【python技巧】替换文件中的某几行

1. 背景描述

最近在写一个后端项目，主要的操作就是根据用户的前端数据，在后端打开项目中的代码文件，修改对应位置的参数，因为在目前的后端项目中经常使用这个操作，所以简单总结一下。

1. 文件路径：./test.c
2. 文件内容
……
case EPA:
      chan_desc->nb_taps        = 7;
      chan_desc->Td             = .410;
      chan_desc->channel_length = (int) (2*chan_desc->sampling_rate*chan_desc->Td + 1 + 2/(M_PI*M_PI)*log(4*M_PI*chan_desc->sampling_rate*chan_desc->Td));
      sum_amps = 0;
      chan_desc->amps           = (double *) malloc(chan_desc->nb_taps*sizeof(double));
      chan_desc->free_flags=chan_desc->free_flags|CHANMODEL_FREE_AMPS ;

      for (i = 0; inb_taps; i++) {
        chan_desc->amps[i]      = pow(10,.1*epa_amps_dB[i]);
        sum_amps += chan_desc->amps[i];
      }

      for (i = 0; inb_taps; i++)
        chan_desc->amps[i] /= sum_amps;

      chan_desc->delays         = epa_delays;
      chan_desc->ricean_factor  = 1;//待修改位置
      chan_desc->aoa            = 0;//待修改位置
      chan_desc->random_aoa     = 0;//待修改位置
      chan_desc->ch             = (struct complexd **) malloc(nb_tx*nb_rx*sizeof(struct complexd *));
      chan_desc->chF            = (struct complexd **) malloc(nb_tx*nb_rx*sizeof(struct complexd *));
      chan_desc->a              = (struct complexd **) malloc(chan_desc->nb_taps*sizeof(struct complexd *));
……

2. 单行修改-操作步骤

1. 读取文件
   使用python中的open()函数进行文件读取，将数据存储在缓冲区。

#1. 读取文件
path='./test.c'
with open(path, 'r') as file:
    file_content = file.read()

2. 查找文件替换位置
   以查找chan_desc->ricean_factor = 1;//待修改位置为例，查找这句话的起点和终点。

## 注：此步骤需要import re
#2. 查找文件替换位置
start_index=file_content.find('chan_desc->ricean_factor  = ')#起点
end_index=file_content.find('chan_desc->aoa            = ',start_index)#终点
if end_index==-1 or start_index==-1:
    print('未找到待修改位置')
#此时得到的两个指针，分别指向了待修改位置的起点和终点，如下图所示：

3. 设置替换文件内容
   假设目前只修改这一行的参数，

#3. 设置替换文件内容
ricean_factor=3#假设这是要修改的参数信息
updata_content=file_content[:start_index]#获取这行代码之前的内容
update_content+='chan_desc->ricean_factor  = '+str(ricean_factor)+';//待修改位置'#修改这行代码
update_content+=file_content[end_index:]#获取这行代码之后的内容
#此时得到的update_content就是修改后的完整文件内容，只修改了ricean_factor这一行的值

4. 写入文件
   同样使用python中的open函数。

#4. 写入文件
if update_content!="":#如果修改内容不为空
    with open(path, 'w') as file:#w表示覆盖写入，之前的内容都会被覆盖
        file.write(update_content)

5. 总代码
   整体的代码如下所示：

import re
#1. 读取文件
path='./test.c'
with open(path, 'r') as file:
    file_content = file.read()
#2. 查找文件替换位置
start_index=file_content.find('chan_desc->ricean_factor  = ')#起点
end_index=file_content.find('chan_desc->aoa            = ',start_index)#终点
if end_index==-1 or start_index==-1:
    print('未找到待修改位置')
#3. 设置替换文件内容
ricean_factor=3#假设这是要修改的参数信息
updata_content=file_content[:start_index]#获取这行代码之前的内容
update_content+='chan_desc->ricean_factor  = '+str(ricean_factor)+';//待修改位置'#修改这行代码
update_content+=file_content[end_index:]#获取这行代码之后的内容
#4. 写入文件
if update_content!="":#如果修改内容不为空
    with open(path, 'w') as file:#w表示覆盖写入，之前的内容都会被覆盖
        file.write(update_content)

3. 多行修改-操作步骤

1. 多行修改思路
   多行修改有两种修改思路，如果修改部分比较集中，则可直接替换一整块的字符串内容，如果修改部分较为分散，则需要单独查找修改位置，然后再分别进行替换。
2. 多行修改-整块替换

try:
    with open(file_path, "r") as file:
            file_content = file.read()
except Exception as e:
    return str(e)
# 设置改写内容
updated_content = ""
 # 查找修改
start_index_1 = file_content.find("start_sentence")#要确保查找元素的唯一性
end_index_1 = file_content.find("end_sentence",start_index_1,) 

if start_index_1 == -1 or end_index_1 == -1:
    print("未找到待修改位置")
     return -1
 # 
 updated_content = file_content[:start_index_1]#获取这行代码之前的内容
 updated_content += "start_sentence和end_sentence之间的sentence_1;\n"
 updated_content += "start_sentence和end_sentence之间的sentence_2;\n"
 updated_content +=file_content[end_index_1:]

 ##此时updated_content就是修改后的完整文件内容
 if updated_content != "":
     with open(file_path, "w") as file:
         file.write(updated_content)
else:
    print("修改失败")
    return -1

3. 多行修改-局部替换

try:
    with open(file_path, "r") as file:
            file_content = file.read()
except Exception as e:
    return str(e)
# 设置改写内容
updated_content = ""
 # 查找修改
start_index_1 = file_content.find("start_sentence_1")#要确保查找元素的唯一性
end_index_1 = file_content.find("end_sentence_1",start_index_1,) 
start_index_2 = file_content.find("start_sentence_2",end_index_1)
end_index_2 = file_content.find("end_sentence_2",start_index_2,)
start_index_3 = file_content.find("start_sentence_3",end_index_2)
end_index_3 = file_content.find("end_sentence_3",start_index_3,)
start_index_4 = file_content.find("start_sentence_4",end_index_3)
end_index_4 = file_content.find("end_sentence_4",start_index_4,)

if (
     start_index_1 == -1
     or end_index_1 == -1
     or start_index_2 == -1
     or end_index_2 == -1
     or start_index_3 == -1
     or end_index_3 == -1
     or start_index_4 == -1
     or end_index_4 == -1
 ):
    print("未找到待修改位置")
     return -1

 # 
 updated_content = file_content[:start_index_1]#获取这行代码之前的内容
 updated_content += "start_sentence_1和end_sentence_1之间的内容"
 updated_content +=file_content[end_index_1:start_index_2]
 updated_content += "start_sentence_2和end_sentence_2之间的内容"
 updated_content +=file_content[end_index_2:start_index_3]
 updated_content += "start_sentence_3和end_sentence_3之间的内容"
 updated_content +=file_content[end_index_3:start_index_4]
 updated_content += "start_sentence_4和end_sentence_4之间的内容"
 updated_content += file_content[end_index_4:]

 ##此时updated_content就是修改后的完整文件内容
 if updated_content != "":
     with open(file_path, "w") as file:
         file.write(updated_content)
else:
    print("修改失败")
    return -1