Initial framework for the SelectionList

Nothing here actually implements a selection list yet; this just sets out
the very basic framework of the widget, as it inherits form OptionList. The
key things here are:

1. It introduces a Selection class, which is an Option with a typed value.
2. The SelectionList is also typed and expects Selections of that type.

src/textual/widgets/__init__.py

@@ -30,6 +30,7 @@ if typing.TYPE_CHECKING:
     from ._radio_button import RadioButton
     from ._radio_set import RadioSet
     from ._select import Select
+    from ._selection_list import SelectionList
     from ._static import Static
     from ._switch import Switch
     from ._tabbed_content import TabbedContent, TabPane
@@ -61,6 +62,7 @@ __all__ = [
     "RadioButton",
     "RadioSet",
     "Select",
+    "SelectionList",
     "Static",
     "Switch",
     "Tab",

src/textual/widgets/__init__.pyi

@@ -20,6 +20,7 @@ from ._progress_bar import ProgressBar as ProgressBar
 from ._radio_button import RadioButton as RadioButton
 from ._radio_set import RadioSet as RadioSet
 from ._select import Select as Select
+from ._selection_list import SelectionList as SelectionList
 from ._static import Static as Static
 from ._switch import Switch as Switch
 from ._tabbed_content import TabbedContent as TabbedContent

src/textual/widgets/_selection_list.py

@@ -0,0 +1,57 @@
+"""Provides a selection list widget, allowing one or more items to be selected."""
+
+from typing import Generic, TypeVar
+
+from rich.console import RenderableType
+
+from ._option_list import Option, OptionList
+
+SelectionType = TypeVar("SelectionType")
+"""The type for the value of a `Selection`"""
+
+
+class Selection(Generic[SelectionType], Option):
+    """A selection for the `SelectionList`."""
+
+    def __init__(
+        self,
+        value: SelectionType,
+        prompt: RenderableType,
+        id: str | None = None,
+        disabled: bool = False,
+    ):
+        """Initialise the selection.
+
+        Args:
+            value: The value for the selection.
+            prompt: The prompt for the selection.
+            id: The optional ID for the selection.
+            disabled: The initial enabled/disabled state. Enabled by default.
+        """
+        super().__init__(prompt, id, disabled)
+        self._value: SelectionType = value
+
+
+class SelectionList(Generic[SelectionType], OptionList):
+    """A vertical option list that allows making multiple selections."""
+
+    def __init__(
+        self,
+        *selections: Selection[SelectionType],
+        name: str | None = None,
+        id: str | None = None,
+        classes: str | None = None,
+        disabled: bool = False,
+    ):
+        """Initialise the selection list.
+
+        Args:
+            *content: The content for the selection list.
+            name: The name of the selection list.
+            id: The ID of the selection list in the DOM.
+            classes: The CSS classes of the selection list.
+            disabled: Whether the selection list is disabled or not.
+        """
+        super().__init__(
+            *selections, name=name, id=id, classes=classes, disabled=disabled
+        )

src/textual/widgets/selection_list.py

@@ -0,0 +1,3 @@
+from ._selection_list import Selection
+
+__all__ = ["Selection"]