python list comprehension twos for loop 嵌套for循环

list comprehension 后面可以有多个for loops，每个for后面可以有if

[(x, y, x * y)for x in(0,1,2,3)for y in(0,1,2,3)if x < y]
等同于：

for x in (0,1,2,3):
    for y in (0,1,2,3):
        if x < y:
            print (x, y, x*y)

It also supports both “if" statements and referencing the outer iterator from the inner one, like so:

>>> seq = ['abc', 'def', 'g', 'hi'] 
... [y for x in seq if len(seq) > 1 for y in x if y != 'e'] 
['a', 'b', 'c', 'd', 'f', 'g', 'h', 'i']

等同于：

>>> result = [] 
... for x in seq: 
...     if len(seq) > 1: 
...         for y in x: 
...             if y != 'e': 
...                 result.append(y) 
... result ['a', 'b', 'c', 'd', 'f', 'g', 'h', 'i']

（说明一下，第一个for loop里面的if从句是多余的，len（seq）一直不变。

参考：http://stackoverflow.com/questions/7847624/list-comprehension-for-loops-python

http://rhodesmill.org/brandon/2009/nested-comprehensions/

http://www.python-course.eu/list_comprehension.php