code: def test_from_column_names_no_label_constructor():
      df = pd.DataFrame({"x": [2]})
      conv = DFtoVW.from_column_names(x=["x"], df=df)
      lines_list = conv.convert_df()
      first_line = lines_list[0]
      assert first_line == "| x:2"
  
imports: import pytest
  import pandas
  from vowpalwabbit.dftovw import ContextualbanditLabel, DFtoVW, Feature, MulticlassLabel, MultiLabel, Namespace, SimpleLabel
label_desc: Use image node_id 6 for calling a global function with example usage: test_from_column_names_no_label_constructor() without return types