ENH: Index optional pivot

doc/source/whatsnew/v0.17.0.txt

@@ -110,6 +110,8 @@ Other enhancements
 
 - ``pd.merge`` will now allow duplicate column names if they are not merged upon (:issue:`10639`).
 
+- ``pd.pivot`` will now allow passing index as ``None`` (:issue:`3962`).
+
 .. _whatsnew_0170.api:
 
 .. _whatsnew_0170.api_breaking:

pandas/core/frame.py

@@ -3460,8 +3460,9 @@ def pivot(self, index=None, columns=None, values=None):
 
         Parameters
         ----------
-        index : string or object
-            Column name to use to make new frame's index
+        index : string or object, optional
+            Column name to use to make new frame's index. If None, uses
+            existing index.
         columns : string or object
             Column name to use to make new frame's columns
         values : string or object, optional

pandas/core/reshape.py

@@ -319,11 +319,17 @@ def pivot(self, index=None, columns=None, values=None):
     See DataFrame.pivot
     """
     if values is None:
-        indexed = self.set_index([index, columns])
+        cols = [columns] if index is None else [index, columns]
+        append = index is None
+        indexed = self.set_index(cols, append=append)
         return indexed.unstack(columns)
     else:
+        if index is None:
+            index = self.index
+        else:
+            index = self[index]
         indexed = Series(self[values].values,
-                         index=MultiIndex.from_arrays([self[index],
+                         index=MultiIndex.from_arrays([index,
                                                        self[columns]]))
         return indexed.unstack(columns)
 

pandas/tests/test_frame.py

@@ -9478,6 +9478,47 @@ def test_pivot_integer_bug(self):
         repr(result)
         self.assert_numpy_array_equal(result.columns, ['A', 'B'])
 
+    def test_pivot_index_none(self):
+        # gh-3962
+        data = {
+            'index': ['A', 'B', 'C', 'C', 'B', 'A'],
+            'columns': ['One', 'One', 'One', 'Two', 'Two', 'Two'],
+            'values': [1., 2., 3., 3., 2., 1.]
+        }
+
+        frame = DataFrame(data).set_index('index')
+        result = frame.pivot(columns='columns', values='values')
+        expected = DataFrame({
+            'One': {'A': 1., 'B': 2., 'C': 3.},
+            'Two': {'A': 1., 'B': 2., 'C': 3.}
+        })
+
+        expected.index.name, expected.columns.name = 'index', 'columns'
+        assert_frame_equal(result, expected)
+
+        # omit values
+        result = frame.pivot(columns='columns')
+
+        expected.columns = pd.MultiIndex.from_tuples([('values', 'One'),
+                                                     ('values', 'Two')],
+                                                     names=[None, 'columns'])
+        expected.index.name = 'index'
+        assert_frame_equal(result, expected, check_names=False)
+        self.assertEqual(result.index.name, 'index',)
+        self.assertEqual(result.columns.names, (None, 'columns'))
+        expected.columns = expected.columns.droplevel(0)
+
+        data = {
+            'index': range(7),
+            'columns': ['One', 'One', 'One', 'Two', 'Two', 'Two'],
+            'values': [1., 2., 3., 3., 2., 1.]
+        }
+
+        result = frame.pivot(columns='columns', values='values')
+
+        expected.columns.name = 'columns'
+        assert_frame_equal(result, expected)
+
     def test_reindex(self):
         newFrame = self.frame.reindex(self.ts1.index)