feat: Raise instead of returning None in unregister

altair/theme.py

@@ -251,16 +251,30 @@ def wrapper(*args: P.args, **kwargs: P.kwargs) -> ThemeConfig:
     return decorate
 
 
-def unregister(name: LiteralString) -> Plugin[ThemeConfig] | None:
+def unregister(name: LiteralString) -> Plugin[ThemeConfig]:
     """
     Remove and return a previously registered theme.
 
     Parameters
     ----------
     name
         Unique name assigned in ``alt.theme.themes``.
+
+    Raises
+    ------
+    TypeError
+        When ``name`` has not been registered.
     """
-    return themes.register(name, None)
+    plugin = themes.register(name, None)
+    if plugin is None:
+        msg = (
+            f"Found no theme named {name!r} in registry.\n"
+            f"Registered themes:\n"
+            f"{names()!r}"
+        )
+        raise TypeError(msg)
+    else:
+        return plugin
 
 
 enable = themes.enable

tests/vegalite/v5/test_theme.py

@@ -53,12 +53,16 @@ def custom_theme() -> ThemeConfig:
 
     assert theme.active == "big square"
     fn = theme.unregister("big square")
-    assert fn is not None
     assert fn() == custom_theme()
     assert theme.active == theme.themes.active
     # BUG: https://github.com/vega/altair/issues/3619
     # assert theme.active != "big square"
 
+    with pytest.raises(
+        TypeError, match=r"Found no theme named 'big square' in registry."
+    ):
+        theme.unregister("big square")
+
 
 @pytest.mark.parametrize(
     ("color_code", "valid"),