Attrs – the Python library everyone needs (2016)

divbzero|4 years ago

The @dataclass decorator [1] as proposed in PEP 557 [2] has also been available since Python 3.7 was released in 2018.

Using @dataclass the example from OP would look like:

  from dataclasses import dataclass
  
  @dataclass
  class Point3D:
      x: float
      y: float
      z: float

[1]: https://docs.python.org/3/library/dataclasses.html

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

hamstergene|4 years ago

As soon as I opened the article I pressed Cmd+F and searched for "dataclasses".

This article was correct and addressed a very real need in Python programming—for year 2016. By now it is obsolete and today's standard library module `dataclasses` does all of that and more.

skrtskrt|4 years ago

IMO Pydantic is way more ergonomic, has great defaults, and easier to bend to your will when you want to use it a little differently.

Lots of love to Attrs, which is a great library and is a component of a lot of great software. It was my go-to library for years before Pydantic matured, but I think a lot of people have rightly started to move on to Pydantic, particularly with the popularity of FastAPI

ledauphin|4 years ago

I'd say the opposite. Specifically, Pydantic tries to do everything, and as a result (partly b/c they favor base classes over higher order functions), it isn't as composable as attrs is.

I've done some truly amazing things with attrs because of that composability. If I'd wanted the same things with Pydantic, it would have had to be a feature request.

sterlinm|4 years ago

I think this was on HN at some point, but this article makes the case for why/when you’d want to use attrs rather than pydantic. https://threeofwands.com/why-i-use-attrs-instead-of-pydantic...

kissgyorgy|4 years ago

Apples and oranges. pydantic is a serialization and input validation library, attrs is a class-generator library. Completely different features, completely different use cases.

Yes, you can do validation in attrs, but it's not meant to be used the same way as pydantic. For serialization, you need cattrs, which is a completely different package.

ahurmazda|4 years ago

Do you have concern with speed or memory footprint of pydantic compared to the rest (attrs, dataclasses etc)? Pydantic seems insistent on parsing/validating the types at runtime (which makes good sense for something like FastAPI).

kaashif|4 years ago

Pydantic is dataclasses, except types are validated at runtime? It's nice and looks just like a normal dataclass looking at https://pydantic-docs.helpmanual.io/

For any larger program, pervasive type annotations and "compile" time checking with mypy is a really good idea though, which somewhat lessens the need for runtime checking.

joshuamorton|4 years ago

Pydantic is useful if you're dealing with parsing unstructured (or sort of weakly untrusted) data. If you just want "things that feel like structs", dataclassess or attrs are going to be just as easy and more performant (and due to using decorators and not metaclasses, more capable of playing nicely with other things).

tddispointless|4 years ago

I used attrs in a large python project, and it was more pain than it was worth. Off the top of my head, knowing the difference between factory vs default in the initializer was a bug that bit us, inheritance was painful because we were forced to redefine the attrs from the parent class in each child class (maybe they fixed this now). Validators were broken somehow. I even made a GitHub issue for this which was never addressed. Attrs are good for simple stuff. I feel plain old classes are more reliable once things get complex. And for simple things, dataclasses do just fine too.

nauticacom|4 years ago

I've never understood the appeal of these "define struct-like-object" libraries (in any language; I've never understood using the standard library's "Struct" in Ruby). My preferred solution for addressing complexity is also to decompose the codebase into small, understandable, single-purpose objects, but so few of them end up being simple value objects like Point3D. Total ordering and value equality make sense for value objects but not much else, so it really doesn't improve understandability or maintenance that much. And concerns like validation I would never want to put in a library for object construction. In web forms where there are a limited subset of rules I always want to treat the same way, sure, but objects have much more complicated relationships with their dependencies that I don't see much value in validating them with libraries.

Overall, I really don't see the appeal. It makes the already simple cases simpler (was that Point3D implementation really that bad?) and does nothing for the more complicated cases which make up the majority of object relationships.

joshuamorton|4 years ago

Ignore all of the validation aspects. In python, you have tuples, (x, y, z), then you have namedtuples and then attrs/dataclasses/pydantic-style shorthand classes.

These are useful even if only due to the "I can take the three related pieces of information I have and stick them next to each other". That is, if I have some object I'm modelling and it has more than a single attribute (a user with a name and age, or an event with a timestamp and message and optional error code), I have a nice way to model them.

Then, the important thing is that these are still classes, so you can start with

    @dataclass
    class User:
        name: str
        age: int

and have that evolve over time to

    @dataclass
    class User:
        name: str
        age: int
        ...
        permissions: PermissionSet
        
        @property
        def location():
            # send off an rpc, or query the database for some complex thing.

and since it's still just a class, it'll still work. It absolutely makes modelling the more complex cases easier too.

atorodius|4 years ago

well one appeal is that you dont have to write constructors, that‘s already enough of a win for me. then you get sane eq, and sane str, and already you remove 90% boilerplate

unknown|4 years ago

[deleted]

NeutralForest|4 years ago

For anyone interested in differences between attrs and pydantic, how to use dataclasses, etc. I can't recommend the mCoding channel enough, this video : https://www.youtube.com/watch?v=vCLetdhswMg goes into the different libraries and there are other videos going more in depth on how to use them.

gjvc|4 years ago

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