humanize elapsed time

elapsed_time.py

def elapsed_time(start, end):
    """
    >>> from datetime import datetime
    >>> elapsed_time(datetime(2000, 1, 1), datetime(2000, 1, 1, 20, 15))
    '20 hours, and 15 minutes'
    >>> elapsed_time(datetime(2000, 1, 1), datetime(2000, 1, 1, 0, 1))
    '1 minute'
    >>> elapsed_time(datetime(2000, 1, 1), datetime(2000, 1, 2, 0, 1))
    '1 day, and 1 minute'
    >>> elapsed_time(datetime(2000, 1, 1), datetime(2000, 1, 2, 2, 3, 4))
    '1 day, 2 hours, 3 minutes, and 4 seconds'
    """

    assert start < end

    td = end - start
    constants = ((3600 * 24, 'day'), (3600, 'hour'), (60, 'minute'), (1, 'second'))
    ts = td.seconds + td.days * 24 * 3600
    out = []
    total = 0
    for coef, name in constants:
        t = math.floor((ts - total) / coef)
        if t > 0:
            out.append("%d %s%s" % (t, name, 's' if t > 1 else ''))
            total += t * coef

    if len(out) > 1:
        out[-1] = "and %s" % out[-1]
    return ', '.join(out)