Search and replace a line in a file in Python

Question

I want to loop over the contents of a text file and do a search and replace on some lines and write the result back to the file  I could first load the whole file in memory and then write it back  but that probably is not the best way to do it   What is the best way to do this  within the following code   f   open file  for line in f      if line contains  foo            newline   line replace  foo    bar             how to write this newline back to the file

User · Answer

Create a new file  copy lines from the old to the new  and do the replacing before you write the lines to the new file

User · Answer

This should work   inplace editing   import fileinput    Does a list of files  and   redirects STDOUT to the file in question for line in fileinput input files  inplace   1          print line replace  foo    bar

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Here s another example that was tested  and will match search  amp  replace patterns   import fileinput import sys  def replaceAll file searchExp replaceExp       for line in fileinput input file  inplace 1           if searchExp in line              line   line replace searchExp replaceExp          sys stdout write line    Example use   replaceAll   fooBar txt   Hello sWorld     Goodbye sWorld

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A more pythonic way would be to use context managers like the code below   from tempfile import mkstemp from shutil import move from os import remove  def replace source file path  pattern  substring       fh  target file path   mkstemp       with open target file path   w   as target file          with open source file path   r   as source file              for line in source file                  target file write line replace pattern  substring       remove source file path      move target file path  source file path    You can find the full snippet here

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I guess something like this should do it  It basically writes the content to a new file and replaces the old file with the new file   from tempfile import mkstemp from shutil import move  copymode from os import fdopen  remove  def replace file path  pattern  subst        Create temp file     fh  abs path   mkstemp       with fdopen fh  w   as new file          with open file path  as old file              for line in old file                  new file write line replace pattern  subst        Copy the file permissions from the old file to the new file     copymode file path  abs path       Remove original file     remove file path       Move new file     move abs path  file path

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Based on the answer by Thomas Watnedal   However  this does not answer the line-to-line part of the original question exactly  The function can still replace on a line-to-line basis   This implementation replaces the file contents without using temporary files  as a consequence file permissions remain unchanged   Also re sub instead of replace  allows regex replacement instead of plain text replacement only   Reading the file as a single string instead of line by line allows for multiline match and replacement   import re  def replace file  pattern  subst         Read contents from file as a single string     file handle   open file   r       file string   file handle read       file handle close          Use RE package to allow for replacement  also allowing for  multiline  REGEX      file string    re sub pattern  subst  file string          Write contents to file        Using mode  w  truncates the file      file handle   open file   w       file handle write file string      file handle close

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Expanding on  Kiran s answer  which I agree is more succinct and Pythonic  this adds codecs to support the reading and writing of UTF-8   import codecs   from tempfile import mkstemp from shutil import move from os import remove   def replace source file path  pattern  substring       fh  target file path   mkstemp        with codecs open target file path   w    utf-8   as target file          with codecs open source file path   r    utf-8   as source file              for line in source file                  target file write line replace pattern  substring       remove source file path      move target file path  source file path

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fileinput is quite straightforward as mentioned on previous answers   import fileinput  def replace in file file path  search text  new text       with fileinput input file path  inplace True  as f          for line in f              new line   line replace search text  new text              print new line  end       Explanation    fileinput can accept multiple files  but I prefer to close each single file as soon as it is being processed  So placed single file path in with statement  print statement does not print anything when inplace True  because STDOUT is being forwarded to the original file  end    in print statement is to eliminate intermediate blank new lines    Can be used as follows   file path     path to my file  replace in file file path   old-text    new-text

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If you re wanting a generic function that replaces any text with some other text  this is likely the best way to go  particularly if you re a fan of regex s   import re def replace  filePath  text  subs  flags 0        with open  filePath   r     as file          fileContents   file read           textPattern   re compile  re escape  text    flags           fileContents   textPattern sub  subs  fileContents           file seek  0           file truncate           file write  fileContents

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As lassevk suggests  write out the new file as you go  here is some example code   fin   open  a txt   fout   open  b txt    wt   for line in fin      fout write  line replace  foo    bar     fin close   fout close

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Using hamishmcn s answer as a template I was able to search for a line in a file that match my regex and replacing it with empty string   import re   fin   open  in txt    r     in file fout   open  out txt    w     out file for line in fin      p   re compile   -  0-9      0-9       -  0-9          pattern     newline   p sub    line    replace matching strings with empty string     print newline     fout write newline  fin close   fout close

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if you remove the indent at the like below  it will search and replace in multiple line  See below for example   def replace file  pattern  subst        Create temp file     fh  abs path   mkstemp       print fh  abs path     new file   open abs path  w       old file   open file      for line in old file          new file write line replace pattern  subst        close temp file     new file close       close fh      old file close        Remove original file     remove file       Move new file     move abs path  file

User · Answer

The shortest way would probably be to use the fileinput module  For example  the following adds line numbers to a file  in-place   import fileinput  for line in fileinput input  test txt   inplace True       print         format fileinput filelineno    line   end       for Python 3       print   d   s     fileinput filelineno    line     for Python 2   What happens here is    The original file is moved to a backup file The standard output is redirected to the original file within the loop Thus any print statements write back into the original file   fileinput has more bells and whistles  For example  it can be used to automatically operate on all files in sys args 1    without your having to iterate over them explicitly  Starting with Python 3 2 it also provides a convenient context manager for use in a with statement     While fileinput is great for throwaway scripts  I would be wary of using it in real code because admittedly it s not very readable or familiar  In real  production  code it s worthwhile to spend just a few more lines of code to make the process explicit and thus make the code readable   There are two options    The file is not overly large  and you can just read it wholly to memory  Then close the file  reopen it in writing mode and write the modified contents back  The file is too large to be stored in memory  you can move it over to a temporary file and open that  reading it line by line  writing back into the original file  Note that this requires twice the storage

[python] Search and replace a line in a file in Python

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