Writelines writes lines without newline Just fills the file

Question

I have a program that writes a list to a file   The list is a list of pipe delimited lines and the lines should be written to the file like this   123 GSV Weather Mean hello joe 43 45 122 GEV temp Mean hello joe 23 45 124 GSI Weather Mean hello Mike 47 45   BUT it wrote them line this ahhhh   123 GSV Weather Mean hello joe 43 45122 GEV temp Mean hello joe 23 45124 GSI Weather Mean hello Mike 47 45   This program wrote all the lines into like one line without any line breaks   This hurts me a lot and I gotta figure-out how to reverse this but anyway  where is my program wrong here  I thought write lines should write lines down the file rather than just write everything to one line    fr   open sys argv 1    r     source file fw   open sys argv 2    masked   sys argv 1    w     Target Directory Location  for line in fr      line   line strip       if line                continue     columns   line strip   split          if columns 0  find       gt  1          looking for   columns 0    this is what we need to search     else          looking for    Dummy dummy com      if looking for in d            by default  iterating over a dictionary will return keys             new line   d looking for          join columns 1                line list append new line      else          new idx   str len d  1          d looking for    new idx         kv   open sys argv 3    a           kv write looking for     new idx   n           kv close           new line   d looking for          join columns 1            line list append new line  fw writelines line list

User · Answer

As others have mentioned  and counter to what the method name would imply  writelines does not add line separators  This is a textbook case for a generator  Here is a contrived example   def item generator things       for item in things          yield item         yield   n   def write things to file things       with open  path to file txt    wb   as f          f writelines item generator things     Benefits  adds newlines explicitly without modifying the input or output values or doing any messy string concatenation  And  critically  does not create any new data structures in memory  IO  writing to a file  is when that kind of thing tends to actually matter  Hope this helps someone

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writelines   does not add line separators  You can alter the list of strings by using map   to add a new  n  line break  at the end of each string   items     abc    123          items   map lambda x  x     n   items  w writelines items

User · Answer

The documentation for writelines   states      writelines   does not add line separators   So you ll need to add them yourself  For example       line list append new line     n     whenever you append a new item to line list

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As we have well established here  writelines does not append the newlines for you   But  what everyone seems to be missing  is that it doesn t have to when used as a direct  counterpart  for readlines   and the initial read persevered the newlines     When you open a file for reading in binary mode  via  rb    then use readlines   to fetch the file contents into memory  split by line  the newlines remain attached to the end of your lines   So  if you then subsequently write them back  you don t likely want writelines to append anything   So if  you do something like   with open  test txt   rb   as f  lines f readlines   with open  test txt   wb   as f  f writelines lines    You should end up with the same file content you started with

User · Answer

As others have noted  writelines is a misnomer  it ridiculously does not add newlines to the end of each line    To do that  explicitly add it to each line   with open dst filename   w   as f      f writelines s     n  for s in lines

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This is actually a pretty common problem for newcomers to Python   especially since  across the standard library and popular third-party libraries  some reading functions strip out newlines  but almost no writing functions  except the log-related stuff  add them   So  there s a lot of Python code out there that does things like   fw write   n  join line list      n     or  fw write line     n  for line in line list    Either one is correct  and of course you could even write your own writelinesWithNewlines function that wraps it up     But you should only do this if you can t avoid it   It s better if you can create keep the newlines in the first place   as in Greg Hewgill s suggestions   line list append new line     n     And it s even better if you can work at a higher level than raw lines of text  e g   by using the csv module in the standard library  as esuaro suggests    For example  right after defining fw  you might do this   cw   csv writer fw  delimiter        Then  instead of this   new line   d looking for          join columns 1    line list append new line    You do this   row list append d looking for    columns 1      And at the end  instead of this   fw writelines line list    You do this   cw writerows row list    Finally  your design is  open a file  then build up a list of lines to add to the file  then write them all at once   If you re going to open the file up top  why not just write the lines one by one  Whether you re using simple writes or a csv writer  it ll make your life simpler  and your code easier to read   Sometimes there can be simplicity  efficiency  or correctness reasons to write a file all at once   but once you ve moved the open all the way to the opposite end of the program from the write  you ve pretty much lost any benefits of all-at-once

User · Answer

As we want to only separate lines  and the writelines function in python does not support adding separator between lines  I have written the simple code below which best suits this problem  sep    quot  n quot    defining the separator new lines   sep join lines    lines as an iterator containing line strings  and finally  with open  quot file name quot    w   as file      file writelines new lines   and you are done

[python] Writelines writes lines without newline, Just fills the file

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