函数的默认参数值，只在第一次调用赋默认值

def foo(x=[]): x.append(1); print(x)
foo(); foo()
得到：
[1]
[1, 1]

第一次调用foo() 得到x =[] ，然后x = [1]，x指向列表[1]内存中的地址。
但是第二次调用foo() , 不执行x = []

参考 Python 官方 Tutorial 中的内容

Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls:

原文地址：http://blog.csdn.net/delphiwcdj/article/details/5719470
Python Default Argument Values

The most useful form is to specify a default value for one or more arguments. This creates a function that can be called with fewer arguments than it is defined to allow. For example:

[1] 
The default values are evaluated at the point of function definition in the defining scope, so that will print 5

>>> def f(arg=i):
    print arg

>>> f()
5
>>> i=2
>>> f

>>> f()
5
>>> i=5
>>> i
5
>>> def f(arg=i):
    print 'i=', i

>>> f()
i= 5
>>> i=6
>>> f()
i= 6
>>>

[2] 
Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls:

>>> def f(a, L=[]):
    L.append(a)
    return L
>>> print f(1)
[1]
>>> print f(2)
[1, 2]
>>> print f(3)
[1, 2, 3]
>>>
# modified as following
>>> def f(a, L=None):
    if L is None:
        L=[]
    L.append(a)
    return L
>>> f(1)
[1]
>>> f(2)
[2]
>>> f(3)
[3]
>>>