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=True1, attrs will also create a full set of ordering methods that are based on the defined fields: __le__, __lt__, __ge__, and __gt__.
Customization
As with other features, you can exclude fields from being involved in comparison operations:
>>> from attrs 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():
>>> @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.
To save you unnecessary boilerplate, attrs comes with the attrs.cmp_using() helper to create such functions.
For NumPy arrays it would look like this:
import numpy
@define
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 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.