[Tutor] Read from large text file, parse, find string, print string + line number to second text file.

[Scurvy Scott]

Hey all how're things?

I'm hoping for some guidance on a problem I'm trying to work through.
I know this has been previously covered on this list but I'm hoping it
won't bother you guys to run through it again.

My basic program I'm attempting to create is like this..

I want to read from a large, very large file.
I want to find a certain string
if it finds the string I would like to select the first 15-20
characters pre and proceeding the string and then output that new
string to a new file along with the line the string was located on
within the file.

It seems fairly straight forward but I'm wondering if y'all can point
me to a direction that would help me accomplish this..

Firstly I know I can read a file and search for the string with (a
portion of this code was found on stackoverflow and is not mine and
some of it is my own)

with open('largeFile', 'r') as inF:
    for line in inF:
        myString = "The String"
        if 'myString' in line:
            f = open(thenewfile', 'w')
            f.write(myString)
            f.close()

I guess what I'm looking for then is tips on A)My stated goal of also
writing the 15-20 characters before and after myString to the new file
and
B)finding the line number and writing that to the file as well.

Any information you can give me or pointers would be awesome, thanks in advance.

I'm on Ubuntu 12.10 running LXDE and working with Python 2.7

Scott