了解"*"“仅关键字"； python3函数中的参数表示法

问题描述

与仅关键字参数功能在python3中配合使用时，我遇到了一些困难行为href ="https://docs.python.org/3.4/library/functools.html#functools.partial">部分.其他仅关键字参数的信息.

I am having some difficulty behaviour of keyword only arguments feature in python3 when used with partial. Other info on keyword only arguments.

这是我的代码:

def awesome_function(a = 0, b = 0, *, prefix):
    print('a ->', a)
    print('b ->', b)
    print('prefix ->', prefix)
    return prefix + str(a+b)

这是我对局部的理解:

>>> two_pow = partial(pow, 2)
>>> two_pow(5)
32
>>>

在上面的示例中，我理解的是，partial使pow的第二个参数成为two_pow的唯一参数.

What I understood is in the above example, partial makes the second argument to pow function as the only argument of two_pow.

我的问题是为什么以下各项起作用:

My question is why does the following work:

>>> g = partial(awesome_function, prefix='$')
>>> g(3, 5)
a -> 3
b -> 5
prefix -> $
'$8'
>>>

但是我得到了错误:

>>> awesome_function(prefix='$', 3, 5)
  File "<stdin>", line 1
SyntaxError: non-keyword arg after keyword arg
>>>

我知道我可以直接致电awesome_function，

I know that I can call awesome_function directly by

>>> awesome_function(prefix='$', a = 3, b = 5)
a -> 3
b -> 5
prefix -> $
'$8'
>>>

推荐答案

根据 Python中的函数调用，要传递的参数的规则如下

As per the semantics of the function calls in Python, the rules for the arguments to be passed are as follows

argument_list   ::=  positional_arguments ["," keyword_arguments]
                       ["," "*" expression] ["," keyword_arguments]
                       ["," "**" expression]
                     | keyword_arguments ["," "*" expression]
                       ["," keyword_arguments] ["," "**"     expression]
                     | "*" expression ["," keyword_arguments] ["," "**" expression]
                     | "**" expression

如您在此处看到的，位置参数应始终出现在函数调用的开头.它们不能出现在其他任何地方.当你做

As you see here, the positional arguments should always appear at the beginning of the function calls. They cannot appear anywhere else. When you do

awesome_function(prefix='$', 3, 5)

它违反了上述规则，因为您在关键字参数(prefix)之后传递了两个位置参数(3和5).这就是为什么得到SyntaxError的原因，因为Python无法解析函数调用表达式.

it violates the above rule, as you are passing two positional arguments (3 and 5) after a keyword argument (prefix). That is why you are getting a SyntaxError, as Python is not able to parse the function call expression.

但是，当您使用partial时，它可以工作，因为partial创建了一个新的函数对象，并存储了所有要传递给它的参数.实际调用partial返回的函数对象时，它将首先应用所有位置参数，然后应用关键字参数.

But, when you are using partial it works, because partial creates a new function object and it stores all the parameters to be passed to it. When you actually invoke the function object returned by partial, it applies all the positional arguments first and then the keyword arguments.

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