BUG: Fix error message for assigning a column with an empty DataFrame (…

…#56120)

doc/source/whatsnew/v2.2.0.rst

@@ -590,6 +590,7 @@ Other
 - Bug in :meth:`Dataframe.from_dict` which would always sort the rows of the created :class:`DataFrame`.  (:issue:`55683`)
 - Bug in rendering ``inf`` values inside a a :class:`DataFrame` with the ``use_inf_as_na`` option enabled (:issue:`55483`)
 - Bug in rendering a :class:`Series` with a :class:`MultiIndex` when one of the index level's names is 0 not having that name displayed (:issue:`55415`)
+- Bug in the error message when assigning an empty dataframe to a column (:issue:`55956`)
 
 .. ***DO NOT USE THIS SECTION***
 

pandas/core/frame.py

@@ -4375,11 +4375,15 @@ def _set_item_frame_value(self, key, value: DataFrame) -> None:
 
             return self.isetitem(locs, value)
 
-        if len(value.columns) != 1:
+        if len(value.columns) > 1:
             raise ValueError(
                 "Cannot set a DataFrame with multiple columns to the single "
                 f"column {key}"
             )
+        elif len(value.columns) == 0:
+            raise ValueError(
+                f"Cannot set a DataFrame without columns to the column {key}"
+            )
 
         self[key] = value[value.columns[0]]
 

pandas/tests/frame/indexing/test_setitem.py

@@ -93,6 +93,11 @@ def test_setitem_error_msmgs(self):
         with pytest.raises(ValueError, match=msg):
             df["gr"] = df.groupby(["b", "c"]).count()
 
+        # GH 55956, specific message for zero columns
+        msg = "Cannot set a DataFrame without columns to the column gr"
+        with pytest.raises(ValueError, match=msg):
+            df["gr"] = DataFrame()
+
     def test_setitem_benchmark(self):
         # from the vb_suite/frame_methods/frame_insert_columns
         N = 10