fix KeyError: "None of [Index(['0_x', '1_x', '0_y', '1_y'], dtype='object')] are in the [columns]" in find_objects

dask_image/ndmeasure/_utils/_find_objects.py

@@ -79,6 +79,17 @@ def _find_objects(ndim, df1, df2):
     if isinstance(df2, Delayed):
         with dask_config.set({'dataframe.convert-string': False}):
             df2 = dd.from_delayed(df2, meta=meta)
-    ddf = dd.merge(df1, df2, how="outer", left_index=True, right_index=True)
+
+    if len(df1) > 0 and len(df2) > 0:
+        ddf = dd.merge(
+            df1, df2,
+            how="outer", left_index=True, right_index=True)
+    elif len(df1) > 0:
+        ddf = df1
+    elif len(df2) > 0:
+        ddf = df2
+    else:
+        ddf = pd.DataFrame()
+
     result = ddf.apply(_merge_bounding_boxes, ndim=ndim, axis=1, meta=meta)
     return result

tests/test_dask_image/test_ndmeasure/test_find_objects.py

@@ -54,6 +54,15 @@ def test_find_objects_err(label_image):
         dask_image.ndmeasure.find_objects(label_image)
 
 
+def test_empty_chunk():
+    test_labels = da.zeros((10, 10), dtype='int', chunks=(3, 3))
+    test_labels[0, 0] = 1
+    computed_result = dask_image.ndmeasure.find_objects(test_labels).compute()
+    expected = pd.DataFrame.from_dict({0: {1: slice(0, 1)},
+                                       1: {1: slice(0, 1)}, })
+    assert computed_result.equals(expected)
+
+
 def test_find_objects(label_image):
     result = dask_image.ndmeasure.find_objects(label_image)
     assert isinstance(result, dd.DataFrame)