avoid "object of type generator has no len()" by checking len of type…

…s instead

tests/test_partitioning.py

@@ -39,13 +39,6 @@ def test_labels_to_indices():
     assert np.array_equal(indices[2], [3, 4])
 
 
-def test_merge_partitions_empty_list():
-    with pytest.raises(
-        ValueError, message="Received empty partitions list, cannot be merged."
-    ):
-        xu.merge_partitions([])
-
-
 class TestGridPartitioning:
     @pytest.fixture(autouse=True)
     def setup(self):
@@ -169,6 +162,11 @@ def test_merge_partitions__errors(self):
         ):
             pt.merge_partitions(partitions)
 
+        with pytest.raises(
+            ValueError, match="Received empty partitions list, cannot be merged."
+        ):
+            xu.merge_partitions([])
+
     def test_merge_partitions_no_duplicates(self):
         part1 = self.uds.isel(mesh2d_nFaces=[0, 1, 2, 3])
         part2 = self.uds.isel(mesh2d_nFaces=[2, 3, 4, 5])

xugrid/ugrid/partitioning.py

@@ -328,10 +328,10 @@ def merge_partitions(partitions, merge_ugrid_chunks: bool = True):
     -------
     merged : UgridDataset
     """
-    if len(partitions) == 0:
-        raise ValueError("Received empty partitions list, cannot be merged.")
     types = {type(obj) for obj in partitions}
     msg = "Expected UgridDataArray or UgridDataset, received: {}"
+    if len(types) == 0:
+        raise ValueError("Received empty partitions list, cannot be merged.")
     if len(types) > 1:
         type_names = [t.__name__ for t in types]
         raise TypeError(msg.format(type_names))