Merge pull request #352 from lincc-frameworks/fix_348

fix add_nested out dtype for empty dataframes

Author: dougbrn

src/nested_pandas/nestedframe/core.py

@@ -408,6 +408,12 @@ def add_nested(
         packed = pack(obj, name=name, on=on, dtype=dtype)
         new_df = self.copy()
         res = new_df.join(packed, how=how, on=on)
+
+        # In some cases join returns a DataFrame, so convert back to NestedFrame
+        # For example, with empty dataframes
+        if not isinstance(res, NestedFrame):
+            res = NestedFrame(res)
+
         return res
 
     def nest_lists(self, columns: list[str], name: str) -> NestedFrame:

tests/nested_pandas/nestedframe/test_nestedframe.py

@@ -2201,6 +2201,16 @@ def test_issue294():
     nf["nested.mag"] = -2.5 * np.log10(nf["nested.flux"])
 
 
+def test_issue348():
+    """https://github.com/lincc-frameworks/nested-pandas/issues/348"""
+    new = NestedFrame.from_flat(
+        pd.DataFrame({"x": np.array([], dtype=float)}),
+        base_columns=[],
+        name="new",
+    )
+    assert isinstance(new, NestedFrame)
+
+
 def test_issue350():
     """https://github.com/lincc-frameworks/nested-pandas/issues/350"""
     nf = generate_data(3, 2)