Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Hynek Schlawack <hs@ox.cx>
Newsgroups: comp.lang.python.announce
Subject: attrs 17.1.0
Date: Tue, 16 May 2017 20:23:31 +0200
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Hi everyone,

fresh for PyCon US 2017, the attrs team is relieved to present you the =
much-delayed attrs 17.1.0!

Full changes: http://www.attrs.org/en/stable/changelog.html

***

First the bad news: until 17.1.0, attrs=E2=80=99 logic regarding when to =
create a __hash__ method was in conflict with Python=E2=80=99s =
specification.  If you use instances as dict keys or put them into sets, =
you have to either make them frozen explicitly by passing `frozen=3DTrue` =
or implicitly by pinky-swearing to not mutate them and force the =
creation of __hash__  using `hash=3DTrue`.

*Please* double check before upgrading!

We=E2=80=99re honestly sorry but didn=E2=80=99t see any better way to =
handle this.  But mind you: hashing mutable objects is a bug so the =
breakage you=E2=80=99ll encounter may very well be the surfacing of =
latent, sneaky bugs.

***

Now the good news!

This release took very long (we promise improvement!) which led to the =
accumulation of a lot of great new features.  A few highlights:


# Decorators for validators

>>> @attr.s
... class C(object):
...     x =3D attr.ib()
...     @x.validator
...     def check(self, attribute, value):
...         if value > 42:
...             raise ValueError("y must be smaller or equal to 42")
>>> C(42)
C(x=3D42)
>>> C(43)
Traceback (most recent call last):
...
ValueError: x must be smaller or equal to 42


# Decorators for defaults & self in factories

One of the most requested features: you can base the default of an =
attribute on a preceding attribute:

>>> @attr.s
... class C(object):
...     x =3D attr.ib(default=3D1)
...     y =3D attr.ib(default=3Dattr.Factory(lambda self: self.x + 1, =
takes_self=3DTrue))
...     z =3D attr.ib()
...     @z.default
...     def name_does_not_matter(self):
...         return self.x + 1
>>> C()
C(x=3D1, y=3D2, z=3D3)


# New Validators: in_() & and_()

in_() allows to check whether a value is part of an enum or any =
container:

>>> import enum
>>> class State(enum.Enum):
...     ON =3D "on"
...     OFF =3D "off"
>>> @attr.s
... class C(object):
...     state =3D attr.ib(validator=3Dattr.validators.in_(State))
...     val =3D attr.ib(validator=3Dattr.validators.in_([1, 2, 3]))
>>> C(State.ON, 1)
C(state=3D<State.ON: 'on'>, val=3D1)
>>> C("on", 1)
Traceback (most recent call last):
...
ValueError: 'state' must be in <enum 'State'> (got 'on')
>>> C(State.ON, 4)
Traceback (most recent call last):
...
ValueError: 'val' must be in [1, 2, 3] (got 4)

and_() allows you to compose multiple validators to one.  As syntactic =
sugar, you can also just pass a list to `validator=3D`.  Therefore the =
following lines are equivalent:

x =3D attr.ib(validator=3Dattr.validators.and_(v1, v2, v3))
x =3D attr.ib(validator=3D[v1, v2, v3])


For the attrs team
Hynek

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