try this :
t['combined']= t.values.tolist()tOut[50]: A B C D combined0 hello 1 GOOD long.kw [hello, 1, GOOD, long.kw]1 1.20 chipotle NaN bingo [1.2, chipotle, nan, bingo]2 various NaN 3000 123.46 [various, nan, 3000, 123.456]