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Comparison
==========
By default, two instances of ``attrs`` classes are equal if all their fields are equal.
For that, ``attrs`` writes ``__eq__`` and ``__ne__`` methods for you.
Additionally, if you pass ``order=True`` (which is the default if you use the `attr.s` decorator), ``attrs`` will also create a full set of ordering methods that are based on the defined fields: ``__le__``, ``__lt__``, ``__ge__``, and ``__gt__``.
.. _custom-comparison:
Customization
-------------
As with other features, you can exclude fields from being involved in comparison operations:
.. doctest::
>>> from attr import define, field
>>> @define
... class C:
... x: int
... y: int = field(eq=False)
>>> C(1, 2) == C(1, 3)
True
Additionally you can also pass a *callable* instead of a bool to both *eq* and *order*.
It is then used as a key function like you may know from `sorted`:
.. doctest::
>>> from attr import define, field
>>> @define
... class S:
... x: str = field(eq=str.lower)
>>> S("foo") == S("FOO")
True
>>> @define(order=True)
... class C:
... x: str = field(order=int)
>>> C("10") > C("2")
True
This is especially useful when you have fields with objects that have atypical comparison properties.
Common examples of such objects are `NumPy arrays <https://github.com/python-attrs/attrs/issues/435>`_.
To save you unnecessary boilerplate, ``attrs`` comes with the `attr.cmp_using` helper to create such functions.
For NumPy arrays it would look like this::
import numpy
@define(order=False)
class C:
an_array = field(eq=attr.cmp_using(eq=numpy.array_equal))
.. warning::
Please note that *eq* and *order* are set *independently*, because *order* is `False` by default in `attrs.define` (but not in `attr.s`).
You can set both at once by using the *cmp* argument that we've undeprecated just for this use-case.