attrs 19.2.0

Classes Without Boilerplate

attrs: Classes Without Boilerplate

attrs is the Python package that will bring back the joy of writing classes by relieving you from the drudgery of implementing object protocols (aka dunder methods).

Its main goal is to help you to write concise and correct software without slowing down your code.

For that, it gives you a class decorator and a way to declaratively define the attributes on that class:

>>> import attr

>>> @attr.s
... class SomeClass(object):
...     a_number = attr.ib(default=42)
...     list_of_numbers = attr.ib(factory=list)
...
...     def hard_math(self, another_number):
...         return self.a_number + sum(self.list_of_numbers) * another_number


>>> sc = SomeClass(1, [1, 2, 3])
>>> sc
SomeClass(a_number=1, list_of_numbers=[1, 2, 3])

>>> sc.hard_math(3)
19
>>> sc == SomeClass(1, [1, 2, 3])
True
>>> sc != SomeClass(2, [3, 2, 1])
True

>>> attr.asdict(sc)
{'a_number': 1, 'list_of_numbers': [1, 2, 3]}

>>> SomeClass()
SomeClass(a_number=42, list_of_numbers=[])

>>> C = attr.make_class("C", ["a", "b"])
>>> C("foo", "bar")
C(a='foo', b='bar')

After declaring your attributes attrs gives you:

• a concise and explicit overview of the class’s attributes,

• a nice human-readable __repr__,

• a complete set of comparison methods (equality and ordering),

• an initializer,

• and much more,

without writing dull boilerplate code again and again and without runtime performance penalties.

On Python 3.6 and later, you can often even drop the calls to attr.ib() by using type annotations.

This gives you the power to use actual classes with actual types in your code instead of confusing tuples or confusingly behaving namedtuples. Which in turn encourages you to write small classes that do one thing well. Never again violate the single responsibility principle just because implementing __init__ et al is a painful drag.

Testimonials

Amber Hawkie Brown, Twisted Release Manager and Computer Owl:

Writing a fully-functional class using attrs takes me less time than writing this testimonial.

Glyph Lefkowitz, creator of Twisted, Automat, and other open source software, in The One Python Library Everyone Needs:

I’m looking forward to is being able to program in Python-with-attrs everywhere. It exerts a subtle, but positive, design influence in all the codebases I’ve see it used in.

Kenneth Reitz, creator of Requests (on paper no less!):

attrs—classes for humans. I like it.

Łukasz Langa, creator of Black, prolific Python core developer, and release manager for Python 3.8 and 3.9:

I’m increasingly digging your attr.ocity. Good job!

Getting Help

Please use the python-attrs tag on StackOverflow to get help.

Answering questions of your fellow developers is also great way to help the project!

Project Information

attrs is released under the MIT license, its documentation lives at Read the Docs, the code on GitHub, and the latest release on PyPI. It’s rigorously tested on Python 2.7, 3.4+, and PyPy.

We collect information on third-party extensions in our wiki. Feel free to browse and add your own!

If you’d like to contribute to attrs you’re most welcome and we’ve written a little guide to get you started!

Release Information

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

Deprecations

• 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

Changes

• 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

Full changelog.

Credits

attrs is written and maintained by Hynek Schlawack.

The development is kindly supported by Variomedia AG.

A full list of contributors can be found in GitHub’s overview.

It’s the spiritual successor of characteristic and aspires to fix some of it clunkiness and unfortunate decisions. Both were inspired by Twisted’s FancyEqMixin but both are implemented using class decorators because subclassing is bad for you, m’kay?