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

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

  • Add @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.

  • Don’t overwrite __name__ with __qualname__ for attr.s(slots=True) classes. #99

16.2.0 (2016-09-17)


  • Add 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)


  • Add 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)


  • Add attr.validators.optional that wraps other validators allowing attributes to be None. #16
  • Fix multi-level inheritance. #24
  • Fix __repr__ to work for non-redecorated subclasses. #20

15.0.0 (2015-04-15)


Initial release.