Versions follow CalVer with a strict backwards-compatibility policy.

The first number of the version is the year. The second number is incremented with each release, starting at 1 for each year. The third number is when we need to start branches for older releases (only for emergencies).

You can find out backwards-compatibility policy here.

Changes for the upcoming release can be found in the “changelog.d” directory in our repository.

23.1.0 - 2023-04-16

Backwards-incompatible Changes

  • Python 3.6 has been dropped and packaging switched to static package data using Hatch. #993


  • The support for zope-interface via the attrs.validators.provides validator is now deprecated and will be removed in, or after, April 2024.

    The presence of a C-based package in our developement dependencies has caused headaches and we‘re not under the impression it’s used a lot.

    Let us know if you're using it and we might publish it as a separate package. #1120


  • attrs.filters.exclude() and attrs.filters.include() now support the passing of attribute names as strings. #1068

  • attrs.has() and attrs.fields() now handle generic classes correctly. #1079

  • Fix frozen exception classes when raised within e.g. contextlib.contextmanager, which mutates their __traceback__ attributes. #1081

  • @frozen now works with type checkers that implement PEP-681 (ex. pyright). #1084

  • Restored ability to unpickle instances pickled before 22.2.0. #1085

  • attrs.asdict()'s and attrs.astuple()'s type stubs now accept the attrs.AttrsInstance protocol. #1090

  • Fix slots class cellvar updating closure in CPython 3.8+ even when __code__ introspection is unavailable. #1092

  • attrs.resolve_types() can now pass include_extras to typing.get_type_hints() on Python 3.9+, and does so by default. #1099

  • Added instructions for pull request workflow to #1105

  • Added type parameter to attrs.field() function for use with attrs.make_class().

    Please note that type checkers ignore type metadata passed into make_class(), but it can be useful if you're wrapping attrs. #1107

  • It is now possible for attrs.evolve() (and attr.evolve()) to change fields named inst if the instance is passed as a positional argument.

    Passing the instance using the inst keyword argument is now deprecated and will be removed in, or after, April 2024. #1117

  • attrs.validators.optional() now also accepts a tuple of validators (in addition to lists of validators). #1122

22.2.0 - 2022-12-21

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

22.1.0 - 2022-07-28

Backwards-incompatible Changes

  • Python 2.7 is not supported anymore.

    Dealing with Python 2.7 tooling has become too difficult for a volunteer-run project.

    We have supported Python 2 more than 2 years after it was officially discontinued and feel that we have paid our dues. All version up to 21.4.0 from December 2021 remain fully functional, of course. #936

  • The deprecated cmp attribute of attrs.Attribute has been removed. This does not affect the cmp argument to attr.s that can be used as a shortcut to set eq and order at the same time. #939


  • Instantiation of frozen slotted classes is now faster. #898
  • If an eq key is defined, it is also used before hashing the attribute. #909
  • Added attrs.validators.min_len(). #916
  • attrs.validators.deep_iterable()'s member_validator argument now also accepts a list of validators and wraps them in an attrs.validators.and_(). #925
  • Added missing type stub re-imports for attrs.converters and attrs.filters. #931
  • Added missing stub for attr(s).cmp_using(). #949
  • attrs.validators._in()'s ValueError is not missing the attribute, expected options, and the value it got anymore. #951
  • Python 3.11 is now officially supported. #969

21.4.0 - 2021-12-29


  • Fixed the test suite on PyPy3.8 where cloudpickle does not work. #892
  • Fixed coverage report for projects that use attrs and don't set a --source. #895, #896

21.3.0 - 2021-12-28

Backward-incompatible Changes

  • When using @define, converters are now run by default when setting an attribute on an instance -- additionally to validators. I.e. the new default is on_setattr=[attrs.setters.convert, attrs.setters.validate].

    This is unfortunately a breaking change, but it was an oversight, impossible to raise a DeprecationWarning about, and it's better to fix it now while the APIs are very fresh with few users. #835, #886

  • import attrs has finally landed! As of this release, you can finally import attrs using its proper name.

    Not all names from the attr namespace have been transferred; most notably attr.s and attr.ib are missing. See attrs.define and attrs.field if you haven't seen our next-generation APIs yet. A more elaborate explanation can be found On The Core API Names

    This feature is at least for one release provisional. We don't plan on changing anything, but such a big change is unlikely to go perfectly on the first strike.

    The API docs have been mostly updated, but it will be an ongoing effort to change everything to the new APIs. Please note that we have not moved -- or even removed -- anything from attr!

    Please do report any bugs or documentation inconsistencies! #887


  • attr.asdict(retain_collection_types=False) (default) dumps collection-esque keys as tuples. #646, #888
  • __match_args__ are now generated to support Python 3.10's Structural Pattern Matching. This can be controlled by the match_args argument to the class decorators on Python 3.10 and later. On older versions, it is never added and the argument is ignored. #815
  • If the class-level on_setattr is set to attrs.setters.validate (default in @define and @mutable) but no field defines a validator, pretend that it's not set. #817
  • The generated __repr__ is significantly faster on Pythons with f-strings. #819
  • Attributes transformed via field_transformer are wrapped with AttrsClass again. #824
  • Generated source code is now cached more efficiently for identical classes. #828
  • Added attrs.converters.to_bool(). #830
  • attrs.resolve_types() now resolves types of subclasses after the parents are resolved. #842 #843
  • Added new validators: lt(val) (< val), le(va) (≤ val), ge(val) (≥ val), gt(val) (> val), and maxlen(n). #845
  • attrs classes are now fully compatible with cloudpickle (no need to disable repr anymore). #857
  • Added new context manager attrs.validators.disabled() and functions attrs.validators.(set|get)_disabled(). They deprecate attrs.(set|get)_run_validators(). All functions are interoperable and modify the same internal state. They are not – and never were – thread-safe, though. #859
  • attrs.validators.matches_re() now accepts pre-compiled regular expressions in addition to pattern strings. #877

21.2.0 - 2021-05-07

Backward-incompatible Changes

  • We had to revert the recursive feature for attr.evolve() because it broke some use-cases -- sorry! #806
  • Python 3.4 is now blocked using packaging metadata because attrs can't be imported on it anymore. To ensure that 3.4 users can keep installing attrs easily, we will yank 21.1.0 from PyPI. This has no consequences if you pin attrs to 21.1.0. #807

21.1.0 - 2021-05-06


  • The long-awaited, much-talked-about, little-delivered import attrs is finally upon us!

    Since the NG APIs have now been proclaimed stable, the next release of attrs will allow you to actually import attrs. We‘re taking this opportunity to replace some defaults in our APIs that made sense in 2015, but don’t in 2021.

    So please, if you have any pet peeves about defaults in attrs‘s APIs, now is the time to air your grievances in #487! We’re not gonna get such a chance for a second time, without breaking our backward-compatibility guarantees, or long deprecation cycles. Therefore, speak now or forever hold you peace! #487

  • The cmp argument to attr.s() and attr.ib() has been undeprecated It will continue to be supported as syntactic sugar to set eq and order in one go.

    I‘m terribly sorry for the hassle around this argument! The reason we’re bringing it back is it's usefulness regarding customization of equality/ordering.

    The cmp attribute and argument on attr.Attribute remains deprecated and will be removed later this year. #773


  • It's now possible to customize the behavior of eq and order by passing in a callable. #435, #627

  • The instant favorite next-generation APIs are not provisional anymore!

    They are also officially supported by Mypy as of their 0.800 release.

    We hope the next release will already contain an (additional) importable package called attrs. #668, #786

  • If an attribute defines a converter, the type of its parameter is used as type annotation for its corresponding __init__ parameter.

    If an attr.converters.pipe is used, the first one's is used. #710

  • Fixed the creation of an extra slot for an attr.ib when the parent class already has a slot with the same name. #718

  • __attrs__init__() will now be injected if init=False, or if auto_detect=True and a user-defined __init__() exists.

    This enables users to do “pre-init” work in their __init__() (such as super().__init__()).

    __init__() can then delegate constructor argument processing to self.__attrs_init__(*args, **kwargs). #731

  • bool(attr.NOTHING) is now False. #732

  • It's now possible to use super() inside of properties of slotted classes. #747

  • Allow for a __attrs_pre_init__() method that -- if defined -- will get called at the beginning of the attrs-generated __init__() method. #750

  • Added forgotten attr.Attribute.evolve() to type stubs. #752

  • attrs.evolve() now works recursively with nested attrs classes. #759

  • Python 3.10 is now officially supported. #763

  • attr.resolve_types() now takes an optional attrib argument to work inside a field_transformer. #774

  • ClassVars are now also detected if they come from typing-extensions. #782

  • To make it easier to customize attribute comparison (#435), we have added the attr.cmp_with() helper.

    See the new docs on comparison for more details. #787

  • Added provisional support for static typing in pyright via PEP 681. Both the pyright specification and attrs implementation may change in future versions of both projects.

    Your constructive feedback is welcome in both attrs#795 and pyright#1782. #796

20.3.0 - 2020-11-05

Backward-incompatible Changes

  • attr.define(), attr.frozen(), attr.mutable(), and attr.field() remain provisional.

    This release does not change anything about them and they are already used widely in production though.

    If you wish to use them together with mypy, you can simply drop this plugin into your project.

    Feel free to provide feedback to them in the linked issue #668.

    We will release the attrs namespace once we have the feeling that the APIs have properly settled. #668


  • attr.s() now has a field_transformer hook that is called for all Attributes and returns a (modified or updated) list of Attribute instances. attr.asdict() has a value_serializer hook that can change the way values are converted. Both hooks are meant to help with data (de-)serialization workflows. #653
  • kw_only=True now works on Python 2. #700
  • raise from now works on frozen classes on PyPy. #703, #712
  • attr.asdict() and attr.astuple() now treat frozensets like sets with regards to the retain_collection_types argument. #704
  • The type stubs for attr.s() and attr.make_class() are not missing the collect_by_mro argument anymore. #711

20.2.0 - 2020-09-05

Backward-incompatible Changes

  • attr.define(), attr.frozen(), attr.mutable(), and attr.field() remain provisional.

    This release fixes a bunch of bugs and ergonomics but they remain mostly unchanged.

    If you wish to use them together with mypy, you can simply drop this plugin into your project.

    Feel free to provide feedback to them in the linked issue #668.

    We will release the attrs namespace once we have the feeling that the APIs have properly settled. #668


  • attr.define() et al now correctly detect __eq__ and __ne__. #671

  • attr.define() et al's hybrid behavior now also works correctly when arguments are passed. #675

  • It's possible to define custom __setattr__ methods on slotted classes again. #681

  • In 20.1.0 we introduced the inherited attribute on the attr.Attribute class to differentiate attributes that have been inherited and those that have been defined directly on the class.

    It has shown to be problematic to involve that attribute when comparing instances of attr.Attribute though, because when sub-classing, attributes from base classes are suddenly not equal to themselves in a super class.

    Therefore the inherited attribute will now be ignored when hashing and comparing instances of attr.Attribute. #684

  • zope.interface is now a “soft dependency” when running the test suite; if zope.interface is not installed when running the test suite, the interface-related tests will be automatically skipped. #685

  • The ergonomics of creating frozen classes using @define(frozen=True) and sub-classing frozen classes has been improved: you don't have to set on_setattr=None anymore. #687

20.1.0 - 2020-08-20

Backward-incompatible Changes

  • Python 3.4 is not supported anymore. It has been unsupported by the Python core team for a while now, its PyPI downloads are negligible, and our CI provider removed it as a supported option.

    It‘s very unlikely that attrs will break under 3.4 anytime soon, which is why we do not block its installation on Python 3.4. But we don’t test it anymore and will block it once someone reports breakage. #608


  • Less of a deprecation and more of a heads up: the next release of attrs will introduce an attrs namespace. That means that you‘ll finally be able to run import attrs with new functions that aren’t cute abbreviations and that will carry better defaults.

    This should not break any of your code, because project-local packages have priority before installed ones. If this is a problem for you for some reason, please report it to our bug tracker and we'll figure something out.

    The old attr namespace isn't going anywhere and its defaults are not changing – this is a purely additive measure. Please check out the linked issue for more details.

    These new APIs have been added provisionally as part of #666 so you can try them out today and provide feedback. Learn more in the API docs. #408


  • Added attr.resolve_types(). It ensures that all forward-references and types in string form are resolved into concrete types.

    You need this only if you need concrete types at runtime. That means that if you only use types for static type checking, you do not need this function. #288, #302

  • Added @attr.s(collect_by_mro=False) argument that if set to True fixes the collection of attributes from base classes.

    It's only necessary for certain cases of multiple-inheritance but is kept off for now for backward-compatibility reasons. It will be turned on by default in the future.

    As a side-effect, attr.Attribute now always has an inherited attribute indicating whether an attribute on a class was directly defined or inherited. #428, #635

  • On Python 3, all generated methods now have a docstring explaining that they have been created by attrs. #506

  • It is now possible to prevent attrs from auto-generating the __setstate__ and __getstate__ methods that are required for pickling of slotted classes.

    Either pass @attr.s(getstate_setstate=False) or pass @attr.s(auto_detect=True) and implement them yourself: if attrs finds either of the two methods directly on the decorated class, it assumes implicitly getstate_setstate=False (and implements neither).

    This option works with dict classes but should never be necessary. #512, #513, #642

  • Fixed a ValueError: Cell is empty bug that could happen in some rare edge cases. #590

  • attrs can now automatically detect your own implementations and infer init=False, repr=False, eq=False, order=False, and hash=False if you set @attr.s(auto_detect=True). attrs will ignore inherited methods. If the argument implies more than one method (e.g. eq=True creates both __eq__ and __ne__), it's enough for one of them to exist and attrs will create neither.

    This feature requires Python 3. #607

  • Added attr.converters.pipe(). The feature allows combining multiple conversion callbacks into one by piping the value through all of them, and retuning the last result.

    As part of this feature, we had to relax the type information for converter callables. #618

  • Fixed serialization behavior of non-slots classes with cache_hash=True. The hash cache will be cleared on operations which make “deep copies” of instances of classes with hash caching, though the cache will not be cleared with shallow copies like those made by copy.copy().

    Previously, copy.deepcopy() or serialization and deserialization with pickle would result in an un-initialized object.

    This change also allows the creation of cache_hash=True classes with a custom __setstate__, which was previously forbidden (#494). #620

  • It is now possible to specify hooks that are called whenever an attribute is set after a class has been instantiated.

    You can pass on_setattr both to @attr.s() to set the default for all attributes on a class, and to @attr.ib() to overwrite it for individual attributes.

    attrs also comes with a new module attr.setters that brings helpers that run validators, converters, or allow to freeze a subset of attributes. #645, #660

  • Provisional APIs called attr.define(), attr.mutable(), and attr.frozen() have been added.

    They are only available on Python 3.6 and later, and call attr.s() with different default values.

    If nothing comes up, they will become the official way for creating classes in 20.2.0 (see above).

    Please note that it may take some time until mypy – and other tools that have dedicated support for attrs – recognize these new APIs. Please do not open issues on our bug tracker, there is nothing we can do about it. #666

  • We have also provisionally added attr.field() that supplants attr.ib(). It also requires at least Python 3.6 and is keyword-only. Other than that, it only dropped a few arguments, but changed no defaults.

    As with attr.s(): attr.ib() is not going anywhere. #669

19.3.0 - 2019-10-15


  • Fixed auto_attribs usage when default values cannot be compared directly with ==, such as numpy arrays. #585

19.2.0 - 2019-10-01

Backward-incompatible Changes

  • Removed deprecated Attribute attribute convert per scheduled removal on 2019/1. This planned deprecation is tracked in issue #307. #504

  • __lt__, __le__, __gt__, and __ge__ do not consider subclasses comparable anymore.

    This has been deprecated since 18.2.0 and was raising a DeprecationWarning for over a year. #570


  • The cmp argument to attr.s() and attr.ib() is now deprecated.

    Please use eq to add equality methods (__eq__ and __ne__) and order to add ordering methods (__lt__, __le__, __gt__, and __ge__) instead – just like with dataclasses.

    Both are effectively True by default but it's enough to set eq=False to disable both at once. Passing eq=False, order=True explicitly will raise a ValueError though.

    Since this is arguably a deeper backward-compatibility break, it will have an extended deprecation period until 2021-06-01. After that day, the cmp argument will be removed.

    attr.Attribute also isn't orderable anymore. #574


  • Updated attr.validators.__all__ to include new validators added in #425. #517
  • Slotted classes now use a pure Python mechanism to rewrite the __class__ cell when rebuilding the class, so super() works even on environments where ctypes is not installed. #522
  • When collecting attributes using @attr.s(auto_attribs=True), attributes with a default of None are now deleted too. #523, #556
  • Fixed attr.validators.deep_iterable() and attr.validators.deep_mapping() type stubs. #533
  • attr.validators.is_callable() validator now raises an exception attr.exceptions.NotCallableError, a subclass of TypeError, informing the received value. #536
  • @attr.s(auto_exc=True) now generates classes that are hashable by ID, as the documentation always claimed it would. #543, #563
  • Added attr.validators.matches_re() that checks string attributes whether they match a regular expression. #552
  • Keyword-only attributes (kw_only=True) and attributes that are excluded from the attrs's __init__ (init=False) now can appear before mandatory attributes. #559
  • The fake filename for generated methods is now more stable. It won't change when you restart the process. #560
  • The value passed to @attr.ib(repr=…) can now be either a boolean (as before) or a callable. That callable must return a string and is then used for formatting the attribute by the generated __repr__() method. #568
  • Added attr.__version_info__ that can be used to reliably check the version of attrs and write forward- and backward-compatible code. Please check out the section on deprecated APIs on how to use it. #580

19.1.0 - 2019-03-03

Backward-incompatible Changes

  • Fixed a bug where deserialized objects with cache_hash=True could have incorrect hash code values. This change breaks classes with cache_hash=True when a custom __setstate__ is present. An exception will be thrown when applying the attrs annotation to such a class. This limitation is tracked in issue #494. #482


  • Add is_callable, deep_iterable, and deep_mapping validators.

    • is_callable: validates that a value is callable
    • deep_iterable: Allows recursion down into an iterable, applying another validator to every member in the iterable as well as applying an optional validator to the iterable itself.
    • deep_mapping: Allows recursion down into the items in a mapping object, applying a key validator and a value validator to the key and value in every item. Also applies an optional validator to the mapping object itself.

    You can find them in the attr.validators package. #425

  • Fixed stub files to prevent errors raised by mypy's disallow_any_generics = True option. #443

  • Attributes with init=False now can follow after kw_only=True attributes. #450

  • attrs now has first class support for defining exception classes.

    If you define a class using @attr.s(auto_exc=True) and subclass an exception, the class will behave like a well-behaved exception class including an appropriate __str__ method, and all attributes additionally available in an args attribute. #500

  • Clarified documentation for hashing to warn that hashable objects should be deeply immutable (in their usage, even if this is not enforced). #503

18.2.0 - 2018-09-01


  • Comparing subclasses using <, >, <=, and >= is now deprecated. The docs always claimed that instances are only compared if the types are identical, so this is a first step to conform to the docs.

    Equality operators (== and !=) were always strict in this regard. #394


  • attrs now ships its own PEP 484 type hints. Together with mypy‘s attrs plugin, you’ve got all you need for writing statically typed code in both Python 2 and 3!

    At that occasion, we've also added narrative docs about type annotations in attrs. #238

  • Added kw_only arguments to attr.ib and attr.s, and a corresponding kw_only attribute to attr.Attribute. This change makes it possible to have a generated __init__ with keyword-only arguments on Python 3, relaxing the required ordering of default and non-default valued attributes. #281, #411

  • The test suite now runs with hypothesis.HealthCheck.too_slow disabled to prevent CI breakage on slower computers. #364, #396

  • attr.validators.in_() now raises a ValueError with a useful message even if the options are a string and the value is not a string. #383

  • attr.asdict() now properly handles deeply nested lists and dictionaries. #395

  • Added attr.converters.default_if_none() that allows to replace None values in attributes. For example attr.ib(converter=default_if_none("")) replaces None by empty strings. #400, #414

  • Fixed a reference leak where the original class would remain live after being replaced when slots=True is set. #407

  • Slotted classes can now be made weakly referenceable by passing @attr.s(weakref_slot=True). #420

  • Added cache_hash option to @attr.s which causes the hash code to be computed once and stored on the object. #426

  • Attributes can be named property and itemgetter now. #430

  • It is now possible to override a base class' class variable using only class annotations. #431

18.1.0 - 2018-05-03


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


  • 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 programmatically.

    #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 incompatibilities.

    #321, #334

  • If attr.s is passed a these argument, it will no longer attempt to remove attributes with the same name from the class body.

    #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.


  • 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

17.4.0 - 2017-12-30

Backward-incompatible Changes

  • The traversal of MROs when using multiple inheritance was backward: If you defined a class C that subclasses A and B like C(A, B), attrs would have collected the attributes from B before those of A.

    This is now fixed and means that in classes that employ multiple inheritance, the output of __repr__ and the order of positional arguments in __init__ changes. Because of the nature of this bug, a proper deprecation cycle was unfortunately impossible.

    Generally speaking, it's advisable to prefer kwargs-based initialization anyways – especially if you employ multiple inheritance and diamond-shaped hierarchies.

    #298, #299, #304

  • The __repr__ set by attrs no longer produces an AttributeError when the instance is missing some of the specified attributes (either through deleting or after using init=False on some attributes).

    This can break code that relied on repr(attr_cls_instance) raising AttributeError to check if any attrs-specified members were unset.

    If you were using this, you can implement a custom method for checking this:

    def has_unset_members(self):
        for field in attr.fields(type(self)):
            except AttributeError:
                return True
        return False



  • The attr.ib(convert=callable) option is now deprecated in favor of attr.ib(converter=callable).

    This is done to achieve consistency with other noun-based arguments like validator.

    convert will keep working until at least January 2019 while raising a DeprecationWarning.



  • Generated __hash__ methods now hash the class type along with the attribute values. Until now the hashes of two classes with the same values were identical which was a bug.

    The generated method is also much faster now.

    #261, #295, #296

  • attr.ib’s metadata argument now defaults to a unique empty dict instance instead of sharing a common empty dict for all. The singleton empty dict is still enforced.


  • ctypes is optional now however if it's missing, a bare super() will not work in slotted classes. This should only happen in special environments like Google App Engine.

    #284, #286

  • The attribute redefinition feature introduced in 17.3.0 now takes into account if an attribute is redefined via multiple inheritance. In that case, the definition that is closer to the base of the class hierarchy wins.

    #285, #287

  • Subclasses of auto_attribs=True can be empty now.

    #291, #292

  • Equality tests are much faster now.


  • All generated methods now have correct __module__, __name__, and (on Python 3) __qualname__ attributes.


17.3.0 - 2017-11-08

Backward-incompatible Changes

  • Attributes are no longer defined on the class body.

    This means that if you define a class C with an attribute x, the class will not have an attribute x for introspection. 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)


  • super() and __class__ now work with slotted classes on Python 3. (#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 base classes.

    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)

17.2.0 - 2017-05-24


  • Validators are hashable again. Note that validators may become frozen in the future, pending availability of no-overhead frozen classes. #192

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!

:alt: 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.

    :::{warning} Please do not upgrade blindly and do test your software! Especially if you use instances as dict keys or put them into sets! :::

  • 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 validator 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 #189
  • 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

16.3.0 - 2016-11-24


  • Attributes now can have user-defined metadata which greatly improves attrs's extensibility. #96

  • Allow for a __attrs_post_init__() method that -- if defined -- will get called at the end of the attrs-generated __init__() method. #111

  • Added @attr.s(str=True) that will optionally create a __str__() method that is identical to __repr__(). This is mainly useful with Exceptions and other classes that rely on a useful __str__() implementation but overwrite the default one through a poor own one. Default Python class behavior is to use __repr__() as __str__() anyways.

    If you tried using attrs with Exceptions and were puzzled by the tracebacks: this option is for you.

  • __name__ is no longer overwritten with __qualname__ for attr.s(slots=True) classes. #99

16.2.0 - 2016-09-17


  • Added attr.astuple() that -- similarly to attr.asdict() -- returns the instance as a tuple. #77
  • Converters now work with frozen classes. #76
  • Instantiation of attrs classes with converters is now significantly faster. #80
  • Pickling now works with slotted classes. #81
  • attr.assoc() now works with slotted classes. #84
  • The tuple returned by attr.fields() now also allows to access the Attribute instances by name. Yes, we‘ve subclassed tuple so you don’t have to! Therefore attr.fields(C).x is equivalent to the deprecated C.x and works with slotted classes. #88

16.1.0 - 2016-08-30

Backward-incompatible Changes:

  • All instances where function arguments were called cl have been changed to the more Pythonic cls. Since it was always the first argument, it‘s doubtful anyone ever called those function with in the keyword form. If so, sorry for any breakage but there’s no practical deprecation path to solve this ugly wart.


  • Accessing Attribute instances on class objects is now deprecated and will stop working in 2017. If you need introspection please use the __attrs_attrs__ attribute or the attr.fields() function that carry them too. In the future, the attributes that are defined on the class body and are usually overwritten in your __init__ method are simply removed after @attr.s has been applied.

    This will remove the confusing error message if you write your own __init__ and forget to initialize some attribute. Instead you will get a straightforward AttributeError. In other words: decorated classes will work more like plain Python classes which was always attrs's goal.

  • The serious-business aliases attr.attributes and attr.attr have been deprecated in favor of attr.attrs and attr.attrib which are much more consistent and frankly obvious in hindsight. They will be purged from documentation immediately but there are no plans to actually remove them.


  • attr.asdict()'s dict_factory arguments is now propagated on recursion. #45
  • attr.asdict(), attr.has() and attr.fields() are significantly faster. #48 #51
  • Add attr.attrs and attr.attrib as a more consistent aliases for attr.s and attr.ib.
  • Add frozen option to attr.s that will make instances best-effort immutable. #60
  • attr.asdict() now takes retain_collection_types as an argument. If True, it does not convert attributes of type tuple or set to list. #69

16.0.0 - 2016-05-23

Backward-incompatible Changes:

  • Python 3.3 and 2.6 are no longer supported. They may work by chance but any effort to keep them working has ceased.

    The last Python 2.6 release was on October 29, 2013 and is no longer supported by the CPython core team. Major Python packages like Django and Twisted dropped Python 2.6 a while ago already.

    Python 3.3 never had a significant user base and wasn‘t part of any distribution’s LTS release.


  • __slots__ have arrived! Classes now can automatically be slotted-style (and save your precious memory) just by passing slots=True. #35
  • Allow the case of initializing attributes that are set to init=False. This allows for clean initializer parameter lists while being able to initialize attributes to default values. #32
  • attr.asdict() can now produce arbitrary mappings instead of Python dicts when provided with a dict_factory argument. #40
  • Multiple performance improvements.

15.2.0 - 2015-12-08


  • Added a convert argument to attr.ib, which allows specifying a function to run on arguments. This allows for simple type conversions, e.g. with attr.ib(convert=int). #26
  • Speed up object creation when attribute validators are used. #28

15.1.0 - 2015-08-20


  • Added attr.validators.optional() that wraps other validators allowing attributes to be None. #16
  • Multi-level inheritance now works. #24
  • __repr__() now works with non-redecorated subclasses. #20

15.0.0 - 2015-04-15


Initial release.