代碼如下:
import random, string
class C(object):    pass
def dangerFunction(msg, l = [], b = {}, c = C()):
    print msg, '-'*10
    print l, b, c.__dict__
    l.append(1)
    b[random.choice(string.ascii_lowercase)] = ''
    c.__dict__[random.choice(string.ascii_lowercase)] = ""
    print l, b, c.__dict__
dangerFunction('1')
dangerFunction('2')
dangerFunction('3')
print '-'*20
def safeFunction(msg, l = None, b = None, c = None):
    if not l:   l = []
    if not b:   b = {}
    if not c:   c = C()
    print msg, '-'*10
    print l, b, c.__dict__
    l.append(1)
    b[random.choice(string.ascii_lowercase)] = ''
    c.__dict__[random.choice(string.ascii_lowercase)] = ""
    print l, b, c.__dict__
safeFunction('1')
safeFunction('2')
safeFunction('3')

運行結果：
代碼如下:
1 ----------
[] {} {}
[1] {'q': ''} {'p': ''}
2 ----------
[1] {'q': ''} {'p': ''}
[1, 1] {'q': '', 'a': ''} {'p': '', 'g': ''}
3 ----------
[1, 1] {'q': '', 'a': ''} {'p': '', 'g': ''}
[1, 1, 1] {'q': '', 'a': '', 'w': ''} {'p': '', 'w': '', 'g': ''}
--------------------
1 ----------
[] {} {}
[1] {'k': ''} {'l': ''}
2 ----------
[] {} {}
[1] {'r': ''} {'c': ''}
3 ----------
[] {} {}
[1] {'q': ''} {'h': ''}

由dangerFunction打印出來的結果來看，缺省值為 [], (), class
再下次調用時，如果繼續參數空缺而使用缺省值，那么缺省值延續上次引用。

可能打印無任何標志無法看清楚，加上文字應該會簡單很多。
代碼如下:
# -*- coding: utf-8 -*-
import random, string
class C(object):    pass
def dangerFunction(msg, l = [], b = {}, c = C()):
    print msg, '-'*10
    print u'操作前', l, b, c.__dict__
    l.append(1)
    b[random.choice(string.ascii_lowercase)] = ''
    c.__dict__[random.choice(string.ascii_lowercase)] = ""
    print u'操作后', l, b, c.__dict__
dangerFunction('1')
dangerFunction('2')
dangerFunction('3')
print '-' * 10, u'我是分隔符', '-' * 10
def safeFunction(msg, l = None, b = None, c = None):
    if not l:   l = []
    if not b:   b = {}
    if not c:   c = C()
    print msg, '-'*10
    print u'操作前', l, b, c.__dict__
    l.append(1)
    b[random.choice(string.ascii_lowercase)] = ''
    c.__dict__[random.choice(string.ascii_lowercase)] = ""
    print u'操作后',l, b, c.__dict__
safeFunction('1')