replace single space between words in python

You can use regular expressions to do this:

>>> import re
>>> x = '09     Web Problem       Any problem has to do with the dept. web sites'
>>> print re.sub(r'([^\s])\s([^\s])', r'\1_\2',x)
09     Web_Problem       Any_problem_has_to_do_with_the_dept._web_sites

The search pattern is (1) any non-white-space character, followed by (2) a single white-space character, followed by (3) another non-white-space character.

Numbers 1 and 3 are captured so that they can be used in the replacement pattern. Number 2 is ignored and we put an underscore in instead.

This leaves the multi-white-space areas alone and simply changes the singly-occurring white-space characters into underscores, which is what I think you were asking for.

If you are trying to maintain the space between the first number and text then:

Updated:

import re
match = re.match("^([0-9]+)(\ +)(.*?)(\ +\ +)(.*)",yourstring)
output = match.group(1)+match.group(2)+match.group(3).replace(' ','_')+match.group(4)+ match.group(5).replace(' ', '_')

To read the file in, you'll want to use the open() function along with a loop (a for loop would make good sense) to read each line.

To break the line into pieces, you can use the nifty string slice syntax. See http://docs.python.org/tutorial/introduction.html#strings for some examples on slices.

The do the actual replacement of spaces to _, the replace method is what you want.

'abc def'.replace(' ', '_')

See http://docs.python.org/library/stdtypes.html#string-methods for more useful string methods.

Since you are just getting started with Python, I highly recommend the following tutorial: http://learnpythonthehardway.org/ Work through the whole thing and you should have a solid foundation to build on.