I am extremely excited to announce the release of attrs 21.1.0.

attrs is the direct ancestor of – and the inspiration for – dataclasses in the standard library and remains the more powerful option for creating regular classes without getting bogged down with writing identical boilerplate again and again: https://www.attrs.org/

Heartfelt thanks go to my generous GitHub sponsors, companies subscribing to attrs on Tidelift, and people who bought me a coffee on Ko-fi! Support like that makes me work on FOSS on a Saturday afternoon – especially when a release drags itself like this one! <3

While this release took a bit longer than I wished for, it comes with many exciting changes. The highlights alone are longer than a usual changelog:

• The next-generation APIs (@attr.define, @attr.mutable, @attr.frozen, @attr.field) are deemed stable now. The old ones aren't going anywhere, but I encourage you to check the new ones out – they're much nicer!

• pyright and pylance support: Eric Traut of Microsoft was kind enough to involve me in their work on the dataclass_transforms spec.

  As a result, Microsoft's type checker pyright will work with this attrs release, and so will their Python language server pylance which should be exciting to VS Code users.

  Currently it only supports a subset of attrs's features, but it's the most important ones and more will most likely follow. Some of the limitations are documented in our documentation on type annotations.

• Customization of field comparison. This is something especially NumPy users have been asking for for a long time: you can now fully customize how a field is compared. We also ship a helper to avoid boilerplate code. So if you'd like to have an object with a NumPy array that compares correctly, this is the way:

  import attr
  import numpy
  
  @attr.define
  class C:
      an_array = attr.field(eq=attr.cmp_using(eq=numpy.array_equal))

  Check out the new documentation on comparison for details.

• To make it more ergonomic, I've decided to un-deprecate the cmp argument again, so you can customize eq and order in one go. Sorry about the trouble! The cmp attribute remains deprecated.

• New powerful __init__ helpers:

  1. If attrs deduces you don't want it to write a __init__ for you, it will create an __attrs_init__ instead that you can call from your custom __init__.
  2. If attrs finds a __attrs_pre_init__, it will call it without any arguments before doing any initializations. This is really only useful if you want to run super().__init__(), but that's a use-case people have asked for for years!

  See Hooking Yourself Into Initialization for details.

• In preparation for the (rescinded) plan to make from __future__ import annotations the default in Python 3.10, attr.resolve_types() can now also be used to resolve types inside of field_transformers.

A Look Ahead

For the next release we've got even bigger plans! By stabilizing the next-generation APIs we can finally go the last step, I've been talking for years (yeah, sorry): import attrs.

attrs's playful APIs (@attr.s, @attr.ib) lost a bit of their charm as the scope of the package grew – especially after the introduction of type annotations.

While the old APIs aren't going anywhere, in the next feature release there will be additionally an attrs package that you can use as an alternative to attr. No more attr.Factory!

The new package gives us the opportunity to rethink the defaults of some functions. So if you have any pet peeves, please air them on #487.

Full Changelog

Deprecations

• The long-awaited, much-talked-about, little-delivered import attrs is finally upon us!

  Since the NG APIs have now been proclaimed stable, the next release of attrs will allow you to actually import attrs. We're taking this opportunity to replace some defaults in our APIs that made sense in 2015, but don't in 2021.

  So please, if you have any pet peeves about defaults in attrs's APIs, now is the time to air your grievances in #487! We're not gonna get such a chance for a second time, without breaking our backward-compatibility guarantees, or long deprecation cycles. Therefore, speak now or forever hold you peace! #487

• The cmp argument to attr.s() and attr.ib() has been undeprecated It will continue to be supported as syntactic sugar to set eq and order in one go.

  I'm terribly sorry for the hassle around this argument! The reason we're bringing it back is it's usefulness regarding customization of equality/ordering.

  The cmp attribute and argument on attr.Attribute remains deprecated and will be removed later this year. #773

Changes

• It's now possible to customize the behavior of eq and order by passing in a callable. #435, #627

• The instant favorite next-generation APIs are not provisional anymore!

  They are also officially supported by Mypy as of their 0.800 release.

  We hope the next release will already contain an (additional) importable package called attrs. #668, #786

• If an attribute defines a converter, the type of its parameter is used as type annotation for its corresponding __init__ parameter.

  If an attr.converters.pipe is used, the first one's is used. #710

• Fixed the creation of an extra slot for an attr.ib when the parent class already has a slot with the same name. #718

• __attrs__init__() will now be injected if init=False, or if auto_detect=True and a user-defined __init__() exists.

  This enables users to do "pre-init" work in their __init__() (such as super().__init__()).

  __init__() can then delegate constructor argument processing to self.__attrs_init__(*args, **kwargs). #731

• bool(attr.NOTHING) is now False. #732

• It's now possible to use super() inside of properties of slotted classes. #747

• Allow for a __attrs_pre_init__() method that -- if defined -- will get called at the beginning of the attrs-generated __init__() method. #750

• Added forgotten attr.Attribute.evolve() to type stubs. #752

• attrs.evolve() now works recursively with nested attrs classes. #759

• Python 3.10 is now officially supported. #763

• attr.resolve_types() now takes an optional attrib argument to work inside a field_transformer. #774

• ClassVars are now also detected if they come from typing-extensions. #782

• To make it easier to customize attribute comparison (#435), we have added the attr.cmp_with() helper.

  See the new docs on comparison for more details. #787

• Added provisional support for static typing in pyright via the dataclass_transforms specification. Both the pyright specification and attrs implementation may change in future versions of both projects.

  Your constructive feedback is welcome in both attrs#795 and pyright#1782. #796