Make ItemAdapter.ADAPTER_CLASSES an Iterable instead of a deque

README.md

@@ -15,7 +15,7 @@ Currently supported types are:
 * [`dict`](https://docs.python.org/3/library/stdtypes.html#dict)
 * [`dataclass`](https://docs.python.org/3/library/dataclasses.html)-based classes
 * [`attrs`](https://www.attrs.org)-based classes
-* [`pydantic`](https://pydantic-docs.helpmanual.io/)-based classes
+* [`pydantic`](https://pydantic-docs.helpmanual.io/)-based classes (`pydantic>=2` not yet supported)
 
 Additionally, interaction with arbitrary types is supported, by implementing
 a pre-defined interface (see [extending `itemadapter`](#extending-itemadapter)).
@@ -27,7 +27,8 @@ a pre-defined interface (see [extending `itemadapter`](#extending-itemadapter)).
 * Python 3.7+
 * [`scrapy`](https://scrapy.org/): optional, needed to interact with `scrapy` items
 * [`attrs`](https://pypi.org/project/attrs/): optional, needed to interact with `attrs`-based items
-* [`pydantic`](https://pypi.org/project/pydantic/): optional, needed to interact with `pydantic`-based items
+* [`pydantic`](https://pypi.org/project/pydantic/): optional, needed to interact with
+  `pydantic`-based items (`pydantic>=2` not yet supported)
 
 ---
 
@@ -151,19 +152,21 @@ interface, providing a `dict`-like API to manipulate data for the object it wrap
 
 **Attributes**
 
-#### class attribute `ADAPTER_CLASSES: collections.deque`
+#### class attribute `ADAPTER_CLASSES: Iterable`
 
-Stores the currently registered adapter classes. Being a
-[`collections.deque`](https://docs.python.org/3/library/collections.html#collections.deque),
-it supports efficient addition/deletion of adapters classes to both ends.
+Stores the currently registered adapter classes.
 
 The order in which the adapters are registered is important. When an `ItemAdapter` object is
 created for a specific item, the registered adapters are traversed in order and the first
 adapter class to return `True` for the `is_item` class method is used for all subsequent
 operations. The default order is the one defined in the
 [built-in adapters](#built-in-adapters) section.
 
-See the section on [extending itemadapter](#extending-itemadapter) for additional information.
+The default implementation uses a
+[`collections.deque`](https://docs.python.org/3/library/collections.html#collections.deque)
+to support efficient addition/deletion of adapters classes to both ends, but if you are
+deriving a subclass (see the section on [extending itemadapter](#extending-itemadapter)
+for additional information), any other iterable (e.g. `list`, `tuple`) will work.
 
 **Methods**
 
@@ -383,7 +386,6 @@ attribute as needed:
 
 **Example**
 ```python
->>> from collections import deque
 >>> from itemadapter.adapter import (
 ...     ItemAdapter,
 ...     AttrsAdapter,
@@ -395,10 +397,10 @@ attribute as needed:
 >>> from scrapy.item import Item, Field
 >>>
 >>> class BuiltinTypesItemAdapter(ItemAdapter):
-...     ADAPTER_CLASSES = deque([DictAdapter, DataclassAdapter])
+...     ADAPTER_CLASSES = [DictAdapter, DataclassAdapter]
 ...
 >>> class ThirdPartyTypesItemAdapter(ItemAdapter):
-...     ADAPTER_CLASSES = deque([AttrsAdapter, PydanticAdapter, ScrapyItemAdapter])
+...     ADAPTER_CLASSES = [AttrsAdapter, PydanticAdapter, ScrapyItemAdapter]
 ...
 >>> class ScrapyItem(Item):
 ...     foo = Field()

itemadapter/adapter.py

@@ -3,7 +3,7 @@
 from collections import deque
 from collections.abc import KeysView, MutableMapping
 from types import MappingProxyType
-from typing import Any, Deque, Iterator, Type, Optional, List
+from typing import Any, Iterable, Iterator, Type, Optional, List
 
 from itemadapter.utils import (
     _get_pydantic_model_metadata,
@@ -271,7 +271,7 @@ class ItemAdapter(MutableMapping):
     to extract and set data without having to take the object's type into account.
     """
 
-    ADAPTER_CLASSES: Deque[Type[AdapterInterface]] = deque(
+    ADAPTER_CLASSES: Iterable[Type[AdapterInterface]] = deque(
         [
             ScrapyItemAdapter,
             DictAdapter,

tests/requirements.txt

@@ -1,5 +1,5 @@
 attrs
-pydantic
+pydantic<2
 pytest-cov>=2.8
 pytest>=5.4
 scrapy>=2.0

tests/test_adapter_pydantic.py

@@ -17,7 +17,7 @@
 )
 
 
-class DataclassTestCase(unittest.TestCase):
+class PydanticTestCase(unittest.TestCase):
     def test_false(self):
         from itemadapter.adapter import PydanticAdapter
 

tests/test_itemadapter.py

@@ -1,11 +1,10 @@
 import unittest
-from collections import deque
 
 from itemadapter.adapter import ItemAdapter, DictAdapter
 
 
 class DictOnlyItemAdapter(ItemAdapter):
-    ADAPTER_CLASSES = deque([DictAdapter])
+    ADAPTER_CLASSES = [DictAdapter]
 
 
 class ItemAdapterTestCase(unittest.TestCase):