在Python中，可以定义包含若干参数的函数，这里有几种可用的形式，也可以混合使用：

1. 默认参数

最常用的一种形式是为一个或多个参数指定默认值。

>>> def ask_ok(prompt,retries=4,complaint='Yes or no Please!'):    while True:        ok=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)

这个函数可以通过几种方式调用：

• 只提供强制参数

>>> ask_ok('Do you really want to quit?')Do you really want to quit?yesTrue

• 提供一个可选参数

>>> ask_ok('OK to overwrite the file',2)OK to overwrite the fileNoYes or no Please!OK to overwrite the filenoFalse

• 提供所有的参数

>>> ask_ok('OK to overwrite the file?',2,'Come on, only yes or no!')OK to overwrite the file? testCome on, only yes or no!OK to overwrite the file?yesTrue

2. 关键字参数

函数同样可以使用keyword=value形式通过关键字参数调用

>>> def parrot(voltage,state='a stiff',action='voom',type='Norwegian Blue'):    print("--This parrot wouldn't", action, end=' ')    print("if you put",voltage,"volts through it.")    print("--Lovely plumage, the",type)    print("--It's",state,"!")    >>> parrot(1000)--This parrot wouldn't voom if you put 1000 volts through it.--Lovely plumage, the Norwegian Blue--It's a stiff !>>> parrot(action="vooooom",voltage=1000000)--This parrot wouldn't vooooom if you put 1000000 volts through it.--Lovely plumage, the Norwegian Blue--It's a stiff !>>> parrot('a thousand',state='pushing up the daisies')--This parrot wouldn't voom if you put a thousand volts through it.--Lovely plumage, the Norwegian Blue--It's pushing up the daisies !

但是以下的调用方式是错误的：

>>> parrot(voltage=5, 'dead')SyntaxError: non-keyword arg after keyword arg>>> parrot()Traceback (most recent call last):  File "
         
          ", line 1, in 
          
               parrot()TypeError: parrot() missing 1 required positional argument: 'voltage'>>> parrot(110, voltage=220)Traceback (most recent call last):  File "
           
            ", line 1, in 
            
                 parrot(110, voltage=220)TypeError: parrot() got multiple values for argument 'voltage'>>> parrot(actor='John')Traceback (most recent call last):  File "
             
              ", line 1, in 
              
                parrot(actor='John')TypeError: parrot() got an unexpected keyword argument 'actor'>>> parrot(voltage=100,action='voom',action='voooooom')SyntaxError: keyword argument repeated
              
             
            
           
          
         

Python的函数定义中有两种特殊的情况，即出现*，**的形式。

*用来传递任意个无名字参数，这些参数会以一个元组的形式访问

**用来传递任意个有名字的参数，这些参数用字典来访问

（*name必须出现在**name之前）

>>> def cheeseshop1(kind,*arguments,**keywords):    print("--Do you have any",kind,"?")    print("--I'm sorry, we're all out of",kind)    for arg in arguments:        print(arg)    print("-"*40)    keys=sorted(keywords.keys())    for kw in keys:        print(kw,":",keywords[kw])        >>> cheeseshop1("Limbuger","It's very runny, sir.","It's really very, very runny, sir.",shopkeeper="Michael Palin",client="John",sketch="Cheese Shop Sketch")--Do you have any Limbuger ?--I'm sorry, we're all out of LimbugerIt's very runny, sir.It's really very, very runny, sir.----------------------------------------client : Johnshopkeeper : Michael Palinsketch : Cheese Shop Sketch>>>

 

3. 可变参数列表

最常用的选择是指明一个函数可以使用任意数目的参数调用。这些参数被包装进一个元组，在可变数目的参数前，可以有零个或多个普通的参数

通常，这些可变的参数在形参列表的最后定义，因为他们会收集传递给函数的所有剩下的输入参数。任何出现在*args参数之后的形参只能是“关键字参数”

>>> def contact(*args,sep='/'):    return sep.join(args)>>> contact("earth","mars","venus")'earth/mars/venus'

 

4. 拆分参数列表

 当参数是一个列表或元组，但函数需要分开的位置参数时，就需要拆分参数

• 调用函数时使用*操作符将参数从列表或元组中拆分出来

>>> list(range(3,6))[3, 4, 5]>>> args=[3,6]>>> list(range(*args))[3, 4, 5]>>>

• 以此类推，字典可以使用**操作符拆分成关键字参数

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

 

5. Lambda

在Python中使用lambda来创建匿名函数，而用def创建的是有名称的。

• python lambda会创建一个函数对象，但不会把这个函数对象赋给一个标识符，而def则会把函数对象赋值给一个变量
• python lambda它只是一个表达式，而def则是一个语句

>>> def make_incrementor(n):    return lambda x:x+n>>> f=make_incrementor(42)>>> f(0)42>>> f(2)44

>>> g=lambda x:x*2>>> print(g(3))6>>> m=lambda x,y,z:(x-y)*z>>> print(m(3,1,2))4

 

6. 文档字符串

关于文档字符串内容和格式的约定：

• 第一行应该总是关于对象用途的摘要，以大写字母开头，并且以句号结束
• 如果文档字符串包含多行，第二行应该是空行

>>> def my_function():    """Do nothing, but document it.    No, really, it doesn't do anything.    """    pass>>> print(my_function.__doc__)Do nothing, but document it.    No, really, it doesn't do anything.    >>>