python - 为什么不能这样使用star运算符？ def foo(* args，此=“default”，** kwargs)

Question

这会很有趣:

def foo(*args, this="default", **kwargs):
    pass

但

  File "foo.py", line 1
    def foo(*args, this="default", **kwargs):
                  ^
SyntaxError: invalid syntax

由于我有一组不可知的位置参数，一组不可知的关键字参数以及一个或两个已知的关键字参数，因此我的另一种选择是:

def foo(*args, **kwargs):
    this = kwargs.pop("this", "default")
    ...

因此，由于似乎总是有一个很好的答案来说明为什么事情是这样，所以我想知道。为什么不允许这样做？

编辑:
谢谢@dano
Added in Python3

Answer 1

您想对this进行的操作是使其成为仅关键字的参数；没有办法提供位置参数。支持此功能是在Python 3.0中添加的。 Here是描述其添加以及其他相关增强的PEP。基本原理部分非常清楚地描述了该问题:

The current Python function-calling paradigm allows arguments to be specified either by position or by keyword. An argument can be filled in either explicitly by name, or implicitly by position.

There are often cases where it is desirable for a function to take a variable number of arguments. The Python language supports this using the 'varargs' syntax ('*name'), which specifies that any 'left over' arguments be passed into the varargs parameter as a tuple.

One limitation on this is that currently, all of the regular argument slots must be filled before the vararg slot can be.

This is not always desirable. One can easily envision a function which takes a variable number of arguments, but also takes one or more 'options' in the form of keyword arguments. Currently, the only way to do this is to define both a varargs argument, and a 'keywords' argument (**kwargs), and then manually extract the desired keywords from the dictionary.

基本上，这只是对varargs原始实现的限制，直到python 3才解决。