Series.value_counts: Preserve original ordering

This fixes pandas-dev#12679.

Author: tomspur

pandas/core/algorithms.py

@@ -458,6 +458,8 @@ def value_counts(values, sort=True, ascending=False, normalize=False,
         if not isinstance(keys, Index):
             keys = Index(keys)
         result = Series(counts, index=keys, name=name)
+        # Use same index as the original values
+        result = result.reindex(values.unique())
 
     if bins is not None:
         # TODO: This next line should be more efficient

pandas/tests/series/test_value_counts.py

@@ -0,0 +1,44 @@
+from pandas import Series
+import pandas.util.testing as tm
+
+
+def test_original_ordering_value_counts():
+    # All items occour exactly once. No matter if sorted or not, the resulting
+    # values should be in the same order.
+    s = Series(list('bacdef'))
+
+    # Garantee the same index if value_counts(sort=False) is used
+    vc = s.value_counts(sort=False)
+    tm.assert_series_equal(Series(vc.index), s)
+
+
+def test_original_ordering_value_counts2():
+    # 'a' is there twice. Unsorted, it should be there at the top, unsorted it
+    # should stay where it is.
+    s = Series(list('bacaef'))
+    ref_nonsorted = Series(list('bacef'))
+
+    # Garantee the same index if value_counts(sort=False) is used
+    vc = s.value_counts(sort=False)
+    tm.assert_series_equal(Series(vc.index), ref_nonsorted)
+
+
+def test_original_ordering_value_counts_ascending():
+    # All items occour exactly once. No matter if sorted or not, the resulting
+    # values should be in the same order.
+    s = Series(list('bacdef'))
+
+    # Garantee the same index if value_counts(sort=False) is used
+    vc = s.value_counts(sort=False, ascending=True)
+    tm.assert_series_equal(Series(vc.index), s)
+
+
+def test_original_ordering_value_counts_ascending2():
+    # 'a' is there twice. Unsorted, it should be there at the bottom, unsorted it
+    # should stay where it is.
+    s = Series(list('bacaef'))
+    ref = Series(list('bacef'))
+
+    # Garantee the same index if value_counts(sort=False) is used
+    vc = s.value_counts(sort=False, ascending=True)
+    tm.assert_series_equal(Series(vc.index), ref)