Versions follow CalVer with a strict backwards compatibility policy. The third digit is only for regressions.

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. #425

  • 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 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 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. Due to 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!

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.


    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.