Test type annotations for Variable.get_axis_num (pydata#9832)

This is a regression test for GH9822.

xarray/tests/test_variable.py

@@ -1604,14 +1604,19 @@ def test_squeeze(self):
         with pytest.raises(ValueError, match=r"cannot select a dimension"):
             v.squeeze("y")
 
-    def test_get_axis_num(self):
+    def test_get_axis_num(self) -> None:
         v = Variable(["x", "y", "z"], np.random.randn(2, 3, 4))
         assert v.get_axis_num("x") == 0
         assert v.get_axis_num(["x"]) == (0,)
         assert v.get_axis_num(["x", "y"]) == (0, 1)
         assert v.get_axis_num(["z", "y", "x"]) == (2, 1, 0)
         with pytest.raises(ValueError, match=r"not found in array dim"):
             v.get_axis_num("foobar")
+        # Test the type annotations: mypy will complain if the inferred
+        # type is wrong
+        v.get_axis_num("x") + 0
+        v.get_axis_num(["x"]) + ()
+        v.get_axis_num(("x", "y")) + ()
 
     def test_set_dims(self):
         v = Variable(["x"], [0, 1])