Help on built-in function filter in module __builtin__:
filter(...)
filter(function or None, sequence) -> list, tuple, or string
Return those items of sequence for which function(item) is true. If
function is None, return the items that are true. If sequence is a tuple
or string, return the same type, else return a list.
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.
See itertools.ifilter() and itertools.ifilterfalse() for iterator versions of this function, including a variation that filters for elements where the function returns false.
中文说明：
该函数的目的是提取出seq中能使func为true的元素序列。func函数是一个布尔函数，filter()函数调用这个函数一次作用于seq中的每一个元素，筛选出符合条件的元素，并以列表的形式返回。
>>> nums=[2,3,6,12,15,18]
>>> def nums_res(x):
... return x%2 == 0 and x%3 == 0
...
>>> print filter(nums_res,nums)
[6, 12, 18]
>>> def is_odd(n):
... return n%2==1
...
>>> filter(is_odd, [1,2,4,5,6,9,10,15])
[1, 5, 9, 15]
>>> def not_empty(s):
... return s and s.strip()
...
>>> filter(not_empty, ['A','','B',None,'C',' '])
['A', 'B', 'C']