python技巧(一)

1.参数unpack

*args表示任何多个无名参数，它是一个tuple；**kwargs表示关键字参数，它是一个 dict。并且同时使用*args和**kwargs时，必须*args参数列要在**kwargs前。

如此可以简单的给函数传递列表和字典

def draw_point(x, y):    # do some magicpoint_foo = (3, 4)point_bar = {'y': 3, 'x': 2}draw_point(*point_foo)draw_point(**point_bar)

2.枚举器

当你在迭代容器，并且需要索引时：

>>> a = ['a', 'b', 'c', 'd', 'e']>>> for index, item in enumerate(a): print index, item...0 a1 b2 c3 d4 e>>>

3.for/else

语法糖，但能是逻辑更清晰

for i in foo:    if i == 0:        breakelse:    print("i was never 0")

相比原始写法

found = Falsefor i in foo:    if i == 0:        found = True        breakif not found:     print("i was never 0")

4.新型格式化

可读性更好，不容易对错位置

found = False>>> print "The %(foo)s is %(bar)i." % {'foo': 'answer', 'bar':42}The answer is 42.>>> foo, bar = 'question', 123>>> print "The %(foo)s is %(bar)i." % locals()The question is 123.

5.使用site-package中的.pth

增加或使用第三方哭，或对其版本配置时，使用PATHPATH环境变量不太方便可以修改site-package中的.pth

"The most convenient way [to modify python's search path] is to add a path configuration file to a directory that's already on Python's path, usually to the .../site-packages/ directory. Path configuration files have an extension of .pth, and each line must contain a single path that will be appended to sys.path. (Because the new paths are appended to sys.path, modules in the added directories will not override standard modules. This means you can't use this mechanism for installing fixed versions of standard modules.)"

6.dict的get函数

使用dict时，如果key不存在，会得到一个异常，使用dict的get函数获取一个key的value，可以对get指定一个值，这样你使用的key不在dict，你会得到默认值

7.两个值交换

python中可以不使用中间值

>>> a = 10>>> b = 5>>> a, b(10, 5)>>> a, b = b, a>>> a, b(5, 10)

8.doctest

单元测试写到注释中

def factorial(n):    """Return the factorial of n, an exact integer >= 0.    If the result is small enough to fit in an int, return an int.    Else return a long.    >>> [factorial(n) for n in range(6)]    [1, 1, 2, 6, 24, 120]    >>> factorial(-1)    Traceback (most recent call last):        ...    ValueError: n must be >= 0    Factorials of floats are OK, but the float must be an exact integer:    """    import math    if not n >= 0:        raise ValueError("n must be >= 0")    if math.floor(n) != n:        raise ValueError("n must be exact integer")    if n+1 == n:  # catch a value like 1e300        raise OverflowError("n too large")    result = 1    factor = 2    while factor <= n:        result *= factor        factor += 1    return resultdef _test():    import doctest    doctest.testmod()    if __name__ == "__main__":    _test()

9.list切块

a = [1,2,3,4,5]>>> a[::2]  # iterate over the whole list in 2-increments[1,3,5]

>>> a[::-1][5,4,3,2,1]

10.三元运算

x = 3 if (y == 1) else 2 if (y == -1) else 1