code: def test_roundtrip_single_column_dataframe():
      categories = DataFrame({"": ["a", "b", "c", "a"]})
      dummies = get_dummies(categories)
      result = from_dummies(dummies, sep="_")
      expected = categories
      tm.assert_frame_equal(result, expected)
  
imports: import numpy
  import pytest
  from pandas import DataFrame, Series, from_dummies, get_dummies
  import pandas._testing
label_desc: Use image node_id 18 for calling a global function with example usage: test_roundtrip_single_column_dataframe() without return types