write versus writelines and concatenated strings

Question

So I m learning Python  I am going through the lessons and ran into a problem where I had to condense a great many target write   into a single write    while having a  quot  n quot  between each user input variable the object of write     I came up with  nl    quot  n quot  lines   line1  nl  line2  nl  line3  nl textdoc writelines lines   If I try to do  textdoc write lines   I get an error  But if I type  textdoc write line1    quot  n quot    line2          Then it works fine  Why am I unable to use a string for a newline in write   but I can use it in writelines    Python 2 7

User · Answer

Exercise 16 from Zed Shaw s book  You can use escape characters as follows   paragraph1     s  n  s  n  s  n     line1  line2  line3  target write paragraph1  target close

User · Answer

Why am I unable to use a string for a newline in write   but I can use it in writelines     The idea is the following  if you want to write a single string you can do this with write    If you have a sequence of strings you can write them all using writelines     write arg  expects a string as argument and writes it to the file  If you provide a list of strings  it will raise an exception  by the way  show errors to us     writelines arg  expects an iterable as argument  an iterable object can be a tuple  a list  a string  or an iterator in the most general sense   Each item contained in the iterator is expected to be a string  A tuple of strings is what you provided  so things worked   The nature of the string s  does not matter to both of the functions  i e  they just write to the file whatever you provide them  The interesting part is that writelines   does not add newline characters on its own  so the method name can actually be quite confusing  It actually behaves like an imaginary method called write all of these strings sequence    What follows is an idiomatic way in Python to write a list of strings to a file while keeping each string in its own line   lines     line1    line2   with open  filename txt    w   as f      f write   n  join lines     This takes care of closing the file for you  The construct   n  join lines  concatenates  connects  the strings in the list lines and uses the character   n  as glue  It is more efficient than using the   operator   Starting from the same lines sequence  ending up with the same output  but using writelines     lines     line1    line2   with open  filename txt    w   as f      f writelines   s n    l for l in lines    This makes use of a generator expression and dynamically creates newline-terminated strings  writelines   iterates over this sequence of strings and writes every item   Edit  Another point you should be aware of   write   and readlines   existed before writelines   was introduced  writelines   was introduced later as a counterpart of readlines    so that one could easily write the file content that was just read via readlines     outfile writelines infile readlines      Really  this is the main reason why writelines has such a confusing name  Also  today  we do not really want to use this method anymore  readlines   reads the entire file to the memory of your machine before writelines   starts to write the data  First of all  this may waste time  Why not start writing parts of data while reading other parts  But  most importantly  this approach can be very memory consuming  In an extreme scenario  where the input file is larger than the memory of your machine  this approach won t even work  The solution to this problem is to use iterators only  A working example   with open  inputfile   as infile      with open  outputfile   as outfile          for line in infile              outfile write line    This reads the input file line by line  As soon as one line is read  this line is written to the output file  Schematically spoken  there always is only one single line in memory  compared to the entire file content being in memory in case of the readlines writelines approach

User · Answer

if you just want to save and load a list try Pickle  Pickle saving   with open  yourFile   wb  as file   pickle dump YourList file    and loading   with open  yourFile   rb  as file   YourList pickle load file

User · Answer

writelines expects an iterable of strings write expects a single string     line1     n    line2 merges those strings together into a single string before passing it to write   Note that if you have many lines  you may want to use   n  join list of lines

User · Answer

Actually  I think the problem is that your variable  lines  is bad  You defined lines as a tuple  but I believe that write   requires a string  All you have to change is your commas into pluses       nl     n  lines   line1 nl line2 nl line3 nl textdoc writelines lines    should work

[python] write() versus writelines() and concatenated strings

Examples related to python

Examples related to string

Examples related to file-io

Examples related to python-2.x