def get_elapsed(start, end, pre='0', post='4'):
    """Elapsed time calculator.

    Method computes the difference between two datetime.datetime objects,
    and returns the difference as a string.

    Arguments:
        start {datetime}    -- start time
        end {datetime}      -- end time

    Keyword Arguments:
        pre {str}   -- leading 0's
                        (default: {'0'})
        post {str}  -- precision
                        (default: {'4'})

    Returns:
        {str} -- elapsed time
    """
    f_str = ''.join(['{', str(pre), ':.', str(post), 'f}'])
    e_str = round((end - start), int(post))
    return f_str.format(e_str)