attrs breaking changes

• 21.2.0 \u2192 21.3.0behavior

  When using `@define`, converters are now run by default when setting an attribute on an instance -- additionally to validators.

• 21.2.0 \u2192 21.3.0behavior

  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.

• 21.2.0 \u2192 21.3.0behavior

  `attr.asdict(retain_collection_types=False)` (default) dumps collection-esque keys as tuples.

• 21.2.0 \u2192 21.3.0breaking

  `attrs.resolve_types()` now resolves types of subclasses after the parents are resolved.

• 21.2.0 \u2192 21.3.0breaking

  `__match_args__` are now generated to support Python 3.10's

• 21.2.0 \u2192 21.3.0breaking

  The generated `__repr__` is significantly faster on Pythons with f-strings.

• 21.2.0 \u2192 21.3.0breaking

  Attributes transformed via `field_transformer` are wrapped with `AttrsClass` again.

• 21.2.0 \u2192 21.3.0breaking

  Generated source code is now cached more efficiently for identical classes.

• 21.2.0 \u2192 21.3.0breaking

  Added `attrs.converters.to_bool()`.

• 21.2.0 \u2192 21.3.0breaking

  Added new validators: `lt(val)` (\< val), `le(va)` (≤ val), `ge(val)` (≥ val), `gt(val)` (> val), and `maxlen(n)`.

• 21.2.0 \u2192 21.3.0breaking

  `attrs` classes are now fully compatible with [cloudpickle](https://github.com/cloudpipe/cloudpickle) (no need to disable `repr` anymore).

• 21.2.0 \u2192 21.3.0breaking

  Added new context manager `attrs.validators.disabled()` and functions `attrs.validators.(set|get)_disabled()`.

• 21.2.0 \u2192 21.3.0breaking

  `attrs.validators.matches_re()` now accepts pre-compiled regular expressions in addition to pattern strings.

• 21.2.0 \u2192 21.3.0breaking

  `import attrs` has finally landed!

• 21.1.0 \u2192 21.2.0breaking

  Python 3.4 is now blocked using packaging metadata because `attrs` can't be imported on it anymore.

• 21.1.0 \u2192 21.2.0breaking

  We had to revert the recursive feature for `attr.evolve()` because it broke some use-cases -- sorry!

• 20.2.0 \u2192 20.3.0api

  The type stubs for `attr.s()` and `attr.make_class()` are not missing the *collect_by_mro* argument anymore.

• 20.2.0 \u2192 20.3.0api

  `attr.asdict()` and `attr.astuple()` now treat `frozenset`s like `set`s with regards to the *retain_collection_types* argument.

• 20.2.0 \u2192 20.3.0breaking

  `attr.s()` now has a *field_transformer* hook that is called for all `Attribute`s and returns a (modified or updated) list of `Attribute` instances.

• 20.2.0 \u2192 20.3.0breaking

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

• 20.2.0 \u2192 20.3.0breaking

  `kw_only=True` now works on Python 2.

• 20.2.0 \u2192 20.3.0breaking

  `raise from` now works on frozen classes on PyPy.

• 20.1.0 \u2192 20.2.0api

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

• 20.1.0 \u2192 20.2.0breaking

  The ergonomics of creating frozen classes using `@define(frozen=True)` and sub-classing frozen classes has been improved:

• 20.1.0 \u2192 20.2.0breaking

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

• 20.1.0 \u2192 20.2.0breaking

  `attr.define()` et al now correctly detect `__eq__` and `__ne__`.

• 20.1.0 \u2192 20.2.0breaking

  It's possible to define custom `__setattr__` methods on slotted classes again.

• 20.1.0 \u2192 20.2.0breaking

  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.

• 20.1.0 \u2192 20.2.0breaking

  `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.

• 19.3.0 \u2192 20.1.0api

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

• 19.3.0 \u2192 20.1.0api

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

• 19.3.0 \u2192 20.1.0behavior

  Fixed serialization behavior of non-slots classes with `cache_hash=True`.

• 19.3.0 \u2192 20.1.0breaking

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

• 19.3.0 \u2192 20.1.0breaking

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

• 19.3.0 \u2192 20.1.0breaking

  `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)`.

• 19.3.0 \u2192 20.1.0breaking

  Added `attr.converters.pipe()`.

• 19.3.0 \u2192 20.1.0breaking

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

• 19.3.0 \u2192 20.1.0breaking

  We have also provisionally added `attr.field()` that supplants `attr.ib()`.

• 19.3.0 \u2192 20.1.0breaking

  Added `attr.resolve_types()`.

• 19.3.0 \u2192 20.1.0breaking

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

• 19.3.0 \u2192 20.1.0breaking

  Python 3.4 is not supported anymore.

• 19.3.0 \u2192 20.1.0deprecated

  Less of a deprecation and more of a heads up: the next release of `attrs` will introduce an `attrs` namespace.

• 19.1.0 \u2192 19.2.0breaking

  Added `attr.__version_info__` that can be used to reliably check the version of `attrs` and write forward- and backward-compatible code.

• 19.1.0 \u2192 19.2.0breaking

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

• 19.1.0 \u2192 19.2.0breaking

  Keyword-only attributes (`kw_only=True`) and attributes that are excluded from the `attrs`'s `__init__` (`init=False`) now can appear before mandatory attributes.

• 19.1.0 \u2192 19.2.0breaking

  The fake filename for generated methods is now more stable.

• 19.1.0 \u2192 19.2.0breaking

  The value passed to `@attr.ib(repr=…)` can now be either a boolean (as before) or a callable.

• 19.1.0 \u2192 19.2.0breaking

  Updated `attr.validators.__all__` to include new validators added in [#425].

• 19.1.0 \u2192 19.2.0breaking

  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.

• 19.1.0 \u2192 19.2.0breaking

  Fixed `attr.validators.deep_iterable()` and `attr.validators.deep_mapping()` type stubs.

• 19.1.0 \u2192 19.2.0breaking

  `attr.validators.is_callable()` validator now raises an exception `attr.exceptions.NotCallableError`, a subclass of `TypeError`, informing the received value.

• 19.1.0 \u2192 19.2.0breaking

  `@attr.s(auto_exc=True)` now generates classes that are hashable by ID, as the documentation always claimed it would.

• 19.1.0 \u2192 19.2.0breaking

  Added `attr.validators.matches_re()` that checks string attributes whether they match a regular expression.

• 19.1.0 \u2192 19.2.0deprecated

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

• 19.1.0 \u2192 19.2.0removed

  When collecting attributes using `@attr.s(auto_attribs=True)`, attributes with a default of `None` are now deleted too.

• 19.1.0 \u2192 19.2.0removed

  Removed deprecated `Attribute` attribute `convert` per scheduled removal on 2019/1.

• 18.2.0 \u2192 19.1.0breaking

  Add `is_callable`, `deep_iterable`, and `deep_mapping` validators.

• 18.2.0 \u2192 19.1.0breaking

  Fixed a bug where deserialized objects with `cache_hash=True` could have incorrect hash code values.

• 18.2.0 \u2192 19.1.0breaking

  `is_callable`: validates that a value is callable

• 18.2.0 \u2192 19.1.0breaking

  `deep_iterable`: Allows recursion down into an iterable,

• 18.2.0 \u2192 19.1.0breaking

  `deep_mapping`: Allows recursion down into the items in a mapping object,

• 18.2.0 \u2192 19.1.0breaking

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

• 18.2.0 \u2192 19.1.0breaking

  Attributes with `init=False` now can follow after `kw_only=True` attributes.

• 18.2.0 \u2192 19.1.0breaking

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

• 18.2.0 \u2192 19.1.0breaking

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

• 17.3.0 \u2192 17.4.0api

  `attr.ib`’s *metadata* argument now defaults to a unique empty `dict` instance instead of sharing a common empty `dict` for all.

• 17.3.0 \u2192 17.4.0breaking

  Generated `__hash__` methods now hash the class type along with the attribute values.

• 17.3.0 \u2192 17.4.0breaking

  `ctypes` is optional now however if it's missing, a bare `super()` will not work in slotted classes.

• 17.3.0 \u2192 17.4.0breaking

  The traversal of MROs when using multiple inheritance was backward:

• 17.3.0 \u2192 17.4.0breaking

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

• 17.3.0 \u2192 17.4.0breaking

  The attribute redefinition feature introduced in 17.3.0 now takes into account if an attribute is redefined via multiple inheritance.

• 17.3.0 \u2192 17.4.0breaking

  Equality tests are *much* faster now.

• 17.3.0 \u2192 17.4.0breaking

  Subclasses of `auto_attribs=True` can be empty now.

• 17.3.0 \u2192 17.4.0deprecated

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

• 17.3.0 \u2192 17.4.0removed

  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).

• 17.2.0 \u2192 17.3.0api

  Added *type* argument to `attr.ib()` and corresponding `type` attribute to `attr.Attribute`.

• 17.2.0 \u2192 17.3.0breaking

  `super()` and `__class__` now work with slotted classes on Python 3.

• 17.2.0 \u2192 17.3.0breaking

  The combination of `str=True` and `slots=True` now works on Python 2.

• 17.2.0 \u2192 17.3.0breaking

  `attr.Factory` is hashable again.

• 17.2.0 \u2192 17.3.0breaking

  Subclasses now can overwrite attribute definitions of their base classes.

• 17.2.0 \u2192 17.3.0breaking

  Added new option *auto_attribs* to `@attr.s` that allows to collect annotated fields without setting them to `attr.ib()`.

• 17.2.0 \u2192 17.3.0breaking

  Instances of classes created using `attr.make_class()` can now be pickled.

• 17.2.0 \u2192 17.3.0breaking

  Attributes are no longer defined on the class body.

• 16.3.0 \u2192 17.1.0api

  For convenience, the *validator* argument of `@attr.s` now can take a list of validators that are wrapped using `and_()`.

• 16.3.0 \u2192 17.1.0api

  Correspondingly, `attr.ib`'s *hash* argument is `None` by default too and mirrors the *cmp* argument as it should.

• 16.3.0 \u2192 17.1.0api

  `attr.make_class()` now accepts the keyword argument `bases` which allows for subclassing.

• 16.3.0 \u2192 17.1.0api

  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.

• 16.3.0 \u2192 17.1.0api

  `attr.Factory()` now has a *takes_self* argument that makes the initializer to pass the partially initialized instance into the factory.

• 16.3.0 \u2192 17.1.0behavior

  Default factories can now also be defined inline using decorators.

• 16.3.0 \u2192 17.1.0behavior

  Fix default hashing behavior.

• 16.3.0 \u2192 17.1.0behavior

  `attrs` will set the `__hash__()` method to `None` by default now.

• 16.3.0 \u2192 17.1.0breaking

  Validators can now be defined conveniently inline by using the attribute as a decorator.

• 16.3.0 \u2192 17.1.0breaking

  Frozen-ness of classes is now inherited.

• 16.3.0 \u2192 17.1.0breaking

  `__attrs_post_init__()` is now run if validation is disabled.

• 16.3.0 \u2192 17.1.0breaking

  Added `attr.validators.in_(options)` that, given the allowed `options`, checks whether the attribute value is in it.

• 16.3.0 \u2192 17.1.0breaking

  Added `attr.validators.and_()` that composes multiple validators into one.

• 16.3.0 \u2192 17.1.0breaking

  Accordingly, `attr.validators.optional()` now can take a list of validators too.

• 16.3.0 \u2192 17.1.0breaking

  Conversion can now be made optional using `attr.converters.optional()`.

• 16.3.0 \u2192 17.1.0breaking

  Metaclasses are now preserved with `slots=True`.

• 16.3.0 \u2192 17.1.0deprecated

  `attr.assoc()` is now deprecated in favor of `attr.evolve()` and will stop working in 2018.

• 16.3.0 \u2192 17.1.0removed

  `FrozenInstanceError` is now raised when trying to delete an attribute from a frozen class.

• 16.0.0 \u2192 16.1.0api

  `attr.asdict()`'s `dict_factory` arguments is now propagated on recursion.

• 16.0.0 \u2192 16.1.0api

  All instances where function arguments were called `cl` have been changed to the more Pythonic `cls`.

• 16.0.0 \u2192 16.1.0api

  `attr.asdict()` now takes `retain_collection_types` as an argument.

• 16.0.0 \u2192 16.1.0breaking

  Add `attr.attrs` and `attr.attrib` as a more consistent aliases for `attr.s` and `attr.ib`.

• 16.0.0 \u2192 16.1.0breaking

  `attr.asdict()`, `attr.has()` and `attr.fields()` are significantly faster.

• 16.0.0 \u2192 16.1.0breaking

  Add *frozen* option to `attr.s` that will make instances best-effort immutable.

• 16.0.0 \u2192 16.1.0deprecated

  Accessing `Attribute` instances on class objects is now deprecated and will stop working in 2017.

• 16.0.0 \u2192 16.1.0deprecated

  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.

• 15.2.0 \u2192 16.0.0api

  `attr.asdict()` can now produce arbitrary mappings instead of Python `dict`s when provided with a `dict_factory` argument.

• 15.2.0 \u2192 16.0.0breaking

  Multiple performance improvements.

• 15.2.0 \u2192 16.0.0breaking

  `__slots__` have arrived!

• 15.2.0 \u2192 16.0.0breaking

  Python 3.3 and 2.6 are no longer supported.

• 15.2.0 \u2192 16.0.0breaking

  Allow the case of initializing attributes that are set to `init=False`.

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