Merge pull request #14 from Giftlin/patch-4

ENH: sort=bool keyword argument for index.difference  pandas-dev#17839

Author: Giftlin

pandas/core/indexes/base.py

@@ -2183,7 +2183,7 @@ def _get_consensus_name(self, other):
                 return self._shallow_copy(name=name)
         return self
 
-    def union(self, other):
+    def union(self, other, sort=True):
         """
         Form the union of two Index objects and sorts if possible.
 
@@ -2241,27 +2241,29 @@ def union(self, other):
                                            allow_fill=False)
                 result = _concat._concat_compat((self._values, other_diff))
 
-                try:
-                    self._values[0] < other_diff[0]
-                except TypeError as e:
-                    warnings.warn("%s, sort order is undefined for "
-                                  "incomparable objects" % e, RuntimeWarning,
-                                  stacklevel=3)
-                else:
-                    types = frozenset((self.inferred_type,
-                                       other.inferred_type))
-                    if not types & _unsortable_types:
-                        result.sort()
+                if sort:
+                    try:
+                        self._values[0] < other_diff[0]
+                    except TypeError as e:
+                        warnings.warn("%s, sort order is undefined for "
+                                      "incomparable objects" % e, RuntimeWarning,
+                                      stacklevel=3)
+                    else:
+                        types = frozenset((self.inferred_type,
+                                           other.inferred_type))
+                        if not types & _unsortable_types:
+                            result.sort()
 
             else:
                 result = self._values
 
-                try:
-                    result = np.sort(result)
-                except TypeError as e:
-                    warnings.warn("%s, sort order is undefined for "
-                                  "incomparable objects" % e, RuntimeWarning,
-                                  stacklevel=3)
+                if sort:
+                    try:
+                        result = np.sort(result)
+                    except TypeError as e:
+                        warnings.warn("%s, sort order is undefined for "
+                                      "incomparable objects" % e, RuntimeWarning,
+                                      stacklevel=3)
 
         # for subclasses
         return self._wrap_union_result(other, result)