REF: do concat on values, avoid blocks

Author: jbrockmendel

pandas/core/internals/concat.py

@@ -1,6 +1,7 @@
 # TODO: Needs a better name; too many modules are already called "concat"
 from collections import defaultdict
 import copy
+from typing import List
 
 import numpy as np
 
@@ -419,13 +420,15 @@ def _get_empty_dtype_and_na(join_units):
     raise AssertionError(msg)
 
 
-def _is_uniform_join_units(join_units) -> bool:
+def _is_uniform_join_units(join_units: List[JoinUnit]) -> bool:
     """
     Check if the join units consist of blocks of uniform type that can
     be concatenated using Block.concat_same_type instead of the generic
     _concatenate_join_units (which uses `concat_compat`).
 
     """
+    # TODO: require dtype match in addition to same type?  e.g. DatetimeTZBlock
+    #  cannot necessarily join
     return (
         # all blocks need to have the same type
         all(type(ju.block) is type(join_units[0].block) for ju in join_units)

pandas/core/internals/managers.py

@@ -20,6 +20,7 @@
 from pandas.core.dtypes.common import (
     DT64NS_DTYPE,
     is_datetimelike_v_numeric,
+    is_dtype_equal,
     is_extension_array_dtype,
     is_list_like,
     is_numeric_v_string_like,
@@ -42,6 +43,7 @@
     DatetimeTZBlock,
     ExtensionBlock,
     ObjectValuesExtensionBlock,
+    _block_shape,
     _extend_blocks,
     _safe_reshape,
     get_block_type,

pandas/tests/extension/test_external_block.py

@@ -32,12 +32,6 @@ def df():
     return pd.DataFrame(block_manager)
 
 
-def test_concat_dataframe(df):
-    # GH17728
-    res = pd.concat([df, df])
-    assert isinstance(res._mgr.blocks[1], CustomBlock)
-
-
 def test_concat_axis1(df):
     # GH17954
     df2 = pd.DataFrame({"c": [0.1, 0.2, 0.3]})