python 关键字参数

原文地址:http://docs.pythontab.com/python/python3.4/controlflow.html#tut-functions

函数可以通过 关键字参数 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, end=' ')

    print("if you put", voltage, "volts through it.")

    print("-- Lovely plumage, the", type)

    print("-- It's", state, "!")

    

接受一个必选参数 (voltage) 以及三个可选参数 (stateaction, 和 type). 

可以用以下方法调用:

parrot(1000)                                          # 1 positional argument

parrot(voltage=1000)                                  # 1 keyword argument

parrot(voltage=1000000, action='VOOOOOM')             # 2 keyword arguments

parrot(action='VOOOOOM', voltage=1000000)             # 2 keyword arguments

parrot('a million', 'bereft of life', 'jump')         # 3 positional arguments

parrot('a thousand', state='pushing up the daisies')  # 1 positional, 1 keyword

不过以下几种调用是无效的:

parrot()                     # required argument missing

parrot(voltage=5.0, 'dead')  # non-keyword argument after a keyword argument

parrot(110, voltage=220)     # duplicate value for the same argument

parrot(actor='John Cleese')  # unknown keyword argument

引入一个形如 **name 的参数时,它接收一个字典(参见 typesmapping ) ,该字典包含了所有未出现在形式参数列表中的关键字参数。这里可能还会组合使用一个形如 *name (下一小节详细介绍) 的形式参数,它接收一个元组(下一节中会详细介绍),包含了所有没有出现在形式参数列表中的参数值。( *name 必须在 **name 之前出现) 例如,我们这样定义一个函数:

def cheeseshop(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])

        

它可以像这样调用:

cheeseshop("Limburger", "It's very runny, sir.",

           "It's really very, VERY runny, sir.",

           shopkeeper="Michael Palin",

           client="John Cleese",

           sketch="Cheese Shop Sketch")

当然它会按如下内容打印:

-- Do you have any Limburger ?

-- I'm sorry, we're all out of Limburger

It's very runny, sir.

It's really very, VERY runny, sir.

----------------------------------------

client : John Cleese

shopkeeper : Michael Palin

sketch : Cheese Shop Sketch

注意在打印 关键字 参数字典的内容前先调用 sort() 方法。否则的话,打印参数时的顺序是未定义的


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