attrs

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.

Sponsors

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!

Example

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

>>> 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.

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.

Check out On The Core API Names for an in-depth explanation!

Hate Type Annotations!?

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

from attrs import define, field

@define
class SomeClass:
    a_number = field(default=42)
    list_of_numbers = field(factory=list)

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, allows more ways to plug into the initialization process, has a replacement for __init_subclass__, and allows for stepping through the generated methods using a debugger.

For more details, please refer to our comparison page, but generally speaking, we are more likely to commit crimes against nature to make things work that one would expect to work, but that are quite complicated in practice.

Project Information

• Changelog
• Documentation
• PyPI
• Source Code
• Contributing
• Third-party Extensions
• Get Help: use the python-attrs tag on Stack Overflow

attrs for Enterprise

Available as part of the Tidelift Subscription.

The maintainers of attrs and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source packages you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact packages you use.

Release Information

Backwards-incompatible Changes

• Class-level kw_only=True behavior is now consistent with dataclasses.

  Previously, a class that sets kw_only=True makes all attributes keyword-only, including those from base classes. If an attribute sets kw_only=False, that setting is ignored, and it is still made keyword-only.

  Now, only the attributes defined in that class that doesn't explicitly set kw_only=False are made keyword-only.

  This shouldn't be a problem for most users, unless you have a pattern like this:

  @attrs.define(kw_only=True)
  class Base:
      a: int
      b: int = attrs.field(default=1, kw_only=False)
  
  @attrs.define
  class Subclass(Base):
      c: int
  

  Here, we have a kw_only=True attrs class (Base) with an attribute that sets kw_only=False and has a default (Base.b), and then create a subclass (Subclass) with required arguments (Subclass.c). Previously this would work, since it would make Base.b keyword-only, but now this fails since Base.b is positional, and we have a required positional argument (Subclass.c) following another argument with defaults. #1457

Changes

• Values passed to the __init__() method of attrs classes are now correctly passed to __attrs_pre_init__() instead of their default values (in cases where kw_only was not specified). #1427

• Added support for Python 3.14 and PEP 749. #1446, #1451

• attrs.validators.deep_mapping() now allows to leave out either key_validator xor value_validator. #1448

• attrs.validators.deep_iterator() and attrs.validators.deep_mapping() now accept lists and tuples for all validators and wrap them into a attrs.validators.and_(). #1449

• Added a new experimental way to inspect classes:

  attrs.inspect(cls) returns the effective class-wide parameters that were used by attrs to construct the class.

  The returned class is the same data structure that attrs uses internally to decide how to construct the final class. #1454

• Fixed annotations for attrs.field(converter=...). Previously, a tuple of converters was only accepted if it had exactly one element. #1461

• The performance of attrs.asdict() has been improved by 45–260%. #1463

• The performance of attrs.astuple() has been improved by 49–270%. #1469

• The type annotation for attrs.validators.or_() now allows for different types of validators.

  This was only an issue on Pyright. #1474

Full changelog →