attrs 17.1.0

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attrs 17.1.0

Hynek Schlawack-6
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’ logic regarding when to create a __hash__ method was in conflict with Python’s specification.  If you use instances as dict keys or put them into sets, you have to either make them frozen explicitly by passing `frozen=True` or implicitly by pinky-swearing to not mutate them and force the creation of __hash__  using `hash=True`.

*Please* double check before upgrading!

We’re honestly sorry but didn’t see any better way to handle this.  But mind you: hashing mutable objects is a bug so the breakage you’ll 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 = 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=42)
>>> 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 = attr.ib(default=1)
...     y = attr.ib(default=attr.Factory(lambda self: self.x + 1, takes_self=True))
...     z = attr.ib()
...     @z.default
...     def name_does_not_matter(self):
...         return self.x + 1
>>> C()
C(x=1, y=2, z=3)


# 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 = "on"
...     OFF = "off"
>>> @attr.s
... class C(object):
...     state = attr.ib(validator=attr.validators.in_(State))
...     val = attr.ib(validator=attr.validators.in_([1, 2, 3]))
>>> C(State.ON, 1)
C(state=<State.ON: 'on'>, val=1)
>>> 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=`.  Therefore the following lines are equivalent:

x = attr.ib(validator=attr.validators.and_(v1, v2, v3))
x = attr.ib(validator=[v1, v2, v3])


For the attrs team
Hynek

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