Merge pull request #199 from Gallaecio/fieldsmixin

Cover FieldsMixin

docs/page-objects/fields.rst

@@ -212,11 +212,11 @@ For example:
     class CustomPage(BasePage, Returns[CustomItem], skip_nonitem_fields=True):
         pass
 
-Alternatively, you can consider :ref:`composition <composition>` for removing
-fields. Composition is more verbose than subclassing, because you need to
-define every field in your page object class, but it can catch some mismatches
-between page object class fields and item class fields that would otherwise be
-hidden by ``skip_nonitem_fields``.
+Alternatively, you can consider :ref:`using a page object as input
+<composition-input>` for removing fields. It is more verbose than subclassing,
+because you need to define every field in your page object class, but it can
+catch some mismatches between page object class fields and item class fields
+that would otherwise be hidden by ``skip_nonitem_fields``.
 
 
 .. _rename-field:
@@ -263,18 +263,27 @@ For example:
         async def new_field(self) -> str:
             return ensure_awaitable(self.old_field)
 
-Alternatively, you can consider :ref:`composition <composition>` for renaming
-fields. Composition is more verbose than subclassing, because you need to
-define every field in your page object class, but it can catch some mismatches
-between page object class fields and item class fields that would otherwise be
-hidden by ``skip_nonitem_fields``.
+Alternatively, you can consider :ref:`using a page object as input
+<composition-input>` for renaming fields. It is more verbose than subclassing,
+because you need to define every field in your page object class, but it can
+catch some mismatches between page object class fields and item class fields
+that would otherwise be hidden by ``skip_nonitem_fields``.
 
 
 .. _composition:
 
 Composition
 ===========
 
+There are 2 forms of composition that you can use when writing a page object:
+:ref:`using a page object as input <composition-input>`, and :ref:`using a
+field mixing <field-mixins>`.
+
+.. _composition-input:
+
+Using a page object as input
+----------------------------
+
 You can reuse a page object class from another page object class using
 composition instead of :ref:`inheritance <inheritance>` by using the original
 page object class as a dependency in a brand new page object class returning a
@@ -339,6 +348,45 @@ On the other hand, all fields of the source page object class will always be
 called to build the entire item, which may be a waste of resources if you only
 need to access some of the item fields.
 
+.. _field-mixins:
+
+Field mixins
+------------
+
+You can subclass :class:`web_poet.fields.FieldsMixin` to create a mixin_ to
+reuse field definitions across multiple, otherwise-unrelated classes. For
+example:
+
+.. _mixin: https://en.wikipedia.org/wiki/Mixin
+
+.. code-block:: python
+
+    import attrs
+    from web_poet import ItemPage, field
+    from web_poet.fields import FieldsMixin
+
+    from my_library import BaseItem1, BaseItem2
+
+    @attrs.define
+    class CustomItem:
+        name: str
+
+
+    class NameMixin(FieldsMixin):
+        @field
+        def name(self) -> str:
+            return f"{self.base.brand}: {self.base.name}"
+
+
+    @attrs.define
+    class CustomPage1(NameMixin, ItemPage[CustomItem]):
+        base: BaseItem1
+
+
+    @attrs.define
+    class CustomPage2(NameMixin, ItemPage[CustomItem]):
+        base: BaseItem2
+
 
 .. _field-processors: