Modify one character in a string

[Roy Smith]

"mp" <mailpitches at email.com> wrote:

> X-No-Archive
> How do I go about modifying one character in a string elegantly?
> In other words, I want a function that will change 'aaaa' to 'aaza',
> given the index 2 of the character in the string.

Strings are immutable, so you can't change a string.  The best you can do 
is create a new string from the old one.  Something like:

>>> old = 'aaaa'
>>> new = old[:2] + 'z' + old[3:]
>>> print new
aaza

or a few variations on that theme.

> Also, how do I do this when dealing with a file ; which file mode
> should I use and what function should I use to modify a single
> character once in that file mode?

This is a much more complicated question, because it depends on the details 
of the operating system you're using.  On Unix, you can seek to a specific 
place in a file, write a single character, seek to EOF, and you've done an 
in-place single character edit of the file.  I don't know if that works on 
other OS's or not.