def format_uptime(uptime_in_seconds):
    """Format number of seconds into human-readable string.

    :param uptime_in_seconds: The server uptime in seconds.
    :returns: A human-readable string representing the uptime.

    >>> uptime = format_uptime('56892')
    >>> print(uptime)
    15 hours 48 min 12 sec
    """

    m, s = divmod(int(uptime_in_seconds), 60)
    h, m = divmod(m, 60)
    d, h = divmod(h, 24)

    uptime_values = []

    for value, unit in ((d, 'days'), (h, 'hours'), (m, 'min'), (s, 'sec')):
        if value == 0 and not uptime_values:
            # Don't include a value/unit if the unit isn't applicable to
            # the uptime. E.g. don't do 0 days 0 hours 1 min 30 sec.
            continue
        elif value == 1 and unit.endswith('s'):
            # Remove the "s" if the unit is singular.
            unit = unit[:-1]
        uptime_values.append('{0} {1}'.format(value, unit))

    uptime = ' '.join(uptime_values)
    return uptime