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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.

We collect information on third-party extensions in our wiki. Feel free to browse and add your own!

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

Release Information

18.1.0 (2018-05-03)

Changes

  • x=X(); x.cycle = x; repr(x) will no longer raise a RecursionError, and will instead show as X(x=...).

    #95

  • attr.ib(factory=f) is now syntactic sugar for the common case of attr.ib(default=attr.Factory(f)).

    #178, #356

  • Added attr.field_dict() to return an ordered dictionary of attrs attributes for a class, whose keys are the attribute names.

    #290, #349

  • The order of attributes that are passed into attr.make_class() or the these argument of @attr.s() is now retained if the dictionary is ordered (i.e. dict on Python 3.6 and later, collections.OrderedDict otherwise).

    Before, the order was always determined by the order in which the attributes have been defined which may not be desirable when creating classes programatically.

    #300, #339, #343

  • In slotted classes, __getstate__ and __setstate__ now ignore the __weakref__ attribute.

    #311, #326

  • Setting the cell type is now completely best effort. This fixes attrs on Jython.

    We cannot make any guarantees regarding Jython though, because our test suite cannot run due to dependency incompatabilities.

    #321, #334

  • If attr.s is passed a these argument, it will not attempt to remove attributes with the same name from the class body anymore.

    #322, #323

  • The hash of attr.NOTHING is now vegan and faster on 32bit Python builds.

    #331, #332

  • The overhead of instantiating frozen dict classes is virtually eliminated. #336

  • Generated __init__ methods now have an __annotations__ attribute derived from the types of the fields.

    #363

  • We have restructured the documentation a bit to account for attrs’ growth in scope. Instead of putting everything into the examples page, we have started to extract narrative chapters.

    So far, we’ve added chapters on initialization and hashing.

    Expect more to come!

    #369, #370

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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