text: import numpy as np
  import pytest
  
  import pandas as pd
  from pandas.core.internals import BlockManager
  from pandas.core.internals.blocks import ExtensionBlock
  
  
  class CustomBlock(ExtensionBlock):
  
      _holder = np.ndarray
      _can_hold_na = False
  
      def concat_same_type(self, to_concat, placement=None):
          """
          Always concatenate disregarding self.ndim as the values are
          always 1D in this custom Block
          """
          values = np.concatenate([blk.values for blk in to_concat])
          placement = self.mgr_locs if self.ndim == 2 else slice(len(values))
          return self.make_block_same_class(values, placement=placement)
  
  
  @pytest.fixture
  def df():
      df1 = pd.DataFrame({"a": [1, 2, 3]})
      blocks = df1._mgr.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_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]})
      res = pd.concat([df, df2], axis=1)
      assert isinstance(res._mgr.blocks[1], CustomBlock)
  