Every so often, people seem to be writing stuff in the Python library again, usually poorly. While the occasional module has a poor interface, it is usually much better to use the rich standard library and data types that come with Python then inventing your own.
A useful module very few people know about is os.path. It always has the correct path arithmetic for your operating system, and will usually be much better then whatever you come up with yourself.
Compare:
# ugh! return dir+"/"+file # better return os.path.join(dir, file)
More useful functions in os.path: basename, dirname and splitext.
There are also many useful builtin functions people seem not to be aware of for some reason: min() and max() can find the minimum/maximum of any sequence with comparable semantics, for example, yet many people write they own max/min. Another highly useful function is reduce(). Classical use of reduce() is something like
import sys, operator nums = map(float, sys.argv[1:]) print reduce(operator.add, nums)/len(nums)
This cute little script prints the average of all numbers given on the command line. The reduce() adds up all the numbers, and the rest is just some pre- and postprocessing.
On the same note, note that float(), int() and long() all accept arguments of type string, and so are suited to parsing -- assuming you are ready to deal with the ValueError they raise.