一个txt文件里，某一行碰到关键词就连带向下复制，直到再碰到关键词或者空行就停止复制到另一个txt

请问
python一个txt文件里，某一行碰到关键词就连带向下复制，直到再碰到关键词或者空行就停止复制到另一个txt要怎么办

######## Pseudocode
with open("file", "r+") as fr:
  fr.readlines()  # read file
  start_row, end_row
  copy_str

for line_data in fr.readlines():
   if(start_row == "" and line_data.find(key_word)):  # get start line
        # start_row = line_data
        copy_str = copy_str + line_data
        continue
    if(start_row != ""):
        copy_str = copy_str + line_data
    if(start_row != "" and end_row == "" and (line_data.find(key_word) || line_data.strip() == "")):  # get end line
        # end_row = line_data
        # copy_str = copy_str + line_data   
        with open("write_file","r+") as fw:
          fw.write(copy_str)  # get start_row and end_row, end current copy, initional data
          start_row = ""
          end_row = ""