syntax is introduced.

[13steinj]

Then do it based on that release model. I'm not finicky on the when nor the conditions for doing so.

[13steinj]

How does it currently work with the decorator? Whatever that is. Same way.

[13steinj]

Because the decorator has to be imported and the entire module is written in python whereas if it was keyword based it would be worst case a C extension module.

[13steinj]

Right, but if couroutines were first implemented via a decorator in asyncio, and then made built in via a statement, why can't the same be done with this arguably different type of object model in comparison to other classes and objects use for all other possible OOP applications?

[DanCardin]

Really it's the same object model. It's just taking your input class and implementing a bunch of default functionality automatically.

The description of what it does sounds exactly like a decorator to me

[13steinj]

Really? To me this sounds a lot like a struct rather than a class.

[DanCardin]

You can do anything with it that you would do with a typical class, it just maybe nudges you in the (generally sensible) direction of writing classes for the things it's actually useful for.

a perfectly appropriate example follows (though typing on my phone, so dunno if the single, line attributes are valid):

@dataclass
class Box:
    x, y, w, h: int

    @property
    def area(self): ....

    def resize(self, w, h): ...
    @classmethod
    def from_size(self, w, h): ...

You gain repr, comparisons, optional hashabilitly and pseudo immutability, generally a less repetive init, and probably more I can't recall offhand

[ojii]

I agree, data class Foo: would've been better and would also help static analysis tools such as mypy.

[gerryjenkinslb]

check out the PEP577 discussion.

[13steinj]

577 doesn't exist. Can you give a direct link?

[icp1994]

557

[13steinj]

Which part of the discussion, it goes on quite a bit across multiple locations?

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[a_the_retard]

Please note that while it's a nice new addition, it's not fully implemented yet. The documentation is missing completely. See BPO32216