python技巧 列表推导

val = [expression for value in collection if condition]

等价于

val = []
for value in collection:
    if condition:
        val.append(expression)


列表推导在python2中存在变量泄漏的问题:
    >>> i=1
    >>> [i for i in range(10)]
    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    >>> i
    9
    
python 3中已经修复:
    in [2]: i=1

    in [3]: [i for i in range(10)]
    out[3]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

    in [4]: i
    out[4]: 1