Python学习 Part2：深入Python函数定义

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?yes
True

• 提供一个可选参数

>>> ask_ok('OK to overwrite the file',2)
OK to overwrite the fileNo
Yes or no Please!
OK to overwrite the fileno
False

• 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? test
Come on, only yes or no!
OK to overwrite the file?yes
True

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 "<pyshell#57>", line 1, in <module>
    parrot()
TypeError: parrot() missing 1 required positional argument: 'voltage'
>>> parrot(110, voltage=220)
Traceback (most recent call last):
  File "<pyshell#58>", line 1, in <module>
    parrot(110, voltage=220)
TypeError: parrot() got multiple values for argument 'voltage'
>>> parrot(actor='John')
Traceback (most recent call last):
  File "<pyshell#59>", line 1, in <module>
    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 Limbuger
It's very runny, sir.
It's really very, very runny, sir.
----------------------------------------
client : John
shopkeeper : Michael Palin
sketch : 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.

>>>