Add tests for placeholder widget.

tests/snapshot_tests/test_snapshots.py

@@ -71,6 +71,11 @@ def test_buttons_render(snap_compare):
     assert snap_compare(WIDGET_EXAMPLES_DIR / "button.py", press=["tab"])
 
 
+def test_placeholder_render(snap_compare):
+    # Testing the rendering of the multiple placeholder variants and labels.
+    assert snap_compare(WIDGET_EXAMPLES_DIR / "placeholder.py")
+
+
 def test_datatable_render(snap_compare):
     press = ["tab", "down", "down", "right", "up", "left"]
     assert snap_compare(WIDGET_EXAMPLES_DIR / "data_table.py", press=press)

tests/test_placeholder.py

@@ -0,0 +1,15 @@
+import pytest
+
+from textual.widgets import Placeholder
+from textual.widgets._placeholder import InvalidPlaceholderVariant
+
+
+def test_invalid_placeholder_variant():
+    with pytest.raises(InvalidPlaceholderVariant):
+        Placeholder("this is clearly not a valid variant!")
+
+
+def test_invalid_reactive_variant_change():
+    p = Placeholder()
+    with pytest.raises(InvalidPlaceholderVariant):
+        p.variant = "this is clearly not a valid variant!"