如何逐行替换（更新）文件中的文本

Question

我试图通过读取每一行，测试它，然后写入是否需要更新来替换文本文件中的文本。我不想另存为新文件，因为我的脚本已经先备份了文件并在备份上进行操作。

这是我目前所拥有的......我从 os.walk() 获得 fpath 并且我保证 pathmatch var 正确返回：

fpath = os.path.join(thisdir, filename)
with open(fpath, 'r+') as f:
    for line in f.readlines():
        if '<a href="' in line:
            for test in filelist:
                pathmatch = file_match(line, test)
                    if pathmatch is not None: 
                        repstring = filelist[test] + pathmatch
                        print 'old line:', line
                        line = line.replace(test, repstring)
                        print 'new line:', line
                        f.write(line)

但最终发生的事情是，我只得到了几行更正（请注意，已正确更新，但从文件的前面部分重复）。我认为这是一个范围界定问题，afaict。

*另外：我想知道如何只在匹配的第一个实例时替换文本，例如，我不想匹配显示文本，只匹配底层的 href。

Answer 1

首先，你想写下它是否匹配模式。否则，您只会写出匹配的行。

其次，在读取行和写入结果之间，您需要截断文件（可以f.seek(0) 然后f.truncate()），或者关闭原始文件并重新打开。选择前者，我最终会得到类似的结果：

fpath = os.path.join(thisdir, filename)
with open(fpath, 'r+') as f:
    lines = f.readlines()
    f.seek(0)
    f.truncate()
    for line in lines:
        if '<a href="' in line:
            for test in filelist:
                pathmatch = file_match(line, test)
                    if pathmatch is not None: 
                        repstring = filelist[test] + pathmatch
                        line = line.replace(test, repstring)
        f.write(line)

Answer 2

1. 打开文件进行读取并将所有行复制到内存中。关闭文件。
2. 将转换应用到内存中的行。
3. 打开要写入的文件并写出内存中的所有文本行。

---

with open(filename, "r") as f:
    lines = (line.rstrip() for line in f)
    altered_lines = [some_func(line) if regex.match(line) else line for line in lines]
with open(filename, "w") as f:
    f.write('\n'.join(altered_lines) + '\n')

Answer 3

一种（相对）安全的方式来替换文件中的一行。

#!/usr/bin/python 
# defensive programming style
# function to replace a line in a file
# and not destroy data in case of error

def replace_line(filepath, oldline, newline ):
  """ 
  replace a line in a temporary file, 
  then copy it over into the 
  original file if everything goes well

  """

 # quick parameter checks 
  assert os.exists(filepath)          # ! 
  assert ( oldline and str(oldline) ) # is not empty and is a string
  assert ( newline and str(newline) )

  replaced = False
  written  = False

  try:

    with open(filepath, 'r+') as f:    # open for read/write -- alias to f       

      lines = f.readlines()            # get all lines in file

      if oldline not in lines:
          pass                         # line not found in file, do nothing

      else:
        tmpfile = NamedTemporaryFile(delete=True)  # temp file opened for writing

        for line in lines:           # process each line
          if line == oldline:        # find the line we want 
            tmpfile.write(newline)   # replace it 
            replaced = True  
          else:
            tmpfile.write(oldline)   # write old line unchanged

        if replaced:                   # overwrite the original file     
          f.seek(0)                    # beginning of file
          f.truncate()                 # empties out original file

          for tmplines in tmpfile: 
            f.write(tmplines)          # writes each line to original file
          written = True  

      tmpfile.close()              # tmpfile auto deleted    
      f.close()                          # we opened it , we close it 

  except IOError, ioe:                 # if something bad happened.
    printf ("ERROR" , ioe)
    f.close()                        
    return False

  return replaced and written        # replacement happened with no errors = True 

（注意：这仅替换整行，以及文件中匹配的所有行）