python中的函数

python doc上的内容

默认参数

def ask_ok(prompt, retries=4, complaint='Yes or no, please!'):    while True:        ok = raw_input(prompt)        if ok in ('y', 'ye', 'yes'):            return True        if ok in ('n', 'no', 'nop', 'nope'):            return False        retries = retries - 1        if retries < 0:            raise IOError('refusenik user')        print complaint

使用的时候可以想下面那样使用：

• giving only the mandatory argument: ask_ok('Doyoureally wanttoquit?')
• giving one of the optional arguments: ask_ok('OKtooverwrite the file?', 2)
• or even giving all arguments: ask_ok('OKtooverwrite the file?', 2, 'Come on, only yes or no!')

注意的是参数只被执行一次，当参数中有可变变量的时候需要注意，例如下面的例子

<pre><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">L</span><span class="o">=</span><span class="p">[]):</span>    <span class="n">L</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>    <span class="k">return</span> <span class="n">L</span><span class="k">print</span> <span class="n">f</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span><span class="k">print</span> <span class="n">f</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span><span class="k">print</span> <span class="n">f</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>

输出将是下面的形式，这显然不是想要的结果。

[1][1, 2][1, 2, 3]

关键字参数

位置参数就如下面函数的vlotage参数，关键字参数就是kwarg=value这种形式，例如：

def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'):    print "-- This parrot wouldn't", action,    print "if you put", voltage, "volts through it."    print "-- Lovely plumage, the", type    print "-- It's", state, "!"

调用可以用下面的格式：

parrot(1000)                                          # 1 positional argumentparrot(voltage=1000)                                  # 1 keyword argumentparrot(voltage=1000000, action='VOOOOOM')             # 2 keyword argumentsparrot(action='VOOOOOM', voltage=1000000)             # 2 keyword argumentsparrot('a million', 'bereft of life', 'jump')         # 3 positional argumentsparrot('a thousand', state='pushing up the daisies')  # 1 positional, 1 keyword

但是下面的就是不合法的调用方式:

parrot()                     # required argument missingparrot(voltage=5.0, 'dead')  # non-keyword argument after a keyword argumentparrot(110, voltage=220)     # duplicate value for the same argumentparrot(actor='John Cleese')  # unknown keyword argument

需要注意的是，一个参数只能进行一次设置，关键字参数的设置左边的变量名必须是定义函数内的变量名，关键字参数的位置可以任意，但是位置参数的位置不能任意。

函数参数中的*name和**name这种格式的参数用法

*name必须出现在**name前面

*name将接收的参数放在tuple中

而**name将接受到的参数放在字典中

例子：

def cheeseshop(kind, *arguments, **keywords):    print "-- Do you have any", kind, "?"    print "-- I'm sorry, we're all out of", kind    for arg in <strong>arguments</strong>:        print arg    print "-" * 40    keys = sorted(<strong>keywords</strong>.keys())    for kw in keys:        print kw, ":", keywords[kw]

调用方式：

cheeseshop("Limburger", "It's very runny, sir.",           "It's really very, VERY runny, sir.",           shopkeeper='Michael Palin',           client="John Cleese",           sketch="Cheese Shop Sketch")

输出：

-- Do you have any Limburger ?-- I'm sorry, we're all out of LimburgerIt's very runny, sir.It's really very, VERY runny, sir.----------------------------------------client : John Cleeseshopkeeper : Michael Palinsketch : Cheese Shop Sketch

Arbitrary Argument Lists

Finally, the least frequently used option is to specify that a function can be called with an arbitrary number of arguments. These arguments will be wrapped up in a tuple (seeTuples and Sequences). Before the variable number of arguments, zero or more normal arguments may occur.

def write_multiple_items(file, separator, *args):    file.write(separator.join(args))

Unpacking Argument Lists

>>> range(3, 6)             # normal call with separate arguments[3, 4, 5]>>> args = [3, 6]>>> range(*args)            # call with arguments unpacked from a list[3, 4, 5]

>>> def parrot(voltage, state='a stiff', action='voom'): ...     print "-- This parrot wouldn't", action,...     print "if you put", voltage, "volts through it.",...     print "E's", state, "!"...>>> d = {"voltage": "four million", "state": "bleedin' demised", "action": "VOOM"} #from a dic>>> parrot(**d)-- This parrot wouldn't VOOM if you put four million volts through it. E's bleedin' demised !

Lambda Expressions

lambdaa,b: a+b : this return the sum of a and b

例子：

>>> def make_incrementor(n):...     return lambda x: x + n...>>> f = make_incrementor(42)>>> f(0)42>>> f(1)43

另外lamda表达式可以作为函数实参使用

>>> pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]>>> pairs.sort(key=lambda pair: pair[1])>>> pairs[(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]