How to modify a text file

Question

I m using Python  and would like to insert a string into a text file without deleting or copying the file  How can I do that

User · Answer

Wrote a small class for doing this cleanly.

import tempfile

class FileModifierError(Exception):
    pass

class FileModifier(object):

    def __init__(self, fname):
        self.__write_dict = {}
        self.__filename = fname
        self.__tempfile = tempfile.TemporaryFile()
        with open(fname, 'rb') as fp:
            for line in fp:
                self.__tempfile.write(line)
        self.__tempfile.seek(0)

    def write(self, s, line_number = 'END'):
        if line_number != 'END' and not isinstance(line_number, (int, float)):
            raise FileModifierError("Line number %s is not a valid number" % line_number)
        try:
            self.__write_dict[line_number].append(s)
        except KeyError:
            self.__write_dict[line_number] = [s]

    def writeline(self, s, line_number = 'END'):
        self.write('%s\n' % s, line_number)

    def writelines(self, s, line_number = 'END'):
        for ln in s:
            self.writeline(s, line_number)

    def __popline(self, index, fp):
        try:
            ilines = self.__write_dict.pop(index)
            for line in ilines:
                fp.write(line)
        except KeyError:
            pass

    def close(self):
        self.__exit__(None, None, None)

    def __enter__(self):
        return self

    def __exit__(self, type, value, traceback):
        with open(self.__filename,'w') as fp:
            for index, line in enumerate(self.__tempfile.readlines()):
                self.__popline(index, fp)
                fp.write(line)
            for index in sorted(self.__write_dict):
                for line in self.__write_dict[index]:
                    fp.write(line)
        self.__tempfile.close()

Then you can use it this way:

with FileModifier(filename) as fp:
    fp.writeline("String 1", 0)
    fp.writeline("String 2", 20)
    fp.writeline("String 3")  # To write at the end of the file

User · Answer

As mentioned by Adam you have to take your system limitations into consideration before you can decide on approach whether you have enough memory to read it all into memory replace parts of it and re-write it    If you re dealing with a small file or have no memory issues this might help   Option 1  Read entire file into memory  do a regex substitution on the entire or part of the line and replace it with that line plus the extra line  You will need to make sure that the  middle line  is unique in the file or if you have timestamps on each line this should be pretty reliable     open file with r b  allow write and binary mode  f   open  file log    r b        read entire content of file into memory f content   f read     basically match middle line and replace it with itself and the extra line f content   re sub r  middle line    r  1 nnew line   f content    return pointer to top of file so we can re-write the content with replaced string f seek 0    clear file content  f truncate     re-write the content with the updated content f write f content    close file f close     Option 2  Figure out middle line  and replace it with that line plus the extra line      open file with r b  allow write and binary mode  f   open  file log     r b        get array of lines f content   f readlines     get middle line middle line   len f content  2   overwrite middle line f content middle line       nnew line    return pointer to top of file so we can re-write the content with replaced string f seek 0    clear file content  f truncate     re-write the content with the updated content f write    join f content     close file f close

User · Answer

Depends on what you want to do   To append you can open it with  a     with open  foo txt    a   as f       f write  new line n     If you want to preprend something you have to read from the file first   with open  foo txt    r    as f       old   f read     read everything in the file      f seek 0    rewind      f write  new line n    old    write the new line before

User · Answer

Python s mmap module will allow you to insert into a file  The following sample shows how it can be done in Unix  Windows mmap may be different   Note that this does not handle all error conditions and you might corrupt or lose the original file  Also  this won t handle unicode strings   import os from mmap import mmap  def insert filename  str  pos       if len str   lt  1            nothing to insert         return      f   open filename   r        m   mmap f fileno    os path getsize filename       origSize   m size          or this could be an error     if pos  gt  origSize          pos   origSize     elif pos  lt  0          pos   0      m resize origSize   len str       m pos len str      m pos origSize      m pos pos len str     str     m close       f close     It is also possible to do this without mmap with files opened in  r   mode  but it is less convenient and less efficient as you d have to read and temporarily store the contents of the file from the insertion position to EOF - which might be huge

User · Answer

Rewriting a file in place is often done by saving the old copy with a modified name   Unix folks add a   to mark the old one   Windows folks do all kinds of things -- add  bak or  old -- or rename the file entirely or put the   on the front of the name   import shutil shutil move  afile  afile        destination  open  aFile   w    source  open  aFile       r    for line in source      destination write  line       if  lt some condition gt           destination write   gt some additional line gt      n    source close   destination close     Instead of shutil  you can use the following   import os os rename  aFile  aFile

User · Answer

The fileinput module of the Python standard library will rewrite a file inplace if you use the inplace 1 parameter   import sys import fileinput    replace all occurrences of  sit  with  SIT  and insert a line after the 5th for i  line in enumerate fileinput input  lorem ipsum txt   inplace 1        sys stdout write line replace  sit    SIT       replace  sit  and write     if i    4  sys stdout write   n      write a blank line after the 5th line

User · Answer

Unfortunately there is no way to insert into the middle of a file without re-writing it  As previous posters have indicated  you can append to a file or overwrite part of it using seek but if you want to add stuff at the beginning or the middle  you ll have to rewrite it   This is an operating system thing  not a Python thing  It is the same in all languages   What I usually do is read from the file  make the modifications and write it out to a new file called myfile txt tmp or something like that  This is better than reading the whole file into memory because the file may be too large for that  Once the temporary file is completed  I rename it the same as the original file   This is a good  safe way to do it because if the file write crashes or aborts for any reason  you still have your untouched original file

User · Answer

If you know some unix you could try the following   Notes    means the command prompt   Say you have a file my data txt with content as such     cat my data txt This is a data file with all of my data in it    Then using the os module you can use the usual sed commands  import os    Identifiers used are  my data file    my data txt  command    sed -i  s all none   my data txt     Execute the command os system command    If you aren t aware of sed  check it out  it is extremely useful

[python] How to modify a text file?

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