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Learning To Write - Python

Now that you know how to create and use Python's tuples, listsand dictionaries, it's time to get your hands dirty. In this article, findout how to read from, and write to, files on the filesystem with built-inPython methods, and also take a look at some of the new features availablein Python 2.x.

TABLE OF CONTENTS:
  1. Python 101 (part 5): Snake Oil For The Soul
  2. Air In A Bottle
  3. Treading The Right Path
  4. Standing In Line
  5. Learning To Write
  6. Desperately Seeking Python
  7. String Theory
  8. Pop() Goes The Weasel
By: Vikram Vaswani, (c) Melonfire
Rating: starstarstarstarstar / 4
July 09, 2001

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Obviously, reading a file is no great shakes - but how about writing to afile?

Well, it isn't all that hard either - you just need to open the file withan optional argument indicating that you would like to write to it.

>>> data = open("snakeoil.txt", "w") >>>
The optional argument specifies the mode in which the file should beopened, and may be either one of "r" (read only), "w" (write) or "a"(append). Opening a file with "w" is akin to creating a new, empty file -the previous contents of the file are erased. Opening a file with "a"retains the previous contents of the file and appends new material to theend of the file.

You can add the "+" operator to the three modes above to indicate that bothread and write operations should be allowed.

Just as Python has the read() and readlines() methods for file read, it hascorresponding write() and writelines() functions for file output. Thewrite() function writes a single string to the file, as demonstrated in thefollowing example:

>>> data = open("snakeoil.txt", "a") >>> data.write("-- end of commercial --") >>> data.close() >>>


$ cat snakeoil.txt After years of research, scientists at AB Labs have come up with a radical new product, once that promises to improve the quality of life for humans and animals everywhere. We call it Air In A Bottle(tm), and we firmly consider it to be one of our best inventions ever. Carry it around with you (the bottle is titanium, the same material used in rocket ships, and the air is, well, air) and inhale from it whenever you need some air. Or give it to your friends as a present (after all, they need air too!) Air In A Bottle(tm). Get some today! -- end of commercial --$
It should be noted that write() does not automatically add a line break toyour string; this needs to be done manually.

The writelines() method takes a list and writes each element of the list tothe file sequentially. The following Python program builds a list from userinput, and then writes the list to the file.

#!/usr/bin/python # empty list comments = [] # ask for three comments and build list for x in range (0,3): rawInput = raw_input("Enter a comment: ") comments.append("User comment:" + rawInput + "\n") # open file data = open("snakeoil.txt", "a") # write list data.writelines(comments) # clean up data.close()
Here's what it looks like:

Enter a comment: This product sucks! Enter a comment: How stupid do you think we are?????? Enter a comment: Can I have some more? $ cat snakeoil.txt After years of research, scientists at AB Labs have come up with a radical new product, once that promises to improve the quality of life for humans and animals everywhere. We call it Air In A Bottle(tm), and we firmly consider it to be one of our best inventions ever. Carry it around with you (the bottle is titanium, the same material used in rocket ships, and the air is, well, air) and inhale from it whenever you need some air. Or give it to your friends as a present (after all, they need air too!) Air In A Bottle(tm). Get some today! User comment:This product sucks! User comment:How stupid do you think we are?????? User comment:Can I have some more?
As you can see, the comments entered at the command line have been appendedto the end of the file.

 
 
>>> More Python Articles          >>> More By Vikram Vaswani, (c) Melonfire
 

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