BUG: pivot/unstack leading to too many items should raise exception (GH20601)

Author: sweb

pandas/core/reshape/reshape.py

@@ -160,9 +160,14 @@ def _make_selectors(self):
         comp_index = ensure_platform_int(comp_index)
         stride = self.index.levshape[self.level] + self.lift
         self.full_shape = ngroups, stride
+        num_entries = np.prod(self.full_shape)
+
+        if num_entries < 0:
+            raise ValueError('Unstack leads to too many (max int32) entries, '
+                             'cannot reshape')
 
         selector = self.sorted_labels[-1] + stride * comp_index + self.lift
-        mask = np.zeros(np.prod(self.full_shape), dtype=bool)
+        mask = np.zeros(num_entries, dtype=bool)
         mask.put(selector, True)
 
         if mask.sum() < len(self.index):

pandas/tests/reshape/test_reshape.py

@@ -584,6 +584,15 @@ def test_get_dummies_duplicate_columns(self, df):
         tm.assert_frame_equal(result, expected)
 
 
+def test_too_many_items_to_unstack_raises_value_error():
+    index = pd.MultiIndex.from_arrays([np.arange(979758),
+                                       np.arange(979758)],
+                                      names=['ind1', 'ind2'])
+    s = pd.Series(0, index=index)
+    with tm.assert_raises_regex(ValueError, 'Unstack leads to too many .*'):
+        s.unstack()
+
+
 class TestCategoricalReshape(object):
 
     @pytest.mark.filterwarnings("ignore:\\nPanel:FutureWarning")