On The Core API Names¶
And what even is
attr.dataclass that’s not documented but commonly used!?
We recommend our modern APIs for new code:
attrs.define()to define a new class,
attrs.frozen()is an alias for
attrs.field()to define an attribute.
They have been added in
attrs 20.1.0, they are expressive, and they have modern defaults like slots and type annotation awareness switched on by default.
They are only available in Python 3.6 and later.
Sometimes they’re referred to as next-generation or NG APIs.
attrs 21.3.0 you can also import them from the
attrs package namespace.
attrs will never force you to use type annotations.
A Short History Lesson¶
At this point,
attrs is an old project.
It had its first release in April 2015 – back when most Python code was on Python 2.7 and Python 3.4 was the first Python 3 release that showed promise.
attrs was always Python 3-first, but type annotations came only into Python 3.5 that was released in September 2015 and were largely ignored until years later.
At this time, if you didn’t want to implement all the dunder methods, the most common way to create a class with some attributes on it was to subclass a
collections.namedtuple, or one of the many hacks that allowed you to access dictionary keys using attribute lookup.
attrs history goes even a bit further back, to the now-forgotten characteristic that came out in May 2014 and already used a class decorator, but was overall too unergonomic.
In the wake of all of that, glyph and Hynek came together on IRC and brainstormed how to take the good ideas of
characteristic, but make them easier to use and read.
At this point the plan was not to make
attrs what it is now – a flexible class building kit.
All we wanted was an ergonomic little library to succinctly define classes with attributes.
Under the impression of the unwieldy
characteristic name, we went to the other side and decided to make the package name part of the API, and keep the API functions very short.
This led to the infamous
attr.ib which some found confusing and pronounced it as “attr dot s” or used a singular
@s as the decorator.
But it was really just a way to say
Some people hated this cutey API from day one, which is why we added aliases for them that we called serious business:
Fans of them usually imported the names and didn’t use the package name in the first place.
attr package name started creaking the moment we added
attr.Factory, since it couldn’t be morphed into something meaningful in any way.
A problem that grew worse over time, as more APIs and even modules were added.
Being able to just write:
@attr.s class Point(object): x = attr.ib() y = attr.ib()
was a big step for those who wanted to write small, focused classes.
Dataclasses Enter The Arena¶
Type annotations for class attributes have just landed in Python 3.6 and Guido felt like it would be a good mechanic to introduce something similar to
attrs to the Python standard library.
The result, of course, was PEP 5572 which eventually became the
dataclasses module in Python 3.7.
attrs at this point was lucky to have several people on board who were also very excited about type annotations and helped implement it; including a Mypy plugin.
And so it happened that
attrs shipped the new method of defining classes more than half a year before Python 3.7 – and thus
dataclasses – were released.
Due to backward-compatibility concerns, this feature is off by default in the
attr.s decorator and has to be activated using
As a little easter egg and to save ourselves some typing, we’ve also added an alias called
attr.dataclasses that just set
It was never documented, but people found it and used it and loved it.
Over the next months and years it became clear that type annotations have become the popular way to define classes and their attributes. However, it has also become clear that some people viscerally hate type annotations. We’re determined to serve both.
Over its existence,
attrs never stood still.
But since we also greatly care about backward compatibility and not breaking our users’ code, many features and niceties have to be manually activated.
That is not only annoying, it also leads to the problem that many of
attrs’s users don’t even know what it can do for them.
We’ve spent years alone explaining that defining attributes using type annotations is in no way unique to
Finally we’ve decided to take the Go route:
instead of fiddling with the old APIs – whose names felt anachronistic anyway – we’d define new ones, with better defaults.
So in July 2018, we looked for better names and came up with
attr.field, and friends.
Then in January 2019, we started looking for inconvenient defaults that we now could fix without any repercussions.
These APIs proved to be very popular, so we’ve finally changed the documentation to them in November of 2021.
All of this took way too long, of course. One reason is the COVID-19 pandemic, but also our fear to fumble this historic chance to fix our APIs.
Finally, in December 2021, we’ve added the
attrs package namespace.
We hope you like the result:
from attrs import define @define class Point: x: int y: int