add feature flag for default key behavior

Author: rmorshea

src/idom/config.py

@@ -52,3 +52,29 @@
     Absolute URL
         ``ABSOLUTE_PATH/my-module.js``
 """
+
+IDOM_FEATURE_INDEX_AS_DEFAULT_KEY = _option.Option(
+    "IDOM_FEATURE_INDEX_AS_DEFAULT_KEY",
+    default=False,
+    mutable=False,
+    validator=lambda x: bool(int(x)),
+)
+"""A feature flag for using the index of a sibling element as its default key
+
+In a future release this flag's default value will be set to true, and after that, this
+flag willbe removed entirely and the indices will always be the default key.
+
+For more information on changes to this feature flag see: https://github.com/idom-team/idom/issues/351
+"""
+
+if not IDOM_FEATURE_INDEX_AS_DEFAULT_KEY.get():  # pragma: no cover
+    from warnings import warn
+
+    warn(
+        (
+            "In a future release 'IDOM_FEATURE_INDEX_AS_DEFAULT_KEY' will be turned on "
+            "by default. For more information on changes to this feature flag, see: "
+            "https://github.com/idom-team/idom/issues/351"
+        ),
+        UserWarning,
+    )

src/idom/core/layout.py

@@ -21,7 +21,7 @@
 from jsonpatch import apply_patch, make_patch
 from typing_extensions import TypedDict
 
-from idom.config import IDOM_DEBUG_MODE
+from idom.config import IDOM_DEBUG_MODE, IDOM_FEATURE_INDEX_AS_DEFAULT_KEY
 
 from .component import AbstractComponent
 from .events import EventHandler
@@ -453,7 +453,7 @@ async def get(self) -> AbstractComponent:
 def _process_child_type_and_key(
     children: List[Any],
 ) -> Iterator[Tuple[Any, int, Any]]:
-    for child in children:
+    for index, child in enumerate(children):
         if isinstance(child, dict):
             child_type = _DICT_TYPE
             key = child.get("key")
@@ -466,11 +466,23 @@ def _process_child_type_and_key(
             key = None
 
         if key is None:
-            key = object()
+            key = _default_key(index)
 
         yield (child, child_type, key)
 
 
+if IDOM_FEATURE_INDEX_AS_DEFAULT_KEY.get():
+
+    def _default_key(index: int) -> Any:  # pragma: no cover
+        return index
+
+
+else:
+
+    def _default_key(index: int) -> Any:
+        return object()
+
+
 # used in _process_child_type_and_key
 _DICT_TYPE = 1
 _COMPONENT_TYPE = 2