Raise ValueError when settings scalars with no index

Author: alanbato

doc/source/whatsnew/v0.21.0.txt

@@ -461,6 +461,7 @@ Other API Changes
 - :class:`Period` is now immutable, and will now raise an ``AttributeError`` when a user tries to assign a new value to the ``ordinal`` or ``freq`` attributes (:issue:`17116`).
 - :func:`to_datetime` when passed a tz-aware ``origin=`` kwarg will now raise a more informative ``ValueError`` rather than a ``TypeError`` (:issue:`16842`)
 - Renamed non-functional ``index`` to ``index_col`` in :func:`read_stata` to improve API consistency (:issue:`16342`)
+- Setting on a column with a scalar value and no index now raises a ``ValueError`` (:issue:`16823`)
 
 
 .. _whatsnew_0210.deprecations:

pandas/core/frame.py

@@ -2499,17 +2499,22 @@ def _ensure_valid_index(self, value):
         passed value
         """
         # GH5632, make sure that we are a Series convertible
-        if not len(self.index) and is_list_like(value):
+        if not len(self.index):
+            if not is_list_like(value):
+                # GH16823, Raise an error due to loss of information
+                raise ValueError('If using all scalar values, you must pass'
+                                 ' an index')
             try:
                 value = Series(value)
             except:
-                raise ValueError('Cannot set a frame with no defined index '
-                                 'and a value that cannot be converted to a '
-                                 'Series')
-
+                raise ValueError('Cannot set a frame with no defined'
+                                 'index and a value that cannot be '
+                                 'converted to a Series')
+    
             self._data = self._data.reindex_axis(value.index.copy(), axis=1,
                                                  fill_value=np.nan)
 
+
     def _set_item(self, key, value):
         """
         Add series to DataFrame in specified column.

pandas/tests/frame/test_indexing.py

@@ -721,6 +721,11 @@ def test_setitem_empty_frame_with_boolean(self):
                 df[df > df2] = 47
                 assert_frame_equal(df, df2)
 
+    def test_setitem_scalars_no_index(self):
+        # GH16823
+        df = DataFrame()
+        pytest.raises(ValueError, df.__setitem__, 'foo', 1)
+
     def test_getitem_empty_frame_with_boolean(self):
         # Test for issue #11859