text: import numpy as np
  import pytest
  
  import pandas as pd
  from pandas import DataFrame, merge_ordered
  import pandas._testing as tm
  
  
  class TestMergeOrdered:
      def setup_method(self, method):
          self.left = DataFrame({"key": ["a", "c", "e"], "lvalue": [1, 2.0, 3]})
  
          self.right = DataFrame({"key": ["b", "c", "d", "f"], "rvalue": [1, 2, 3.0, 4]})
  
      def test_basic(self):
          result = merge_ordered(self.left, self.right, on="key")
          expected = DataFrame(
              {
                  "key": ["a", "b", "c", "d", "e", "f"],
                  "lvalue": [1, np.nan, 2, np.nan, 3, np.nan],
                  "rvalue": [np.nan, 1, 2, 3, np.nan, 4],
              }
          )
  
          tm.assert_frame_equal(result, expected)
  
      def test_ffill(self):
          result = merge_ordered(self.left, self.right, on="key", fill_method="ffill")
          expected = DataFrame(
              {
                  "key": ["a", "b", "c", "d", "e", "f"],
                  "lvalue": [1.0, 1, 2, 2, 3, 3.0],
                  "rvalue": [np.nan, 1, 2, 3, 3, 4],
              }
          )
          tm.assert_frame_equal(result, expected)
  
      def test_multigroup(self):
          left = pd.concat([self.left, self.left], ignore_index=True)
  
          left["group"] = ["a"] * 3 + ["b"] * 3
  
          result = merge_ordered(
              left, self.right, on="key", left_by="group", fill_method="ffill"
          )
          expected = DataFrame(
              {
                  "key": ["a", "b", "c", "d", "e", "f"] * 2,
                  "lvalue": [1.0, 1, 2, 2, 3, 3.0] * 2,
                  "rvalue": [np.nan, 1, 2, 3, 3, 4] * 2,
              }
          )
          expected["group"] = ["a"] * 6 + ["b"] * 6
  
          tm.assert_frame_equal(result, expected.loc[:, result.columns])
  
          result2 = merge_ordered(
              self.right, left, on="key", right_by="group", fill_method="ffill"
          )
          tm.assert_frame_equal(result, result2.loc[:, result.columns])
  
          result = merge_ordered(left, self.right, on="key", left_by="group")
          assert result["group"].notna().all()
  
      def test_merge_type(self):
          class NotADataFrame(DataFrame):
              @property
              def _constructor(self):
                  return NotADataFrame
  
          nad = NotADataFrame(self.left)
          result = nad.merge(self.right, on="key")
  
          assert isinstance(result, NotADataFrame)
  
      def test_empty_sequence_concat(self):
          # GH 9157
          empty_pat = "[Nn]o objects"
          none_pat = "objects.*None"
          test_cases = [
              ((), empty_pat),
              ([], empty_pat),
              ({}, empty_pat),
              ([None], none_pat),
              ([None, None], none_pat),
          ]
          for df_seq, pattern in test_cases:
              with pytest.raises(ValueError, match=pattern):
                  pd.concat(df_seq)
  
          pd.concat([pd.DataFrame()])
          pd.concat([None, pd.DataFrame()])
          pd.concat([pd.DataFrame(), None])
  
      def test_doc_example(self):
          left = DataFrame(
              {
                  "group": list("aaabbb"),
                  "key": ["a", "c", "e", "a", "c", "e"],
                  "lvalue": [1, 2, 3] * 2,
              }
          )
  
          right = DataFrame({"key": ["b", "c", "d"], "rvalue": [1, 2, 3]})
  
          result = merge_ordered(left, right, fill_method="ffill", left_by="group")
  
          expected = DataFrame(
              {
                  "group": list("aaaaabbbbb"),
                  "key": ["a", "b", "c", "d", "e"] * 2,
                  "lvalue": [1, 1, 2, 2, 3] * 2,
                  "rvalue": [np.nan, 1, 2, 3, 3] * 2,
              }
          )
  
          tm.assert_frame_equal(result, expected)
  