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# 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__`.
Both for equality and 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.
[^default]: That's the default if you use the {func}`attr.s` decorator, but not with {func}`~attrs.define`.
(custom-comparison)=
## Customization
As with other features, you can exclude fields from being involved in comparison operations:
```{doctest}
>>> 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`:
```{doctest}
>>> @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 {func}`attrs.cmp_using` helper to create such functions.
For NumPy arrays it would look like this:
```python
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.
:::