foo = 'aa' bar = np.array(['aaa', 'aab', 'aca']) out = [i for i, v in enumerate(bar) if foo in v] # out = [0, 1] bar = ['aca', 'bba', 'baa', 'aaf', 'ccc'] out = [i for i, v in enumerate(bar) if foo in v] # out = [2, 3]