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Classes Without Boilerplate

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attrs: Classes Without Boilerplate

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attrs is the Python package that will bring back the joy of writing classes by relieving you from the drudgery of implementing object protocols (aka dunder methods).

Its main goal is to help you to write concise and correct software without slowing down your code.

For that, it gives you a class decorator and a way to declaratively define the attributes on that class:

>>> import attr

>>> @attr.s
... class SomeClass(object):
...     a_number = attr.ib(default=42)
...     list_of_numbers = attr.ib(default=attr.Factory(list))
...
...     def hard_math(self, another_number):
...         return self.a_number + sum(self.list_of_numbers) * another_number


>>> sc = SomeClass(1, [1, 2, 3])
>>> sc
SomeClass(a_number=1, list_of_numbers=[1, 2, 3])

>>> sc.hard_math(3)
19
>>> sc == SomeClass(1, [1, 2, 3])
True
>>> sc != SomeClass(2, [3, 2, 1])
True

>>> attr.asdict(sc)
{'a_number': 1, 'list_of_numbers': [1, 2, 3]}

>>> SomeClass()
SomeClass(a_number=42, list_of_numbers=[])

>>> C = attr.make_class("C", ["a", "b"])
>>> C("foo", "bar")
C(a='foo', b='bar')

After declaring your attributes attrs gives you:

  • a concise and explicit overview of the class’s attributes,
  • a nice human-readable __repr__,
  • a complete set of comparison methods,
  • an initializer,
  • and much more,

without writing dull boilerplate code again and again and without runtime performance penalties.

This gives you the power to use actual classes with actual types in your code instead of confusing tuples or confusingly behaving namedtuples. Which in turn encourages you to write small classes that do one thing well. Never again violate the single responsibility principle just because implementing __init__ et al is a painful drag.

Testimonials

Amber Hawkie Brown, Twisted Release Manager and Computer Owl:

Writing a fully-functional class using attrs takes me less time than writing this testimonial.

Glyph Lefkowitz, creator of Twisted, Automat, and other open source software, in The One Python Library Everyone Needs:

I’m looking forward to is being able to program in Python-with-attrs everywhere. It exerts a subtle, but positive, design influence in all the codebases I’ve see it used in.

Kenneth Reitz, author of requests, Python Overlord at Heroku, on paper no less:

attrs—classes for humans. I like it.

Łukasz Langa, prolific CPython core developer and Production Engineer at Facebook:

I’m increasingly digging your attr.ocity. Good job!

Getting Help

Please use the python-attrs tag on StackOverflow to get help.

Answering questions of your fellow developers is also great way to help the project!

Project Information

attrs is released under the MIT license, its documentation lives at Read the Docs, the code on GitHub, and the latest release on PyPI. It’s rigorously tested on Python 2.7, 3.4+, and PyPy.

If you’d like to contribute you’re most welcome and we’ve written a little guide to get you started!

Release Information

17.3.0 (2017-11-08)

Backward-incompatible Changes

  • Attributes are not defined on the class body anymore.

    This means that if you define a class C with an attribute x, the class will not have an attribute x for introspection anymore. Instead of C.x, use attr.fields(C).x or look at C.__attrs_attrs__. The old behavior has been deprecated since version 16.1. (#253)

Changes

  • super() and __class__ now work on Python 3 when slots=True. (#102, #226, #269, #270, #272)

  • Added type argument to attr.ib() and corresponding type attribute to attr.Attribute.

    This change paves the way for automatic type checking and serialization (though as of this release attrs does not make use of it). In Python 3.6 or higher, the value of attr.Attribute.type can alternately be set using variable type annotations (see PEP 526). (#151, #214, #215, #239)

  • The combination of str=True and slots=True now works on Python 2. (#198)

  • attr.Factory is hashable again. (#204)

  • Subclasses now can overwrite attribute definitions of their superclass.

    That means that you can – for example – change the default value for an attribute by redefining it. (#221, #229)

  • Added new option auto_attribs to @attr.s that allows to collect annotated fields without setting them to attr.ib().

    Setting a field to an attr.ib() is still possible to supply options like validators. Setting it to any other value is treated like it was passed as attr.ib(default=value) – passing an instance of attr.Factory also works as expected. (#262, #277)

  • Instances of classes created using attr.make_class() can now be pickled. (#282)

Full changelog.

Credits

attrs is written and maintained by Hynek Schlawack.

The development is kindly supported by Variomedia AG.

A full list of contributors can be found in GitHub’s overview.

It’s the spiritual successor of characteristic and aspires to fix some of it clunkiness and unfortunate decisions. Both were inspired by Twisted’s FancyEqMixin but both are implemented using class decorators because sub-classing is bad for you, m’kay?

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