Allow passing a Selection into a SelctionList

Up until now I've only been allowing tuples; mostly a hangover from the
initial take on this. Things have drifted a bit now and I feel it makes
sense to allow Selection instances in too.

src/textual/widgets/_selection_list.py

@@ -199,7 +199,8 @@ class SelectionList(Generic[SelectionType], OptionList):
 
     def __init__(
         self,
-        *selections: tuple[TextType, SelectionType]
+        *selections: Selection
+        | tuple[TextType, SelectionType]
         | tuple[TextType, SelectionType, bool],
         name: str | None = None,
         id: str | None = None,
@@ -397,7 +398,8 @@ class SelectionList(Generic[SelectionType], OptionList):
 
     def _make_selection(
         self,
-        selection: tuple[TextType, SelectionType]
+        selection: Selection
+        | tuple[TextType, SelectionType]
         | tuple[TextType, SelectionType, bool],
     ) -> Selection[SelectionType]:
         """Turn incoming selection data into a `Selection` instance.
@@ -411,6 +413,10 @@ class SelectionList(Generic[SelectionType], OptionList):
         Raises:
             SelectionError: If the selection was badly-formed.
         """
+
+        # If we've been given a tuple of some sort, turn that into a proper
+        # Selection.
+        if isinstance(selection, tuple):
             if len(selection) == 3:
                 label, value, selected = cast(
                     "tuple[TextType, SelectionType, bool]", selection
@@ -421,9 +427,17 @@ class SelectionList(Generic[SelectionType], OptionList):
                 )
             else:
                 raise SelectionError(f"Expected 2 or 3 values, got {len(selection)}")
-        if selected:
-            self._select(value)
-        return Selection(label, value)
+            selection = Selection(label, value, selected)
+
+        # At this point we should have a proper selection.
+        assert isinstance(selection, Selection)
+
+        # If the initial state for this is that it's selected, add it to the
+        # selected collection.
+        if selection.initial_state:
+            self._select(selection.value)
+
+        return selection
 
     def _toggle_highlighted_selection(self) -> None:
         """Toggle the state of the highlighted selection.

tests/selection_list/test_selection_list_create.py

@@ -25,6 +25,8 @@ class SelectionListApp(App[None]):
             ("0", 0),
             ("1", 1, False),
             ("2", 2, True),
+            Selection("3", 3, id="3"),
+            Selection("4", 4, True, id="4"),
         )
 
 
@@ -32,8 +34,8 @@ async def test_all_parameters_become_selctions() -> None:
     """All input parameters to a list should become selections."""
     async with SelectionListApp().run_test() as pilot:
         selections = pilot.app.query_one(SelectionList)
-        assert selections.option_count == 3
-        for n in range(3):
+        assert selections.option_count == 5
+        for n in range(5):
             assert isinstance(selections.get_option_at_index(n), Selection)
 
 
@@ -41,24 +43,34 @@ async def test_get_selection_by_index() -> None:
     """It should be possible to get a selection by index."""
     async with SelectionListApp().run_test() as pilot:
         option_list = pilot.app.query_one(SelectionList)
-        for n in range(3):
+        for n in range(5):
             assert option_list.get_option_at_index(n).prompt == Text(str(n))
-        assert option_list.get_option_at_index(-1).prompt == Text("2")
+        assert option_list.get_option_at_index(-1).prompt == Text("4")
+
+
+async def test_get_selection_by_id() -> None:
+    """It should be possible to get a selection by ID."""
+    async with SelectionListApp().run_test() as pilot:
+        option_list = pilot.app.query_one(SelectionList)
+        assert option_list.get_option("3").prompt == Text("3")
+        assert option_list.get_option("4").prompt == Text("4")
 
 
 async def test_add_later() -> None:
     """It should be possible to add more items to a selection list."""
     async with SelectionListApp().run_test() as pilot:
         selections = pilot.app.query_one(SelectionList)
-        assert selections.option_count == 3
-        selections.add_option(("3", 3))
-        assert selections.option_count == 4
-        selections.add_option(Selection("4", 4))
         assert selections.option_count == 5
-        selections.add_options([Selection("5", 5), ("6", 6), ("7", 7, True)])
-        assert selections.option_count == 8
+        selections.add_option(("5", 5))
+        assert selections.option_count == 6
+        selections.add_option(Selection("6", 6))
+        assert selections.option_count == 7
+        selections.add_options(
+            [Selection("7", 7), Selection("8", 8, True), ("9", 9), ("10", 10, True)]
+        )
+        assert selections.option_count == 11
         selections.add_options([])
-        assert selections.option_count == 8
+        assert selections.option_count == 11
 
 
 async def test_add_non_selections() -> None: