attrs/docs/comparison.md

Comparison

By default, two instances of attrs classes are equal if they have the same type and all their fields are equal. For that, attrs writes __eq__ and __ne__ methods for you.

Additionally, if you pass order=True, attrs will also create a complete set of ordering methods: __le__, __lt__, __ge__, and __gt__.

For equality, attrs will generate a statement comparing the types of both instances, and then comparing each attribute in turn using ==.

For order, attrs will:

• Check if the types of the instances you're comparing are equal,
• if so, create a tuple of all field values for each instance,
• and finally perform the desired comparison operation on those tuples.

(custom-comparison)=

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 {func}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 {func}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=attrs.cmp_using(eq=numpy.array_equal))

:::{warning} Please note that eq and order are set independently, because order is False by default in {func}~attrs.define (but not in {func}attr.s). You can set both at once by using the cmp argument that we've undeprecated just for this use-case. :::