Writing a list to a file with Python

Question

Is this the cleanest way to write a list to a file  since writelines   doesn t insert newline characters   file writelines    s n    item  for item in list     It seems like there would be a standard way

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What are you going to do with the file  Does this file exist for humans  or other programs with clear interoperability requirements   If you are just trying to serialize a list to disk for later use by the same python app  you should be pickleing the list   import pickle  with open  outfile    wb   as fp      pickle dump itemlist  fp    To read it back   with open   outfile    rb   as fp      itemlist   pickle load fp

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Because i m lazy      import json a    1 2 3  with open  test txt    w   as f      f write json dumps a     Now read the file back into a Python list object with open  test txt    r   as f      a   json loads f read

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with open   test txt   w  as fp     for line in list12         fp write line   n

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file write   n  join list

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Yet another way   Serialize to json using simplejson  included as json in python 2 6     gt  gt  gt  import simplejson  gt  gt  gt  f   open  output txt    w    gt  gt  gt  simplejson dump  1 2 3 4   f   gt  gt  gt  f close     If you examine output txt       1  2  3  4    This is useful because the syntax is pythonic  it s human readable  and it can be read by other programs in other languages

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You can also use the print function if you re on python3 as follows   f   open  myfile txt   wb   print mylist  file f

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In python 3 you can use print and   for argument unpacking   with open  fout txt    w   as fout      print  my list  sep   n   file fout

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Why don t you try  file write str list

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Let avg be the list  then   In  29   a   n array  avg   In  31   a tofile  avgpoints dat  sep   n  dtype     f     You can use  e or  s depending on your requirement

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I thought it would be interesting to explore the benefits of using a genexp  so here s my take   The example in the question uses square brackets to create a temporary list  and so is equivalent to   file writelines  list    s n    item for item in list       Which needlessly constructs a temporary list of all the lines that will be written out  this may consume significant amounts of memory depending on the size of your list and how verbose the output of str item  is   Drop the square brackets  equivalent to removing the wrapping list   call above  will instead pass a temporary generator to file writelines     file writelines    s n    item for item in list     This generator will create newline-terminated representation of your item objects on-demand  i e  as they are written out   This is nice for a couple of reasons    Memory overheads are small  even for very large lists If str item  is slow there s visible progress in the file as each item is processed     This avoids memory issues  such as   In  1   import os  In  2   f   file os devnull   w    In  3    timeit f writelines    s n    item for item in xrange 2  20    1 loops  best of 3  385 ms per loop  In  4    timeit f writelines     s n    item for item in xrange 2  20     ERROR  Internal Python error in the inspect module  Below is the traceback from this internal error   Traceback  most recent call last       MemoryError    I triggered this error by limiting Python s max  virtual memory to  100MB with ulimit -v 102400    Putting memory usage to one side  this method isn t actually any faster than the original   In  4    timeit f writelines    s n    item for item in xrange 2  20    1 loops  best of 3  370 ms per loop  In  5    timeit f writelines     s n    item for item in xrange 2  20     1 loops  best of 3  360 ms per loop    Python 2 6 2 on Linux

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The simpler way is   with open  outfile    w   as outfile      outfile write   n  join itemlist     You could ensure that all items in item list are strings using a generator expression   with open  outfile    w   as outfile      outfile write   n  join str item  for item in itemlist     Remember that all itemlist list need to be in memory  so  take care about the memory consumption

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Another way of iterating and adding newline     for item in items      filewriter write f  item       n

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Redirecting stdout to a file might also be useful for this purpose  from contextlib import redirect stdout with open  test txt    w   as f    with redirect stdout f        for i in range mylst size           print mylst i

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You can also go through following  Example  my list  1 2 3 4 5  quot abc quot   quot def quot   with open  your file txt    w   as file      for item in my list          file write  quot  s n quot    item   Output  In your file txt items are saved like  1  2  3  4  5  abc  def  Your script also saves as above  Otherwise  you can use pickle import pickle my list  1 2 3 4 5  quot abc quot   quot def quot    to write with open  your file txt    wb   as file      pickle dump my list  file   to read with open   your file txt    rb   as file      Outlist   pickle load file  print Outlist   Output   1  2  3  4  5   abc    def   It save dump the list same as a list when we load it we able to read  Also by simplejson possible same as above output import simplejson as sj my list  1 2 3 4 5  quot abc quot   quot def quot    To write with open  your file txt    w   as file      sj dump my list  file    To save with open  your file txt    r   as file      mlist sj load file  print mlist

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This logic will first convert the items in list to string str   Sometimes the list contains a tuple like   alist     i12 tiger     113 lion     This logic will write to file each tuple in a new line  We can later use eval while loading each tuple when reading the file    outfile   open  outfile txt    w     open a file in write mode for item in list to persistence       iterate over the list items    outfile write str item      n     write to the file outfile close       close the file

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Using numpy savetxt is also an option  import numpy as np  np savetxt  list txt   list  delimiter  quot  n quot   fmt  quot  s quot

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In Python3 You Can use this loop  with open  your file txt    w   as f      for item in list          f print     item

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Serialize list into text file with comma sepparated value  mylist   dir   with open  filename txt   w   as f      f write      join  mylist

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In General  Following is the syntax for writelines   method  fileObject writelines  sequence     Example     usr bin python    Open a file fo   open  foo txt    rw    seq     This is 6th line n    This is 7th line      Write sequence of lines at the end of the file  line   fo writelines  seq      Close opend file fo close     Reference  http   www tutorialspoint com python file writelines htm

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poem        Programming is fun When the work is done if you wanna make your work also fun  use Python      f   open  poem txt    w     open for  w riting f write poem    write text to file f close     close the file   How It Works  First  open a  le by using the built-in open function and specifying the name of the  le and the mode in which we want to open the  le  The mode can be a read mode     r      write mode     w     or append mode     a      We can also specify whether we are reading  writing  or appending in text mode     t     or binary mode     b      There are actually many more modes available and help open  will give you more details about them  By default  open   considers the  le to be a    t   ext  le and opens it in    r   ead mode  In our example  we  rst open the  le in write text mode and use the write method of the  le object to write to the  le and then we  nally close the  le   The above example is from the book  A Byte of Python  by Swaroop C H  swaroopch com

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You can use a loop   with open  your file txt    w   as f      for item in my list          f write   s n    item    In Python 2  you can also use  with open  your file txt    w   as f      for item in my list          print  gt  gt  f  item   If you re keen on a single function call  at least remove the square brackets     so that the strings to be printed get made one at a time  a genexp rather than a listcomp  -- no reason to take up all the memory required to materialize the whole list of strings

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Using Python 3 and Python 2 6  syntax   with open filepath   w   as file handler      for item in the list          file handler write     n  format item     This is platform-independent  It also terminates the final line with a newline character  which is a UNIX best practice   Starting with Python 3 6      n  format item  can be replaced with an f-string  f  item  n

[python] Writing a list to a file with Python

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