Python 基础-函数

>>> def fib2(n): #return Fibonacci series up to n         #如何从函数中返回一个包含菲波那契数列的数值链表
...     """ Return a list containing the Fibonacci series up to n. """
...     result = []
...     a,b=0,1
...     while b<n:
...             result.append(b)  # see below
...             a,b = b,a+b
...     return result
...                                #这里要回车 不要输入
>>> f100 = fib2(100)  # call it
>>> f100  # write the result
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
>>> 

4.7 深入函数定义
4.7.1 参数默认值 ：

>>> def ask_ok(prompt, retries=4,complaint='Yes or no,please!'):
...     while True:
...             ok=raw_input(prompt)
...             if ok in ('y','ye','yes'): return 1
...             if ok in ('n','no','nop','nope'): return 0
...             retries = retries-1
...             if retries <0: raise IOError,'refusenik user'
...             print complaint
... 
>>> ask_ok('Do you really want to quit?')
Do you really want to quit?y
1
>>> ask_ok('Do you really want to quit?')
Do you really want to quit?no
0
>>> ask_ok('Do you really want to quit?')
Do you really want to quit?
Yes or no,please!
Do you really want to quit?i
Yes or no,please!
Do you really want to quit?y
1
>>> ask_ok('Do you really want to quit?',1)
Do you really want to quit?d
Yes or no,please!
Do you really want to quit?y
1
>>> ask_ok('Do you really want to quit?',0)
Do you really want to quit?
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in ask_ok
IOError: refusenik user
>>> 

>>> 
>>> i=5
>>> def f(arg=i):  
...     print arg
... 
>>> i =6
>>> f()  #默认值在函数定义段被解析，如下所示：
5
>>> 

重要警告：默认值只会解析一次。当默认值是一个可变对象，诸如链表、字典或大部分类实例时，会产生一些差异。例如，以下函数在后继的调用中会积累它的参数值：

>>> def f(a,L=[]):
...     L.append(a)
...     return L
... 
>>> print f(1)
[1]
>>> print f(2)
[1, 2]
>>> print f(3)
[1, 2, 3]
>>> print f(1)
[1, 2, 3, 1]
>>> 

如果你不想在不同的函数调用之间共享参数默认值，可以如下面的实例一样编写函数：

>>> def f(a,L=None):
...     if L is None:
...             L=[]
...     L.append(a)
...     return L
... 
>>> print f(1)
[1]
>>> print f(2)
[2]
>>> print f(3)
[3]
>>> 

4.7.2 参数关键字
    #函数可以通过参数关键字的形式来调用，形如“keyword = value”。例如，以下的函数：

>>> 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,"!"
... 
>>> 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='V0000M',voltage=1000000)
-- This parrot wouldn't  V0000M 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 !
>>> ##以下是错误的调用
>>> parrot() # required argument missing（缺少必要参数）
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: parrot() takes at least 1 argument (0 given)
>>> parrot(voltage=5.0,'dead')
  File "<stdin>", line 1
SyntaxError: non-keyword arg after keyword arg
>>> parrot(voltage=5,'dead')  # non-keyword argument following keyword（在关键字后面有非关键字参数）
  File "<stdin>", line 1
SyntaxError: non-keyword arg after keyword arg
>>> parrot(voltage=5,action='dead')
-- This parrot wouldn't  dead if you put 5 Volts through it.
--Lovely plumage the  Norwegian Blue
-- It's  a stiff !
>>> 
>>> parrot(110,voltage=220)                      # duplicate value for argument（对参数进行了重复赋值）
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: parrot() got multiple values for keyword argument 'voltage'
>>> parrot(action =110,voltage=220)
-- This parrot wouldn't  110 if you put 220 Volts through it.
--Lovely plumage the  Norwegian Blue
-- It's  a stiff !
>>> parrot(actor='John Cleese')                    # unknown keyword（未知关键字）
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: parrot() got an unexpected keyword argument 'actor'
>>> parrot(action='John Cleese')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: parrot() takes at least 1 non-keyword argument (0 given)
>>> parrot(11,action='John Cleese')
-- This parrot wouldn't  John Cleese if you put 11 Volts through it.
--Lovely plumage the  Norwegian Blue
-- It's  a stiff !
>>> 

4.7.5 文档字符串

这里介绍文档字符串的概念和格式。

第一行应该是关于对象用途的简介。简短起见，不用明确的陈述对象名或类型，因为它们可以从别的途径了解到（除非这个名字碰巧就是描述这个函数操作的动词）。这一行应该以大写字母开头，以句号结尾。

如果文档字符串有多行，第二行应该空出来，与接下来的详细描述明确分隔。接下来的文档应该有一或多段描述对象的调用约定、边界效应等。

Python的解释器不会从多行的文档字符串中去除缩进，所以必要的时候应当自己清除缩进。这符合通常的习惯。第一行之后的第一个非空行决定了整个文档的缩进格式。（我们不用第一行是因为它通常紧靠着起始的引号，缩进格式显示的不清楚。）留白“相当于”是字符串的起始缩进。每一行都不应该有缩进，如果有缩进的话，所有的留白都应该清除掉。相当于留白就是验证后的制表符扩展（通常是8个空格）。（这一段译得不通，有疑问的读者请参见原文－－译者）

以下是一个多行文档字符串的示例：

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

        No,really,it doesn't do anything.

>>>