Series.value_counts: Preserve original ordering when using sort=False

Ensure that value_counts returns the same ordering of the indices than the input object
when sorting the values no matter if it is ascending or descending.

This fixes pandas-dev#12679.

Author: tomspur

doc/source/whatsnew/v0.24.0.rst

@@ -1634,6 +1634,7 @@ Other
 
 - Bug where C variables were declared with external linkage causing import errors if certain other C libraries were imported before Pandas. (:issue:`24113`)
 - Require at least 0.28.2 version of ``cython`` to support read-only memoryviews (:issue:`21688`)
+- :meth:`Series.value_counts` returns the counts in the same ordering as the original series when using ``sort=False``
 
 .. _whatsnew_0.24.0.contributors:
 

pandas/core/algorithms.py

@@ -666,7 +666,7 @@ def value_counts(values, sort=True, ascending=False, normalize=False,
     value_counts : Series
 
     """
-    from pandas.core.series import Series, Index
+    from pandas import Series, Index, CategoricalIndex
     name = getattr(values, 'name', None)
 
     if bins is not None:
@@ -708,6 +708,10 @@ def value_counts(values, sort=True, ascending=False, normalize=False,
 
     if sort:
         result = result.sort_values(ascending=ascending)
+    else:
+        uniq = unique(values)
+        if not isinstance(result.index, CategoricalIndex):
+            result = result.reindex(uniq)
 
     if normalize:
         result = result / float(counts.sum())

pandas/tests/test_algos.py

@@ -962,6 +962,42 @@ def test_value_counts_uint64(self):
         if not compat.is_platform_32bit():
             tm.assert_series_equal(result, expected)
 
+    def test_value_counts_nonsorted(self):
+        # 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=False)
+        tm.assert_series_equal(Series(vc.index), s)
+        vc = s.value_counts(sort=False, ascending=True)
+        tm.assert_series_equal(Series(vc.index), s)
+
+        # Garantee does not hold yet for the sort=True case
+        # vc = s.value_counts(sort=True, ascending=False)
+        # tm.assert_series_equal(Series(vc.index), s)
+        # vc = s.value_counts(sort=True, ascending=True)
+        # tm.assert_series_equal(Series(vc.index), s)
+
+        # 'a' is there twice. Sorted, it should be there at the top.
+        # Unsorted it should stay where it is.
+        s = Series(list('bacaef'))
+        ref_nonsorted = Series(list('bacef'))
+        ref_sorted = Series(list('abcef'))
+
+        # Garantee the same index if value_counts(sort=False) is used
+        vc = s.value_counts(sort=False, ascending=False)
+        tm.assert_series_equal(Series(vc.index), ref_nonsorted)
+        vc = s.value_counts(sort=False, ascending=True)
+        tm.assert_series_equal(Series(vc.index), ref_nonsorted)
+
+        # Garantee does not hold yet for the sort=True case
+        # vc = s.value_counts(sort=True, ascending=False)
+        # tm.assert_series_equal(Series(vc.index), ref_sorted)
+        # vc = s.value_counts(sort=True, ascending=True)
+        # tm.assert_series_equal(Series(vc.index), ref_sorted)
+
 
 class TestDuplicated(object):