Python学习 Part2：深入Python函数定义

花蜻宽

　　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>> 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 "", 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 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.
>>>