Merge pull request #236 from lincc-frameworks/fix-flat_length

Fix NestedExtensionArray.flat_length for empty arrays

Author: dougbrn

src/nested_pandas/series/ext_array.py

@@ -798,7 +798,10 @@ def list_lengths(self) -> np.ndarray:
     @property
     def flat_length(self) -> int:
         """Length of the flat arrays"""
-        return pa.compute.sum(self.list_lengths).as_py()
+        sum_result = pa.compute.sum(self.list_lengths).as_py()
+        if sum_result is None:
+            sum_result = 0
+        return sum_result
 
     @property
     def num_chunks(self) -> int:

tests/nested_pandas/nestedframe/test_nestedframe.py

@@ -1277,3 +1277,9 @@ def test_issue193():
     ndf.query("nested.flux / nested.t > 0")
     # This failed with numpy 1 with:
     # TypeError: Cannot interpret 'double[pyarrow]' as a data type
+
+
+def test_issue235():
+    """https://github.com/lincc-frameworks/nested-pandas/issues/235"""
+    nf = generate_data(3, 10).iloc[:0]
+    nf["nested.x"] = []