A couple of functions for formatting timedeltas

format_timedelta.py

"""
Functions for flexibly formatting Python timedelta objects.

The following field codes apply:
 d - days
 M - minutes
 h - hours
 s - seconds
 m - milliseconds
 u - microseconds
"""

import re

# Microsecond conversion table
mc = {'u': 1}
mc['m'] = mc['u'] * 1000
mc['s'] = mc['m'] * 1000
mc['M'] = mc['s'] * 60
mc['h'] = mc['M'] * 60
mc['d'] = mc['h'] * 24

def timedelta_parts(td, *codes):
    """
    Returns a dict containing value components of a timedelta.
    The parts always add up to the total size of the timedelta.
    """
    td = abs(td)
    microseconds = (td.days * mc['d']) + (td.seconds * mc['s']) + td.microseconds
    parts = {}
    for unit in ['d', 'h', 'M', 's', 'm', 'u']:
        if unit in codes:
            parts[unit] = microseconds / mc[unit]
            microseconds -= parts[unit] * mc[unit]
    return parts

fmt_codes = re.compile(r'%(?P<args>[^a-zA-Z]*)(?P<code>[dhMsmu])')

def format_timedelta(td, fmt):
    """
    Formats a timedelta object according to the specified format string.

    >>> import datetime
    >>> td = datetime.timedelta(days=2, hours=19, minutes=33, seconds=5)
    >>> format_timedelta(td, '%02h:%02M:%02s')
    '67:33:05'
    >>> format_timedelta(td, '%d days, %s seconds')
    '2 days, 70385 seconds'
    """
    codes = [code for args, code in fmt_codes.findall(fmt)]
    parts = timedelta_parts(td, *codes)
    
    # Normalise special codes into named keys, i.e. %02h -> %(h)02d
    norm_fmt = fmt_codes.sub(
        lambda m: '%%(%(code)s)%(args)sd' % (m.groupdict()), fmt)
    return norm_fmt % parts

if __name__ == '__main__':
    import doctest
    doctest.testmod()

Two functions: one for decomposing a timedelta into discrete parts (hours, mins, etc.), and another for formatting timedeltas using 'standard' Python string interpolation.