Most pythonic way to format a number with commas?

[Ned Batchelder]

For dir/ls replacement script, I wanted to format file sizes with
commas as a thousands separator.  I didn't find anything in the
standard library, but figured it couldn't be too hard to do with list
manipulation.  I came up with this:

def formatNumber(num):
    """Format a number for display"""
    sl = [c for c in str(num)]
    for i in range(len(sl)-3, 0, -3):
        sl[i:i] = [',']
    return string.join(sl, '')

While this works, and uses list ops to do it, I thought it would be
possible to get a simpler solution.

Is there one?

--Ned.
http://www.nedbatchelder.com