BUG: ticket numpy#2063, make unique return consistent index.

Make unique use  mergesort when return_index is true. This quarantees that
the returned indices are of the first occurrence of the unique elements and
makes the behavior better defined and consistent.

numpy/core/tests/test_regression.py

@@ -1611,5 +1611,14 @@ def test_assign_obj_listoflists(self):
         a[...] = [[1,2]]
         assert_equal(a, [[1,2], [1,2]])
 
+    def test_unique_stable(self):
+        # Ticket #2063 must always choose stable sort for argsort to
+        # get consistent results
+        v=np.array([0,0,0,0,0,1,1,1,1,1,1,2,2,2,2,2,2]*4)
+        w=np.array([0,0,0,0,0,1,1,1,1,1,1,2,2,2,2])
+        resv = np.unique(v,return_index=True)
+        resw = np.unique(w,return_index=True)
+        assert_equal(resv, resw)
+
 if __name__ == "__main__":
     run_module_suite()

numpy/lib/arraysetops.py

@@ -112,8 +112,8 @@ def unique(ar, return_index=False, return_inverse=False):
     unique : ndarray
         The sorted unique values.
     unique_indices : ndarray, optional
-        The indices of the unique values in the (flattened) original array.
-        Only provided if `return_index` is True.
+        The indices of the first occurrences of the unique values in the
+        (flattened) original array. Only provided if `return_index` is True.
     unique_inverse : ndarray, optional
         The indices to reconstruct the (flattened) original array from the
         unique array. Only provided if `return_inverse` is True.
@@ -174,7 +174,10 @@ def unique(ar, return_index=False, return_inverse=False):
             return ar
 
     if return_inverse or return_index:
-        perm = ar.argsort()
+        if return_index:
+            perm = ar.argsort(kind='mergesort')
+        else:
+            perm = ar.argsort()
         aux = ar[perm]
         flag = np.concatenate(([True], aux[1:] != aux[:-1]))
         if return_inverse: