为什么在使用* args语法的参数列表中尾随逗号是SyntaxError？

Question

为什么不能*args在Python中使用尾随逗号？换句话说，这有效

>>> f(1, 2, b=4,)

但这不是

>>> f(*(1, 2), b=4,)

  File "<stdin>", line 1

    f(*(1, 2), b=4,)

                   ^

SyntaxError: invalid syntax

Python 2和Python 3都是这种情况。

Answer 1

让我们看一下语言规范：call&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;::=&nbsp; primary "(" [argument_list [","]&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | expression genexpr_for] ")"argument_list&nbsp; &nbsp; &nbsp; &nbsp; ::=&nbsp; positional_arguments ["," keyword_arguments]&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ["," "*" expression] ["," keyword_arguments]&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ["," "**" expression]&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | keyword_arguments ["," "*" expression]&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ["," "**" expression]&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | "*" expression ["," "*" expression] ["," "**" expression]&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | "**" expressionpositional_arguments ::=&nbsp; expression ("," expression)*keyword_arguments&nbsp; &nbsp; ::=&nbsp; keyword_item ("," keyword_item)*keyword_item&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;::=&nbsp; identifier "=" expression让我们筛选一下我们关心的部分：call&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;::=&nbsp; primary "(" [argument_list [","]] ")"argument_list&nbsp; &nbsp; &nbsp; &nbsp; ::=&nbsp; positional_arguments ["," keyword_arguments]&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ["," "*" expression] ["," keyword_arguments]&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ["," "**" expression]positional_arguments ::=&nbsp; expression ("," expression)*keyword_arguments&nbsp; &nbsp; ::=&nbsp; keyword_item ("," keyword_item)*keyword_item&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;::=&nbsp; identifier "=" expression因此，看起来像在调用函数的任何参数之后，我们都被允许了extra ,。因此，这看起来像是cpython实现中的错误。类似的东西：f(1, *(2,3,4), )应该按照这种语法工作，但在CPython中却不行。在较早的答案中，Eric链接到CPython语法规范，该规范包括上述语法的CPython实现。它在下面：arglist: (argument ',')* ( argument [',']&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;| '*' test (',' argument)* [',' '**' test]&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;| '**' test&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;)请注意，这个语法是不一样的由语言规范提出的一个。我认为这是一个实现错误。请注意，CPython实现还有其他问题。这也应该得到支持：f(*(1,2,3), *(4,5,6))奇怪的是，该规范不允许 f(*(1,2,3), *(4,5,6), *(7,8,9))当我进一步研究时，我认为规范的这一部分需要进行一些修复。这是允许的：f(x=1, *(2,3))，但是这不是：f(x=1, 2, 3)。为了对原始问题有所帮助，在CPython中，如果不使用*args或**kwargs功能，则可以使用逗号结尾。我同意这很la脚。

Answer 2

在对问题9232中的此错误进行了一些讨论之后，Guido van Rossum评论：我对此添加+1。我认为这不需要PEP。某些地方已经支持定义中的逗号结尾，因此我不赞成它会捕获错误的说法。在暂停期间，我们也许太严格了。