BUG: DataFrame.__setitem__ not raising ValueError when rhs is df and has wrong number of columns

doc/source/whatsnew/v1.3.0.rst

@@ -286,6 +286,7 @@ Indexing
 - Bug in :meth:`DataFrame.__setitem__` raising ``ValueError`` with empty :class:`DataFrame` and specified columns for string indexer and non empty :class:`DataFrame` to set (:issue:`38831`)
 - Bug in :meth:`DataFrame.loc.__setitem__` raising ValueError when expanding unique column for :class:`DataFrame` with duplicate columns (:issue:`38521`)
 - Bug in :meth:`DataFrame.iloc.__setitem__` and :meth:`DataFrame.loc.__setitem__` with mixed dtypes when setting with a dictionary value (:issue:`38335`)
+- Bug in :meth:`DataFrame.__setitem__` not raising ``ValueError`` when right hand side is a :class:`DataFrame` with wrong number of columns (:issue:`38604`)
 - Bug in :meth:`DataFrame.loc` dropping levels of :class:`MultiIndex` when :class:`DataFrame` used as input has only one row (:issue:`10521`)
 - Bug in setting ``timedelta64`` values into numeric :class:`Series` failing to cast to object dtype (:issue:`39086`)
 - Bug in setting :class:`Interval` values into a :class:`Series` or :class:`DataFrame` with mismatched :class:`IntervalDtype` incorrectly casting the new values to the existing dtype (:issue:`39120`)

pandas/core/frame.py

@@ -3223,7 +3223,7 @@ def _setitem_array(self, key, value):
             self._check_setitem_copy()
             self.iloc[indexer] = value
         else:
-            if isinstance(value, DataFrame):
+            if isinstance(value, DataFrame) and self.columns.is_unique:
                 if len(value.columns) != len(key):
                     raise ValueError("Columns must be same length as key")
                 for k1, k2 in zip(key, value.columns):
@@ -3256,14 +3256,20 @@ def _setitem_frame(self, key, value):
     def _set_item_frame_value(self, key, value: DataFrame) -> None:
         self._ensure_valid_index(value)
 
-        # align right-hand-side columns if self.columns
-        # is multi-index and self[key] is a sub-frame
-        if isinstance(self.columns, MultiIndex) and key in self.columns:
+        if key in self.columns:
             loc = self.columns.get_loc(key)
-            if isinstance(loc, (slice, Series, np.ndarray, Index)):
-                cols = maybe_droplevels(self.columns[loc], key)
-                if len(cols) and not cols.equals(value.columns):
-                    value = value.reindex(cols, axis=1)
+            cols = self.columns[loc]
+            len_cols = 1 if is_scalar(cols) else len(cols)
+            if len_cols != len(value.columns):
+                raise ValueError("Columns must be same length as key")
+
+            # align right-hand-side columns if self.columns
+            # is multi-index and self[key] is a sub-frame
+            if isinstance(self.columns, MultiIndex):
+                if isinstance(loc, (slice, Series, np.ndarray, Index)):
+                    cols = maybe_droplevels(cols, key)
+                    if len(cols) and not cols.equals(value.columns):
+                        value = value.reindex(cols, axis=1)
 
         # now align rows
         value = _reindex_for_setitem(value, self.index)

pandas/tests/frame/indexing/test_setitem.py

@@ -366,6 +366,23 @@ def test_setitem_frame_duplicate_columns(self):
         )
         tm.assert_frame_equal(df, expected)
 
+    @pytest.mark.parametrize("cols", [["a", "b", "c"], ["a", "a", "a"]])
+    def test_setitem_df_wrong_column_number(self, cols):
+        # GH#38604
+        df = DataFrame([[1, 2, 3]], columns=cols)
+        rhs = DataFrame([[10, 11]], columns=["d", "e"])
+        msg = "Columns must be same length as key"
+        with pytest.raises(ValueError, match=msg):
+            df["a"] = rhs
+
+    def test_setitem_listlike_indexer_duplicate_columns(self):
+        # GH#38604
+        df = DataFrame([[1, 2, 3]], columns=["a", "b", "b"])
+        rhs = DataFrame([[10, 11, 12]], columns=["d", "e", "c"])
+        df[["a", "b"]] = rhs
+        expected = DataFrame([[10, 11, 12]], columns=["a", "b", "b"])
+        tm.assert_frame_equal(df, expected)
+
 
 class TestDataFrameSetItemWithExpansion:
     def test_setitem_listlike_views(self):