Learning To Write - Python

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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