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Classes Without Boilerplate

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Documentation License: MIT Downloads per month DOI

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). Trusted by NASA for Mars missions since 2020!

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


attrs would not be possible without our amazing sponsors. Especially those generously supporting us at the The Organization tier and higher:

Please consider joining them to help make attrs’s maintenance more sustainable!


attrs gives you a class decorator and a way to declaratively define the attributes on that class:

>>> from attrs import asdict, define, make_class, Factory

>>> @define
... class SomeClass:
...     a_number: int = 42
...     list_of_numbers: list[int] = 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)
>>> sc == SomeClass(1, [1, 2, 3])
>>> sc != SomeClass(2, [3, 2, 1])

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

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

>>> C = 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__,
  • equality-checking methods,
  • an initializer,
  • and much more,

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

Hate type annotations!? No problem! Types are entirely optional with attrs. Simply assign attrs.field() to the attributes instead of annotating them with types.

This example uses attrs's modern APIs that have been introduced in version 20.1.0, and the attrs package import name that has been added in version 21.3.0. The classic APIs (@attr.s, attr.ib, plus their serious-business aliases) and the attr package import name will remain indefinitely.

Please check out On The Core API Names for a more in-depth explanation.

Data Classes

On the tin, attrs might remind you of dataclasses (and indeed, dataclasses are a descendant of attrs). In practice it does a lot more and is more flexible. For instance it allows you to define special handling of NumPy arrays for equality checks, or allows more ways to plug into the initialization process.

For more details, please refer to our comparison page.

Project Information

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Changes in This Release

Backwards-incompatible Changes

  • Python 3.5 is not supported anymore. #988


  • Python 3.6 is now deprecated and support will be removed in the next release. #1017


  • attrs.field() now supports an alias option for explicit __init__ argument names.

    Get __init__ signatures matching any taste, peculiar or plain! The PEP 681 compatible alias option can be use to override private attribute name mangling, or add other arbitrary field argument name overrides. #950

  • attrs.NOTHING is now an enum value, making it possible to use with e.g. typing.Literal. #983

  • Added missing re-import of attr.AttrsInstance to the attrs namespace. #987

  • Fix slight performance regression in classes with custom __setattr__ and speedup even more. #991

  • Class-creation performance improvements by switching performance-sensitive templating operations to f-strings.

    You can expect an improvement of about 5% -- even for very simple classes. #995

  • attrs.has() is now a TypeGuard for AttrsInstance. That means that type checkers know a class is an instance of an attrs class if you check it using attrs.has() (or attr.has()) first. #997

  • Made attrs.AttrsInstance stub available at runtime and fixed type errors related to the usage of attrs.AttrsInstance in Pyright. #999

  • On Python 3.10 and later, call abc.update_abstractmethods() on dict classes after creation. This improves the detection of abstractness. #1001

  • attrs's pickling methods now use dicts instead of tuples. That is safer and more robust across different versions of a class. #1009

  • Added attrs.validators.not_(wrapped_validator) to logically invert wrapped_validator by accepting only values where wrapped_validator rejects the value with a ValueError or TypeError (by default, exception types configurable). #1010

  • The type stubs for attrs.cmp_using() now have default values. #1027

  • To conform with PEP 681, attr.s() and attrs.define() now accept unsafe_hash in addition to hash. #1065

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