BUG: punt to user when passing overlapping replacement values in a nested dict

doc/source/release.rst

@@ -164,6 +164,8 @@ Bug Fixes
 - ``eval``/``query`` expressions with strings containing the ``@`` character
   will now work (:issue:`6366`).
 - Bug in ``Series.reindex`` when specifying a ``method`` with some nan values was inconsistent (noted on a resample) (:issue:`6418`)
+- Bug in :meth:`DataFrame.replace` where nested dicts were erroneously
+  depending on the order of dictionary keys and values (:issue:`5338`).
 
 pandas 0.13.1
 -------------

pandas/core/generic.py

@@ -2327,8 +2327,12 @@ def replace(self, to_replace=None, value=None, inplace=False, limit=None,
                 value_dict = {}
 
                 for k, v in items:
-                    to_rep_dict[k] = list(v.keys())
-                    value_dict[k] = list(v.values())
+                    keys, values = zip(*v.items())
+                    if set(keys) & set(values):
+                        raise ValueError("Replacement not allowed with "
+                                         "overlapping keys and values")
+                    to_rep_dict[k] = list(keys)
+                    value_dict[k] = list(values)
 
                 to_replace, value = to_rep_dict, value_dict
             else:

pandas/tests/test_frame.py

@@ -8089,6 +8089,19 @@ def test_replace_with_dict_with_bool_keys(self):
         with tm.assertRaisesRegexp(TypeError, 'Cannot compare types .+'):
             df.replace({'asdf': 'asdb', True: 'yes'})
 
+    def test_replace_int_to_int_chain(self):
+        df = DataFrame({'a': lrange(1, 5)})
+        with tm.assertRaisesRegexp(ValueError, "Replacement not allowed .+"):
+            df.replace({'a': dict(zip(range(1, 5), range(2, 6)))})
+
+    def test_replace_str_to_str_chain(self):
+        a = np.arange(1, 5)
+        astr = a.astype(str)
+        bstr = np.arange(2, 6).astype(str)
+        df = DataFrame({'a': astr})
+        with tm.assertRaisesRegexp(ValueError, "Replacement not allowed .+"):
+            df.replace({'a': dict(zip(astr, bstr))})
+
     def test_combine_multiple_frames_dtypes(self):
 
         # GH 2759