71 lines
2.1 KiB
Markdown
71 lines
2.1 KiB
Markdown
# 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:
|
|
|
|
```{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.
|
|
:::
|