如何替换文件中第m行的第n个字符串？

def replace_string_in_file(file_path, m, n, new_string):
    with open(file_path, 'r') as file:
        lines = file.readlines()

    # 定位要替换的行
    line_to_replace = lines[m-1]

    # 分割行为单词列表
    words = line_to_replace.split()

    # 替换指定位置的字符串
    words[n-1] = new_string

    # 更新替换后的行
    updated_line = ' '.join(words)

    # 替换原文件或写入新文件
    lines[m-1] = updated_line

    with open(file_path, 'w') as file:
        file.writelines(lines)

# 示例用法
file_path = 'example.txt'
m = 3
n = 2
new_string = 'replacement'

replace_string_in_file(file_path, m, n, new_string)