How do you append to a file

Question

How do you append to the file instead of overwriting it  Is there a special function that appends to the file

User · Answer

You probably want to pass  a  as the mode argument   See the docs for open     with open  foo    a   as f      f write  cool beans        There are other permutations of the mode argument for updating      truncating  w  and binary  b  mode but starting with just  a  is your best bet

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Python has many variations off of the main three modes  these three modes are    w    write text  r    read text  a    append text   So to append to a file it s as easy as   f   open  filename txt    a    f write  whatever you want to write here  in append mode  here      Then there are the modes that just make your code fewer lines    r    read   write text  w    read   write text  a    append   read text   Finally  there are the modes of reading writing in binary format     rb   read binary  wb   write binary  ab   append binary  rb   read   write binary  wb   read   write binary  ab   append   read binary

User · Answer

with open  test txt    a   as myfile      myfile write  appended text

User · Answer

You need to open the file in append mode  by setting  a  or  ab  as the mode  See open     When you open with  a  mode  the write position will always be at the end of the file  an append   You can open with  a   to allow reading  seek backwards and read  but all writes will still be at the end of the file     Example    gt  gt  gt  with open  test1   wb   as f          f write  test    gt  gt  gt  with open  test1   ab   as f          f write  koko    gt  gt  gt  with open  test1   rb   as f          f read    testkoko    Note  Using  a  is not the same as opening with  w  and seeking to the end of the file - consider what might happen if another program opened the file and started writing between the seek and the write  On some operating systems  opening the file with  a  guarantees that all your following writes will be appended atomically to the end of the file  even as the file grows by other writes      A few more details about how the  a  mode operates  tested on Linux only   Even if you seek back  every write will append to the end of the file    gt  gt  gt  f   open  test   a      Not using  with  just to simplify the example REPL session  gt  gt  gt  f write  hi    gt  gt  gt  f seek 0   gt  gt  gt  f read    hi   gt  gt  gt  f seek 0   gt  gt  gt  f write  bye     Will still append despite the seek 0    gt  gt  gt  f seek 0   gt  gt  gt  f read    hibye    In fact  the fopen manpage states      Opening a file in append mode  a as the first character of mode    causes all subsequent write operations to this stream to occur at   end-of-file  as if preceded the call   fseek stream  0  SEEK END        Old simplified answer  not using with    Example   in a real program use with to close the file - see the documentation    gt  gt  gt  open  test   wb   write  test    gt  gt  gt  open  test   a b   write  koko    gt  gt  gt  open  test   rb   read    testkoko

User · Answer

The simplest way to append more text to the end of a file would be to use   with open   path to file    a    as file      file write  Additions to file   file close     The a  in the open      statement instructs to open the file in append mode and allows read and write access   It is also always good practice to use file close   to close any files that you have opened once you are done using them

User · Answer

if you want to append to a file   with open  test txt    a   as myfile      myfile write  append me     We declared the variable myfile to open a file named test txt  Open takes 2 arguments  the file that we want to open and a string that represents the kinds of permission or operation we want to do on the file  here is file mode options   Mode    Description   r  This is the default mode  It Opens file for reading   w  This Mode Opens file for writing   If file does not exist  it creates a new file  If file exists it truncates the file   x  Creates a new file  If file already exists  the operation fails   a  Open file in append mode   If file does not exist  it creates a new file   t  This is the default mode  It opens in text mode   b  This opens in binary mode      This will open a file for reading and writing  updating

User · Answer

You can also open the file in r  mode and then set the file position to the end of the file   import os  with open  text txt    r    as f      f seek 0  os SEEK END      f write  text to add     Opening the file in r  mode will let you write to other file positions besides the end  while a and a  force writing to the end

User · Answer

when we using this line open filename   a    that a indicates the appending the file  that means allow to insert extra data to the existing file   You can just use this following lines to append the text in your file  def FileSave filename content       with open filename   a   as myfile          myfile write content   FileSave  test txt   test1  n   FileSave  test txt   test2  n

User · Answer

Here s my script  which basically counts the number of lines  then appends  then counts them again so you have evidence it worked    shortPath        file to be appended  short   open shortPath   r       this counts how many line are originally in the file  long path       file to be appended to   long   open long path   r   for i l in enumerate long        pass print   s has  i lines initially    long path i  long close    long   open long path   a      now open long file to append l   True    will be a line c   0    count the number of lines you write while l       try           l   short next      when you run out of lines  this breaks and the except statement is run         c    1         long write l       except           l   None         long close           print  Done   wrote  s lines   c      finally  count how many lines are left   long   open long path   r   for i l in enumerate long        pass print   s has  i lines after appending new lines    long path  i  long close

User · Answer

I always do this   f   open  filename txt    a   f write  stuff   f close     It s simple  but very useful

User · Answer

The  a  parameter signifies append mode  If you don t want to use with open each time  you can easily write a function to do it for you   def append txt   nFunction Successfully Executed   file       with open file   a   as f          f write txt    If you want to write somewhere else other than the end  you can use  r        import os  with open file   r    as f      f seek 0  os SEEK END      f write  text to add     Finally  the  w   parameter grants even more freedom  Specifically  it allows you to create the file if it doesn t exist  as well as empty the contents of a file that currently exists         Credit for this function goes to  Primusa

User · Answer

You can also do it with print instead of write   with open  test txt    a   as f      print  appended text   file f    If test txt doesn t exist  it will be created

User · Answer

If multiple processes are writing to the file  you must use append mode or the data will be scrambled  Append mode will make the operating system put every write  at the end of the file irrespective of where the writer thinks his position in the file is  This is a common issue for multi-process services like nginx or apache where multiple instances of the same process  are writing to the same log file  Consider what happens if you try to seek  then write  Example does not work well with multiple processes    f   open  quot logfile quot    quot w quot    f seek 0  os SEEK END   f write  quot data to write quot     writer1  seek to end of file            position 1000  for example  writer2  seek to end of file            position 1000 writer2  write data at position 1000    end of file is now 1000   length of data  writer1  write data at position 1000    writer1 s data overwrites writer2 s data   By using append mode  the operating system will place any write at the end of the file  f   open  quot logfile quot    quot a quot    f seek 0  os SEEK END   f write  quot data to write quot     Append most does not mean   quot open file  go to end of the file once after opening it quot   It means   quot open file  every write I do will be at the end of the file quot   WARNING  For this to work you must write all your record in one shot  in one write call  If you split the data between multiple writes  other writers can and will get their writes in between yours and mangle your data

[python] How do you append to a file?

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