使用 *args、**kwargs 和可选/默认参数调用 Python 函数 [英] Calling a Python function with *args,**kwargs and optional / default arguments

问题描述

在python中，我可以定义一个函数如下:

In python, I can define a function as follows:

def func(kw1=None,kw2=None,**kwargs):
   ...

在这种情况下，我可以调用 func 为:

In this case, i can call func as:

func(kw1=3,kw2=4,who_knows_if_this_will_be_used=7,more_kwargs=Ellipsis)

我还可以将函数定义为:

I can also define a function as:

def func(arg1,arg2,*args):
    ...

可以称为

func(3,4,additional,arguments,go,here,Ellipsis)

最后，我可以将两种形式结合起来

Finally, I can combine the two forms

def func(arg1,arg2,*args,**kwargs):
    ...

但是，调用是行不通的:

But, what does not work is calling:

func(arg1,arg2,*args,kw1=None,kw2=None,**kwargs):  #SYNTAX ERROR (in python 2 only,  apparently this works in python 3)
    ...

我最初的想法是这可能是因为一个函数

My original thought was that this was probably because a function

def func(arg1,arg2,*args,kw1=None):
    ...

可以称为

func(1,2,3) #kw1 will be assigned 3

所以这会给 3 应该被打包到 args 还是 kwargs 中引入一些歧义.但是，使用 python 3，可以指定仅关键字参数:

So this would introduce some ambiguity as to whether 3 should be packed into args or kwargs. However, with python 3, there is the ability to specify keyword only arguments:

def func(a,b,*,kw=None):  #can be called as func(1,2), func(1,2,kw=3), but NOT func(1,2,3)
   ...

有了这个，似乎没有语法歧义:

With this, it seems that there is no syntactic ambiguity with:

def func(a,b,*args,*,kw1=None,**kwargs):
    ...

然而，这仍然会带来语法错误(使用 Python3.2 测试).我失踪有什么原因吗?并且，有没有办法获得我上面描述的行为(使用带有默认参数的 *args)——我知道我可以通过操作函数内部的 kwargs 字典来模拟这种行为.

However, this still brings up a syntax error (tested with Python3.2). Is there a reason for this that I am missing? And, is there a way to get the behavior I described above (Having *args with default arguments) -- I know I can simulate that behavior by manipulating the kwargs dictionary inside the function.

推荐答案

您可以在 Python 3 中做到这一点.

You can do that in Python 3.

def func(a,b,*args,kw1=None,**kwargs):

裸* 仅用于指定关键字参数而不接受带有*args 的可变数量的位置参数.您不使用两个 *.

The bare * is only used when you want to specify keyword only arguments without accepting a variable number of positional arguments with *args. You don't use two *s.

引用语法，在 Python 2 中，你有

To quote from the grammar, in Python 2, you have

parameter_list ::=  (defparameter ",")*
                    (  "*" identifier [, "**" identifier]
                    | "**" identifier
                    | defparameter [","] )

在 Python 3 中，您有

parameter_list ::=  (defparameter ",")*
                    (  "*" [parameter] ("," defparameter)*
                    [, "**" parameter]
                    | "**" parameter
                    | defparameter [","] )

包括在 * 参数之后提供附加参数的规定.

which includes a provision for additional parameters after the * parameter.

更新:

最新的 Python 3 文档此处.

Latest Python 3 documentation here.

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