BUG: in as_matrix with subset of columns with single-block

Author: wesm

pandas/core/frame.py

@@ -822,6 +822,9 @@ def _insert_item(self, key, value):
         Series/TimeSeries will be conformed to the DataFrame's index to
         ensure homogeneity.
         """
+        # Need to make sure new columns (which go into the BlockManager as new
+        # blocks) are always copied
+
         if hasattr(value, '__iter__'):
             if isinstance(value, Series):
                 if value.index.equals(self.index):

pandas/core/internals.py

@@ -421,6 +421,8 @@ def as_matrix(self, columns=None):
             if columns is None or blk.columns.equals(columns):
                 # if not, then just call interleave per below
                 mat = blk.values
+            else:
+                mat = self.reindex_columns(columns).as_matrix()
         else:
             if columns is None:
                 mat = _interleave(self.blocks, self.columns)

pandas/tests/test_frame.py

@@ -1284,6 +1284,11 @@ def test_as_matrix(self):
         mat = self.mixed_frame.as_matrix(['foo', 'A'])
         self.assertEqual(mat[0, 0], 'bar')
 
+        # single block corner case
+        mat = self.frame.as_matrix(['A', 'B'])
+        expected = self.frame.reindex(columns=['A', 'B']).values
+        assert_almost_equal(mat, expected)
+
     def test_values(self):
         pass