http://www.cnblogs.com/BeginMan/p/3164937.html

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filter}===>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.

一、列表解析

列表解析来自函数式编程语言(haskell),语法如下：

[expr for iter_var in iterable][expr for iter_var in iterable if cond_expr]

第一种语法：首先迭代iterable里所有内容，每一次迭代，都把iterable里相应内容放到iter_var中，再在表达式中应用该iter_var的内容，最后用表达式的计算值生成一个列表。

第二种语法：加入了判断语句，只有满足条件的内容才把iterable里相应内容放到iter_var中，再在表达式中应用该iter_var的内容，最后用表达式的计算值生成一个列表。

例子如下：

>>> seq = [11,10,9,9,5,35,8,20,31,72,54,53]>>> filter(lambda x:x%2,seq)[11, 9, 9, 5, 35, 31, 53]>>> [x for x in seq if x%2] [11, 9, 9, 5, 35, 31, 53]

更复杂的如求矩阵：

#3行5列的矩阵>>> [(x+1,y+1) for x in range(3) for y in range(5)] [(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5)] #简单的3行3列 >>> [(x,y) for x in range(3) for y in range(3)] [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)] #模拟 >>> for x in range(3): for y in range(3): print x,y 0 0 0 1 0 2 1 0 1 1 1 2 2 0 2 1

计算一个文件的字数和大小：

>>> import os>>> f = open('demo.txt','r') >>> #计算单词的个数 >>> len([word for line in f for word in line.split()]) 1942 >>> #计算文件大小 >>> os.stat('demo.txt').st_size 14051L >>> f.seek(0) #seek()函数回到文件头部，因为迭代器已经访问完了文件的所有行 >>> sum([len(word) for line in f for word in line.split()]) 10806 >>> f.close() >>>

 经典例子：

books=[        {
     "name":u"C#从入门到精通","price":23.7,"store":u"卓越"}, { "name":u"ASP.NET高级编程","price":44.5,"store":u"卓越"}, { "name":u"Python核心编程","price":24.7,"store":u"当当"}, { "name":u"JavaScript大全","price":45.7,"store":u"当当"}, { "name":u"Django简明教程","price":26.7,"store":u"新华书店"}, { "name":u"深入Python","price":55.7,"store":u"新华书店"}, ]

1、书籍信息中价格最低的

#No1:原始方式：>>> price=[]>>> for item in books: 　　for p in item: if p == 'price': price.append(item[p]) >>> min(price) 23.699999999999999 #No2:列表解析： >>> min([item[p] for item in books for p in item if p=='price']) 23.699999999999999

#Python相关书籍检索for item in books:    for p in item: if item['name'].find('Python')>=0: print item[p], #24.7 Python核心编程 当当 55.7 深入Python 新华书店 total = [item[p] for item in books for p in item if item['name'].find('Python')>=0] print total #[24.699999999999999, u'Python\u6838\u5fc3\u7f16\u7a0b', u'\u5f53\u5f53', 55.700000000000003, u'\u6df1\u5165Python', u'\u65b0\u534e\u4e66\u5e97']

 

二、生成器表达式

1、为什么要用到它?

2、它与列表解析的区别?