这个已经介绍的很明了，
reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])
相当于计算
((((1+2)+3)+4)+5)
而：
reduce(lambda x, y: x+y, [1, 2, 3, 4, 5],6)
相当于计算
(((((6+1)+2)+3)+4)+5)
4、filter
在http://docs.python.org/2/library/functions.html#filter中如下介绍filter函数：
filter(function, iterable)
Construct a list from those elements of iterable for which function returns true. iterable may be either a sequence, a container which supports iteration, or an iterator. If iterable is a string or a tuple, the result also has that type; otherwise it is always a list. If function is None, the identity function is assumed, that is, all elements of iterable that are false are removed.
Note that filter(function, iterable) is equivalent to [item for item in iterable if function(item)] if function is not None and [item for item in iterable if item] if function is None.
参数function（是函数）用于处理iterable中的每个元素，如果function处理某元素时候返回true，那么该元素将作为list的成员而返回。比如，过滤掉字符串中的字符a：
def func(x):
''' '''
return x != 'a'
print filter(func, 'awake')
运行结果是：
wke
这也可以通过列表推导来实现：
print ''.join([x for x in 'awake' if x != 'a'])
