在Python中，可以定義包含若干參數的函數，這里有幾種可用的形式，也可以混合使用：

1. 默認參數

最常用的一種形式是為一個或多個參數指定默認值。

>>> 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)

這個函數可以通過幾種方式調用：

只提供強制參數

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

Do you really want to quit?yes

True

提供一個可選參數

>>> ask_ok('OK to overwrite the file',2)

OK to overwrite the fileNo

Yes or no Please!

OK to overwrite the fileno

False

提供所有的參數

>>> 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. 關鍵字參數

函數同樣可以使用keyword=value形式通過關鍵字參數調用

>>> 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 !

但是以下的調用方式是錯誤的：

>>> 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的函數定義中有兩種特殊的情況，即出現*，**的形式。

*用來傳遞任意個無名字參數，這些參數會以一個元組的形式訪問

**用來傳遞任意個有名字的參數，這些參數用字典來訪問

(*name必須出現在**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. 可變參數列表

最常用的選擇是指明一個函數可以使用任意數目的參數調用。這些參數被包裝進一個元組，在可變數目的參數前，可以有零個或多個普通的參數

通常，這些可變的參數在形參列表的最后定義，因為他們會收集傳遞給函數的所有剩下的輸入參數。任何出現在*args參數之后的形參只能是“關鍵字參數”

>>> def contact(*args,sep='/'):

return sep.join(args)

>>> contact("earth","mars","venus")

'earth/mars/venus'

4. 拆分參數列表

當參數是一個列表或元組，但函數需要分開的位置參數時，就需要拆分參數

調用函數時使用*操作符將參數從列表或元組中拆分出來

>>> list(range(3,6))

[3, 4, 5]

>>> args=[3,6]

>>> list(range(*args))

[3, 4, 5]

>>>

以此類推，字典可以使用**操作符拆分成關鍵字參數

>>> 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來創建匿名函數，而用def創建的是有名稱的。

python lambda會創建一個函數對象，但不會把這個函數對象賦給一個標識符，而def則會把函數對象賦值給一個變量

python lambda它只是一個表達式，而def則是一個語句

>>> 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. 文檔字符串

關于文檔字符串內容和格式的約定：

第一行應該總是關于對象用途的摘要，以大寫字母開頭，并且以句號結束

如果文檔字符串包含多行，第二行應該是空行

>>> 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.

>>>

總結

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