Add tests for first-child and last-child pseudo-classes

Author: Sergi Pons Freixes

tests/snapshot_tests/__snapshots__/test_snapshots/test_child_pseudo_classes.svg

@@ -2554,6 +2554,38 @@ def on_mount(self) -> None:
 
     assert snap_compare(PSApp())
 
+def test_child_pseudo_classes(snap_compare):
+    """Test pseudo classes added in https://github.com/Textualize/textual/pull/XXXX
+
+    You should see 2 labels and 3 buttons
+
+    The first label should have a red border.
+
+    The last button should have a green border.
+    """
+
+    class CPSApp(App):
+        CSS = """
+        Label { width: 1fr; height: 1fr; }
+        Button { width: 1fr; height: 1fr; }
+        Label:first-child { border:heavy red; }
+        Label:last-child { border:heavy orange; }
+        Button:first-child { border:heavy yellow; }
+        Button:last-child { border:heavy green; }
+        """
+
+        def compose(self) -> ComposeResult:
+            yield Label("Label 1")
+            yield Label("Label 2")
+            yield Button("Button 1")
+            yield Button("Button 2")
+
+        def on_mount(self) -> None:
+            # Mounting a new widget should update previous widgets, as the last child has changed
+            self.mount(Button("HELLO"))
+
+    assert snap_compare(CPSApp())
+
 
 def test_split_segments_infinite_loop(snap_compare):
     """Regression test for https://github.com/Textualize/textual/issues/5151

tests/snapshot_tests/test_snapshots.py

@@ -43,6 +43,7 @@ async def test_hover_update_styles():
             "enabled",
             "first-of-type",
             "last-of-type",
+            "last-child",
             "even",
         }
 
@@ -59,6 +60,7 @@ async def test_hover_update_styles():
             "hover",
             "first-of-type",
             "last-of-type",
+            "last-child",
             "even",
         }
         assert button.styles.background != initial_background

tests/test_app.py

@@ -646,26 +646,36 @@ def compose(self) -> ComposeResult:
         labels = list(app.query(Label))
         assert labels[0].first_of_type
         assert not labels[0].last_of_type
+        assert labels[0].first_child
+        assert not labels[0].last_child
         assert labels[0].is_odd
         assert not labels[0].is_even
 
         assert not labels[1].first_of_type
         assert not labels[1].last_of_type
+        assert not labels[1].first_child
+        assert not labels[1].last_child
         assert not labels[1].is_odd
         assert labels[1].is_even
 
         assert not labels[2].first_of_type
         assert not labels[2].last_of_type
+        assert not labels[2].first_child
+        assert not labels[2].last_child
         assert labels[2].is_odd
         assert not labels[2].is_even
 
         assert not labels[3].first_of_type
         assert not labels[3].last_of_type
+        assert not labels[3].first_child
+        assert not labels[3].last_child
         assert not labels[3].is_odd
         assert labels[3].is_even
 
         assert not labels[4].first_of_type
         assert labels[4].last_of_type
+        assert not labels[4].first_child
+        assert labels[4].last_child
         assert labels[4].is_odd
         assert not labels[4].is_even