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Tin Tvrtkovic 2016-03-30 22:54:46 +02:00
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@ -382,11 +382,9 @@ Slots
By default, instances of classes have a dictionary for attribute storage. By default, instances of classes have a dictionary for attribute storage.
This wastes space for objects having very few instance variables. This wastes space for objects having very few instance variables.
The space consumption can become acute The space consumption can become acute when creating large numbers of instances.
when creating large numbers of instances.
Normal Python classes can avoid using a separate dictionary Normal Python classes can avoid using a separate dictionary for each instance of a class by defining ``__slots__``.
for each instance of a class by defining ``__slots__``.
``attrs`` classes should just use ``slots=True``. ``attrs`` classes should just use ``slots=True``.
@ -400,57 +398,47 @@ for each instance of a class by defining ``__slots__``.
.. note:: .. note::
``attrs`` slot classes can inherit from other classes ``attrs`` slot classes can inherit from other classes just like non-slot classes, but some of the benefits of slot classes are lost if you do that.
just like non-slot classes, If you must inherit from other classes, try to inherit only from other slot classes.
but some of the benefits of slot classes are lost if you do that.
If you must inherit from other classes,
try to inherit only from other slot classes.
Slot classes are a little different than ordinary, dictionary-backed classes: Slot classes are a little different than ordinary, dictionary-backed classes:
* assigning to a non-existent attribute of an instance - Assigning to a non-existent attribute of an instance will result in an ``AttributeError`` being raised.
will result in an ``AttributeError`` being raised. Depending on your needs, this might be a good thing since it will let you catch typos early.
Depending on your needs, this might actually be a good thing
since it will let you catch typos early.
This is not the case if your class inherits from any non-slot classes. This is not the case if your class inherits from any non-slot classes.
.. doctest:: .. doctest::
>>> @attr.s(slots=True) >>> @attr.s(slots=True)
... class Coordinates(object): ... class Coordinates(object):
... x = attr.ib() ... x = attr.ib()
... y = attr.ib() ... y = attr.ib()
... ...
>>> c = Coordinates(x=1, y=2) >>> c = Coordinates(x=1, y=2)
>>> c.z = 3 >>> c.z = 3
Traceback (most recent call last): Traceback (most recent call last):
... ...
AttributeError: 'Coordinates' object has no attribute 'z' AttributeError: 'Coordinates' object has no attribute 'z'
* slot classes cannot share attribute names with their instances, - Slot classes cannot share attribute names with their instances, while non-slot classes can.
while non-slot classes can.
The following behaves differently if slot classes are used: The following behaves differently if slot classes are used:
.. doctest:: .. doctest::
>>> @attr.s >>> @attr.s
... class C(object): ... class C(object):
... x = attr.ib() ... x = attr.ib()
>>> C.x >>> C.x
Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None) Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None)
>>> @attr.s(slots=True) >>> @attr.s(slots=True)
... class C(object): ... class C(object):
... x = attr.ib() ... x = attr.ib()
>>> C.x >>> C.x
<member 'x' of 'C' objects> <member 'x' of 'C' objects>
* since non-slot classes cannot be turned into slot classes - Since non-slot classes cannot be turned into slot classes after they have been created, ``attr.s(.., slots=True)`` will actually replace the class it is applied to with a copy.
after they have been created, In almost all cases this isn't a problem, but we mention it for the sake of completeness.
``attr.s(.., slots=True)`` will actually replace the class
it is applied to with a copy.
In almost all cases this isn't a problem,
but we mention it for the sake of completeness.
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