How to insert a newline at middle of a file in python

i am a new user of python. Just trying to write a script using python. Can someone help me out.

I calculate some values using data from a file, and 开发者_JAVA技巧i want to replace the existing value with the new value in the file along with that i would like to comment out the existing value so that i can verify whether the new calculated value does make sense. I did parse the required files and am messed up with the insertion of newline and commenting out the old one.

Would be greatful if someone posts an example.

Thanks n regards ss213

Output to a new file, line by line, changing what you need. Then replace the old file with the new one.

input = open('input.txt','r')
output = open('temp.txt','w')
for line in input.readlines():
    if line == 'xxx':
        output.write('#' + line)
        output.write('yyy')
    else:
        output.write(line)

A bit vague. Let's suppose you have a string

a = 'ABCDEF'

and you want to insert a newline in the middle, then you can use:

a = "%s%s%s" % (a [:3], '\n', a [3:])

Or three bazillion other ways of doing it. Could you post an example input file and the desired output?