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

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

17.3.0 (2017-11-08)

Backward-incompatible Changes

  • Attributes are not defined on the class body anymore.

    This means that if you define a class C with an attribute x, the class will not have an attribute x for introspection anymore. 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 on Python 3 when slots=True. (#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 superclass.

    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 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 not overwritten with __qualname__ for attr.s(slots=True) classes anymore. #99

16.2.0 (2016-09-17)


  • Added attr.astuple() that – similarly to attr.asdict() – returns the instance as a tuple. #77
  • Converts now work with frozen classes. #76
  • Instantiation of attrs classes with converters is now significantly faster. #80
  • Pickling now works with __slots__ classes. #81
  • attr.assoc() now works with __slots__ 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 __slots__ 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 aren’t supported anymore. 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 isn’t supported by the CPython core team anymore. 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 slots-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.