新手对python default不是很理解它有什么用途，pythondefault,以下这段代码的defau

新手对python default不是很理解它有什么用途，pythondefault,以下这段代码的defau

以下这段代码的default怎么理解，是有个参数为default吗？那么这个default有什么用途呢？我是从c#转来学python的，看到这个函数觉得很奇怪，觉得default是多余的。（代码是从开源程序中copy来的）

def pretty_date(dt, default=None):    """    Returns string representing "time since" e.g.    3 days ago, 5 hours ago etc.    """    if default is None:        default = 'just now'    now = datetime.utcnow()    diff = now - dt    periods = (        (diff.days / 365, 'year', 'years'),        (diff.days / 30, 'month', 'months'),        (diff.days / 7, 'week', 'weeks'),        (diff.days, 'day', 'days'),        (diff.seconds / 3600, 'hour', 'hours'),        (diff.seconds / 60, 'minute', 'minutes'),        (diff.seconds, 'second', 'seconds'),    )    for period, singular, plural in periods:        if not period:            continue        if period == 1:            return u'%d %s ago' % (period, singular)        else:            return u'%d %s ago' % (period, plural)    return default

CREATE TABLE customer(First_Name char(50),Last_Name char(50),Address char(50) default 'Unknown',City char(50) default 'Mumbai',Country char(25),Birth_Date date)

编橙之家文章，