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Classes Without Boilerplate

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attrs Logo

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 Point(object):
...     x = attr.ib(default=42)
...     y = attr.ib(default=attr.Factory(list))
...     def hard_math(self, z):
...         return self.x * self.y * z
>>> pt = Point(x=1, y=2)
>>> pt
Point(x=1, y=2)
>>> pt.hard_math(3)
>>> pt == Point(1, 2)
>>> pt != Point(2, 1)
>>> attr.asdict(pt)
{'x': 1, 'y': 2}
>>> Point()
Point(x=42, y=[])
>>> 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.


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.

—Glyph Lefkowitz, creator of Twisted and Software Developer at Rackspace in The One Python Library Everyone Needs

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

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

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

—Amber Hawkie Brown, Twisted Release Manager and Computer Owl

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

17.1.0 (2017-05-16)

To encourage more participation, the project has also been moved into a dedicated GitHub organization and everyone is most welcome to join!

attrs also has a logo now!

attrs logo

Backward-incompatible changes:

  • attrs will set the __hash__() method to None by default now. The way hashes were handled before was in conflict with Python’s specification. This may break some software although this breakage is most likely just surfacing of latent bugs. You can always make attrs create the __hash__() method using @attr.s(hash=True). See #136 for the rationale of this change.

  • Correspondingly, attr.ib’s hash argument is None by default too and mirrors the cmp argument as it should.


  • attr.assoc() is now deprecated in favor of attr.evolve() and will stop working in 2018.


  • Fix default hashing behavior. Now hash mirrors the value of cmp and classes are unhashable by default. #136 #142

  • Added attr.evolve() that, given an instance of an attrs class and field changes as keyword arguments, will instantiate a copy of the given instance with the changes applied. evolve() replaces assoc(), which is now deprecated. evolve() is significantly faster than assoc(), and requires the class have an initializer that can take the field values as keyword arguments (like attrs itself can generate). #116 #124 #135

  • FrozenInstanceError is now raised when trying to delete an attribute from a frozen class. #118

  • Frozen-ness of classes is now inherited. #128

  • __attrs_post_init__() is now run if validation is disabled. #130

  • Added attr.validators.in_(options) that, given the allowed options, checks whether the attribute value is in it. This can be used to check constants, enums, mappings, etc. #181

  • Added attr.validators.and_() that composes multiple validators into one. #161

  • For convenience, the validator argument of @attr.s now can take a list of validators that are wrapped using and_(). #138

  • Accordingly, attr.validators.optional() now can take a list of validators too. #161

  • Validators can now be defined conveniently inline by using the attribute as a decorator. Check out the examples to see it in action! #143

  • attr.Factory() now has a takes_self argument that makes the initializer to pass the partially initialized instance into the factory. In other words you can define attribute defaults based on other attributes. #165

  • Default factories can now also be defined inline using decorators. They are always passed the partially initialized instance. #165

  • Conversion can now be made optional using attr.converters.optional(). #105 #173

  • attr.make_class() now accepts the keyword argument bases which allows for subclassing. #152

  • Metaclasses are now preserved with slots=True. #155

Full changelog.


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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