BUG: Adding empty dataframes should result in empty blocks pandas-dev#10181

Author: rekcahpassyla

pandas/core/internals.py

@@ -3530,11 +3530,15 @@ def construction_error(tot_items, block_shape, axes, e=None):
 def create_block_manager_from_blocks(blocks, axes):
     try:
         if len(blocks) == 1 and not isinstance(blocks[0], Block):
-            # It's OK if a single block is passed as values, its placement is
-            # basically "all items", but if there're many, don't bother
-            # converting, it's an error anyway.
-            blocks = [make_block(values=blocks[0],
-                                 placement=slice(0, len(axes[0])))]
+            # if blocks[0] is of length 0, return empty blocks
+            if not len(blocks[0]):
+                blocks = []
+            else:
+                # It's OK if a single block is passed as values, its placement is
+                # basically "all items", but if there're many, don't bother
+                # converting, it's an error anyway.
+                blocks = [make_block(values=blocks[0],
+                                     placement=slice(0, len(axes[0])))]
 
         mgr = BlockManager(blocks, axes)
         mgr._consolidate_inplace()

pandas/tests/test_frame.py

@@ -2821,7 +2821,7 @@ def custom_frame_function(self):
         data = {'col1': range(10),
                 'col2': range(10)}
         cdf = CustomDataFrame(data)
-        
+
         # Did we get back our own DF class?
         self.assertTrue(isinstance(cdf, CustomDataFrame))
 
@@ -2833,7 +2833,7 @@ def custom_frame_function(self):
         # Do we get back our own DF class after slicing row-wise?
         cdf_rows = cdf[1:5]
         self.assertTrue(isinstance(cdf_rows, CustomDataFrame))
-        self.assertEqual(cdf_rows.custom_frame_function(), 'OK')        
+        self.assertEqual(cdf_rows.custom_frame_function(), 'OK')
 
         # Make sure sliced part of multi-index frame is custom class
         mcol = pd.MultiIndex.from_tuples([('A', 'A'), ('A', 'B')])
@@ -5129,6 +5129,11 @@ def test_operators(self):
         df = DataFrame({'a': ['a', None, 'b']})
         assert_frame_equal(df + df, DataFrame({'a': ['aa', np.nan, 'bb']}))
 
+        df = DataFrame()
+        self.assertTrue((df + df).equals(DataFrame()))
+        assert_frame_equal(df + df, DataFrame())
+
+
     def test_ops_np_scalar(self):
         vals, xs = np.random.rand(5, 3), [nan, 7, -23, 2.718, -3.14, np.inf]
         f = lambda x: DataFrame(x, index=list('ABCDE'),