Update grouping flattening methods (#46)

Ensure tuples of (str, value) are *always* created for the "flat" list
of groupings used for the `options` attribute and trait.

Remove superfluous static method.

examples/examples.ipynb

@@ -245,9 +245,9 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.8.6"
+   "version": "3.8.10"
   }
  },
  "nbformat": 4,
  "nbformat_minor": 4
-}
+}

ipywidgets_extended/dropdown.py

@@ -1,5 +1,5 @@
 """Dropdown Widget Extension"""
-from typing import Any, List, Tuple, Union
+from typing import Any, List, Tuple
 
 from ipywidgets.widgets.widget_selection import Dropdown, _make_options
 from traitlets import traitlets
@@ -117,7 +117,7 @@ def __init__(self, *args, **kwargs):
                 kwargs["index"] = kwargs["label"] = kwargs["value"] = None
 
         if "grouping" in kwargs:
-            kwargs["options"] = self._create_grouping_options(grouping)
+            kwargs["options"] = self._flat_groupings(grouping)
         super().__init__(*args, **kwargs)
         self._initializing_traits_ = True
 
@@ -192,7 +192,7 @@ def _validate_grouping(self, proposal) -> Tuple[Tuple[str, List[str]]]:
     def _set_grouping(self, change) -> None:
         """Put options into desired grouping, updating `options`"""
         grouping = self._grouping_full
-        self.options = self._create_grouping_options(grouping)
+        self.options = self._flat_groupings(grouping)
         self.set_trait(
             "_grouping_labels",
             tuple(
@@ -222,15 +222,15 @@ def _group_headers(self) -> List[str]:
         return [_[0] for _ in self._grouping_labels]
 
     def _flat_groupings(
-        self, grouping: Tuple[Tuple[str, Tuple[Any]]] = None
-    ) -> List[Union[str, Any]]:
-        """Get grouping similar to dropdown - a flat list of entries"""
+        self, grouping: Tuple[Tuple[str, Tuple[Tuple[str, Any]]]] = None
+    ) -> List[Tuple[str, Any]]:
+        """Get grouping similar to dropdown - a flat list of entries."""
         grouping = grouping if grouping is not None else self._grouping_labels
 
         res = []
         for header, options in grouping:
             if header:
-                res.append(header)
+                res.append((header, None))
             res.extend(options)
         return res
 
@@ -246,18 +246,8 @@ def _get_grouping_label_value(
         grouping = grouping if grouping is not None else self._grouping_full
 
         res = self._flat_groupings(grouping)[index]
-        if len(res) == 1:
-            return res, None
-        return res
-
-    @staticmethod
-    def _create_grouping_options(
-        grouping: Tuple[Tuple[str, Tuple[Tuple[str, Any]]]]
-    ) -> Tuple[Tuple[str, Any]]:
-        """Create and return a standard list of options from a grouping."""
-        res = []
-        for header, options in grouping:
-            if header:
-                res.append((header, None))
-            res.extend(options)
+        if not isinstance(res, tuple) or len(res) != 2:
+            raise ValueError(
+                f"Found a grouped value that is not a tuple with length 2: {res!r}"
+            )
         return res