code: def test_with_prefix_basic(dummies_basic):
      expected = DataFrame(
          {"col1": ["a", "b", "a"], "col2": ["b", "a", "c"]}
      )
      result = from_dummies(dummies_basic, sep="_")
      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 28 for calling a global function with example usage: test_with_prefix_basic(dummies_basic) without return types