BUG: don't use partial setting with scalars (GH5720)

doc/source/release.rst

@@ -246,7 +246,7 @@ API Changes
     (:issue:`4390`)
   - allow ``ix/loc`` for Series/DataFrame/Panel to set on any axis even when
     the single-key is not currently contained in the index for that axis
-    (:issue:`2578`, :issue:`5226`, :issue:`5632`)
+    (:issue:`2578`, :issue:`5226`, :issue:`5632`, :issue:`5720`)
   - Default export for ``to_clipboard`` is now csv with a sep of `\t` for
     compat (:issue:`3368`)
   - ``at`` now will enlarge the object inplace (and return the same)

pandas/core/frame.py

@@ -1904,16 +1904,17 @@ def _ensure_valid_index(self, value):
         if not len(self.index):
 
             # GH5632, make sure that we are a Series convertible
-            try:
-                value = Series(value)
-            except:
-                pass
+            if is_list_like(value):
+                try:
+                    value = Series(value)
+                except:
+                    pass
 
-            if not isinstance(value, Series):
-                raise ValueError('Cannot set a frame with no defined index '
-                                 'and a value that cannot be converted to a '
-                                 'Series')
-            self._data.set_axis(1, value.index.copy(), check_axis=False)
+                if not isinstance(value, Series):
+                    raise ValueError('Cannot set a frame with no defined index '
+                                     'and a value that cannot be converted to a '
+                                     'Series')
+                self._data.set_axis(1, value.index.copy(), check_axis=False)
 
     def _set_item(self, key, value):
         """

pandas/tests/test_indexing.py

@@ -1730,6 +1730,14 @@ def f():
         str(df)
         assert_frame_equal(df,expected)
 
+        # GH5720
+        # don't create rows when empty
+        df = DataFrame({"A": [1, 2, 3], "B": [1.2, 4.2, 5.2]})
+        y = df[df.A > 5]
+        y['New'] = np.nan
+        expected = DataFrame(columns=['A','B','New'])
+        assert_frame_equal(y, expected)
+
     def test_cache_updating(self):
         # GH 4939, make sure to update the cache on setitem