Multiple itemadapter classes

README.md

@@ -374,6 +374,46 @@ class attribute in order to handle custom item classes:
 >>>
 ```
 
+### Multiple adapter classes
+
+If you need to have different handlers and/or priorities for different cases
+you can subclass the `ItemAdapter` class and set the `ADAPTER_CLASSES`
+attribute as needed:
+
+
+**Example**
+```python
+>>> from collections import deque
+>>> from itemadapter.adapter import (
+...     ItemAdapter,
+...     AttrsAdapter,
+...     DataclassAdapter,
+...     DictAdapter,
+...     PydanticAdapter,
+...     ScrapyItemAdapter,
+... )
+>>> from scrapy.item import Item, Field
+>>>
+>>> class BuiltinTypesItemAdapter(ItemAdapter):
+...     ADAPTER_CLASSES = deque([DictAdapter, DataclassAdapter])
+...
+>>> class ThirdPartyTypesItemAdapter(ItemAdapter):
+...     ADAPTER_CLASSES = deque([AttrsAdapter, PydanticAdapter, ScrapyItemAdapter])
+...
+>>> class ScrapyItem(Item):
+...     foo = Field()
+...
+>>> BuiltinTypesItemAdapter.is_item(dict())
+True
+>>> ThirdPartyTypesItemAdapter.is_item(dict())
+False
+>>> BuiltinTypesItemAdapter.is_item(ScrapyItem(foo="bar"))
+False
+>>> ThirdPartyTypesItemAdapter.is_item(ScrapyItem(foo="bar"))
+True
+>>>
+```
+
 ---
 
 ## More examples

itemadapter/adapter.py

@@ -361,17 +361,16 @@ def asdict(self) -> dict:
         """Return a dict object with the contents of the adapter. This works slightly different
         than calling `dict(adapter)`: it's applied recursively to nested items (if there are any).
         """
-        return {key: _asdict(value) for key, value in self.items()}
-
-
-def _asdict(obj: Any) -> Any:
-    """Helper for ItemAdapter.asdict()."""
-    if isinstance(obj, dict):
-        return {key: _asdict(value) for key, value in obj.items()}
-    if isinstance(obj, (list, set, tuple)):
-        return obj.__class__(_asdict(x) for x in obj)
-    if isinstance(obj, ItemAdapter):
-        return obj.asdict()
-    if ItemAdapter.is_item(obj):
-        return ItemAdapter(obj).asdict()
-    return obj
+        return {key: self._asdict(value) for key, value in self.items()}
+
+    @classmethod
+    def _asdict(cls, obj: Any) -> Any:
+        if isinstance(obj, dict):
+            return {key: cls._asdict(value) for key, value in obj.items()}
+        if isinstance(obj, (list, set, tuple)):
+            return obj.__class__(cls._asdict(x) for x in obj)
+        if isinstance(obj, cls):
+            return obj.asdict()
+        if cls.is_item(obj):
+            return cls(obj).asdict()
+        return obj

pylintrc

@@ -7,6 +7,7 @@ disable=
       missing-function-docstring,
       missing-module-docstring,
       raise-missing-from,
+      too-many-return-statements,
       unused-argument,
 
 

tests/test_itemadapter.py

@@ -1,10 +1,11 @@
 import unittest
+from collections import deque
 
-from itemadapter.adapter import ItemAdapter
+from itemadapter.adapter import ItemAdapter, DictAdapter
 
 
-class SubclassedItemAdapter(ItemAdapter):
-    pass
+class DictOnlyItemAdapter(ItemAdapter):
+    ADAPTER_CLASSES = deque([DictAdapter])
 
 
 class ItemAdapterTestCase(unittest.TestCase):
@@ -13,5 +14,5 @@ def test_repr(self):
         self.assertEqual(repr(adapter), "<ItemAdapter for dict(foo='bar')>")
 
     def test_repr_subclass(self):
-        adapter = SubclassedItemAdapter(dict(foo="bar"))
-        self.assertEqual(repr(adapter), "<SubclassedItemAdapter for dict(foo='bar')>")
+        adapter = DictOnlyItemAdapter(dict(foo="bar"))
+        self.assertEqual(repr(adapter), "<DictOnlyItemAdapter for dict(foo='bar')>")