问为什么我得到TypeError:序列项0:预期的str实例，列表找到

EN

import re

def get_words_from_file(filename):
    """gets words from file and applies reg expression (brokey)"""
    with open(filename, 'r', encoding="utf-8") as file:
        all_lines = [line.strip() for line in file]

    bound_start = 0
    bound_end = 0
    bound_num = 0

    for line in all_lines:
        bound_num = bound_num + 1
        if "*** START" in line:
            bound_start = bound_num
        elif "*** END" in line:
            bound_end = bound_num
        else:
            pass
    print(bound_start, bound_end)


    bound_num = 0
    pre_re_list = []
    for line in all_lines:
        bound_num = bound_num + 1
        if bound_num in range(bound_start, bound_end):
            pre_re_list.append(line)
        else:
            pass
    print(pre_re_list)

    bound_num = 0
    final_list = []
    for j in pre_re_list:
        
        word_on_line = re.findall("[a-z]+[-'][a-z]+|[a-z]+[']?|[a-z]+", pre_re_list[bound_num])
        bound_num = bound_num + 1
        final_list.append(word_on_line)
        

    return final_list


filename = "abc.txt"
words2 = get_words_from_file(filename)
print(filename, "loaded ok.")
print("{} valid words found.".format(len(words2)))
print("Valid word list:")
print("\n".join(words2))

abc.txt loaded ok.
2 valid words found.
Valid word list:
*** RUN TIME ERROR(S) ***
Traceback (most recent call last):
  File "__source.py", line 50, in <module>
    print("\n".join(words2))
TypeError: sequence item 0: expected str instance, list found