code: def test_invalid_separator(self):
      # if an invalid separator is supplied a empty data frame is returned
      sep = "nope!"
      df = DataFrame(
          {
              "A2010": [1.0, 2.0],
              "A2011": [3.0, 4.0],
              "B2010": [5.0, 6.0],
              "X": ["X1", "X2"],
          }
      )
      df["id"] = df.index
      exp_data = {
          "X": "",
          "A2010": [],
          "A2011": [],
          "B2010": [],
          "id": [],
          "year": [],
          "A": [],
          "B": [],
      }
      expected = DataFrame(exp_data).astype({"year": np.int64})
      expected = expected.set_index(["id", "year"])[
          ["X", "A2010", "A2011", "B2010", "A", "B"]
      ]
      expected.index = expected.index.set_levels([0, 1], level=0)
      result = wide_to_long(df, ["A", "B"], i="id", j="year", sep=sep)
      tm.assert_frame_equal(
          result.sort_index(axis=1), expected.sort_index(axis=1)
      )
  
imports: import re
  import numpy
  import pytest
  import pandas
  from pandas import DataFrame, Index, date_range, lreshape, melt, wide_to_long
  import pandas._testing
label_desc: Use image node_id 7 for calling the TestWideToLong obj's underlying member method code with example usage: obj.test_invalid_separator() without return types