如何按行替换（更新）文件中的文本

Question

我试图通过读取每一行来替换文本文件中的文本，对其进行测试，然后在需要更新时进行编写。我不想保存为新文件，因为我的脚本已经先备份文件并在备份上运行。

这里是我迄今为止...我从os.walk（获得fpath），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) 

但是最终情况是，我只得到几行（正确更新，介意你，但从文件中的前一个重复）更正。我认为这是一个范围界定问题，缺点。

*另外：我想知道如何只替换匹配的第一个实例上的文本，例如，我不想匹配显示文本，只有基础href。

Answer 1

首先，你要写行是否符合模式。否则，你只写出匹配的行。其次，在读取行和写入结果之间，您需要截断文件（可以是f.seek(0)，然后f.truncate()），或者关闭原始文件并重新打开。采摘前，我想最终是这样的：

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. 打开读文件和所有的线复制到内存中。关闭文件。
2. 对内存中的行应用您的转换。
3. 打开文件进行写入，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

（相对）安全的方式，以取代在一个文件中的行。

#!/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：这将只替换整个行，所有符合该文件中，该行的）