Add basic selection list creation unit tests

tests/selection_list/test_selection_list_create.py

@@ -0,0 +1,73 @@
+"""Core selection list unit tests, aimed at testing basic list creation.
+
+Note that the vast majority of the API *isn't* tested in here as
+`SelectionList` inherits from `OptionList` and so that would be duplicated
+effort. Instead these tests aim to just test the things that have been
+changed or wrapped in some way.
+"""
+
+from __future__ import annotations
+
+import pytest
+from rich.text import Text
+
+from textual.app import App, ComposeResult
+from textual.widgets import SelectionList
+from textual.widgets.option_list import Option
+from textual.widgets.selection_list import Selection, SelectionError
+
+
+class SelectionListApp(App[None]):
+    """Test selection list application."""
+
+    def compose(self) -> ComposeResult:
+        yield SelectionList[int](
+            ("0", 0),
+            ("1", 1, False),
+            ("2", 2, True),
+        )
+
+
+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 isinstance(selections.get_option_at_index(n), Selection)
+
+
+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):
+            assert option_list.get_option_at_index(n).prompt == Text(str(n))
+        assert option_list.get_option_at_index(-1).prompt == Text("2")
+
+
+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_options([])
+        assert selections.option_count == 8
+
+
+async def test_add_non_selections() -> None:
+    """Adding options that aren't selections should result in errors."""
+    async with SelectionListApp().run_test() as pilot:
+        selections = pilot.app.query_one(SelectionList)
+        with pytest.raises(SelectionError):
+            selections.add_option(None)
+        with pytest.raises(SelectionError):
+            selections.add_option(Option("Nope"))
+        with pytest.raises(SelectionError):
+            selections.add_option("Nope")