REF/INT: preserve block type in joining when only having a single block (#17954)

* REF/INT: preserve block type in joining when only having a single block

* add test

* remove checking of values.base

* clean-up comment

Author: jorisvandenbossche

pandas/core/internals.py

@@ -5182,7 +5182,15 @@ def concatenate_block_managers(mgrs_indexers, axes, concat_axis, copy):
 
     for placement, join_units in concat_plan:
 
-        if is_uniform_join_units(join_units):
+        if len(join_units) == 1 and not join_units[0].indexers:
+            b = join_units[0].block
+            values = b.values
+            if copy:
+                values = values.copy()
+            elif not copy:
+                values = values.view()
+            b = b.make_block_same_class(values, placement=placement)
+        elif is_uniform_join_units(join_units):
             b = join_units[0].block.concat_same_type(
                 [ju.block for ju in join_units], placement=placement)
         else:

pandas/tests/internals/test_external_block.py

@@ -7,6 +7,8 @@
 from pandas.core.internals import (
     Block, BlockManager, SingleBlockManager, NonConsolidatableMixIn)
 
+import pytest
+
 
 class CustomBlock(NonConsolidatableMixIn, Block):
 
@@ -25,6 +27,17 @@ def concat_same_type(self, to_concat, placement=None):
             values, placement=placement or slice(0, len(values), 1))
 
 
+@pytest.fixture
+def df():
+    df1 = pd.DataFrame({'a': [1, 2, 3]})
+    blocks = df1._data.blocks
+    values = np.arange(3, dtype='int64')
+    custom_block = CustomBlock(values, placement=slice(1, 2))
+    blocks = blocks + (custom_block,)
+    block_manager = BlockManager(blocks, [pd.Index(['a', 'b']), df1.index])
+    return pd.DataFrame(block_manager)
+
+
 def test_custom_repr():
     values = np.arange(3, dtype='int64')
 
@@ -51,14 +64,14 @@ def test_concat_series():
     assert isinstance(res._data.blocks[0], CustomBlock)
 
 
-def test_concat_dataframe():
+def test_concat_dataframe(df):
     # GH17728
-    df = pd.DataFrame({'a': [1, 2, 3]})
-    blocks = df._data.blocks
-    values = np.arange(3, dtype='int64')
-    custom_block = CustomBlock(values, placement=slice(1, 2))
-    blocks = blocks + (custom_block, )
-    block_manager = BlockManager(blocks, [pd.Index(['a', 'b']), df.index])
-    df = pd.DataFrame(block_manager)
     res = pd.concat([df, df])
     assert isinstance(res._data.blocks[1], CustomBlock)
+
+
+def test_concat_axis1(df):
+    # GH17954
+    df2 = pd.DataFrame({'c': [.1, .2, .3]})
+    res = pd.concat([df, df2], axis=1)
+    assert isinstance(res._data.blocks[1], CustomBlock)