pythongrammarpython-typing

Strange syntax specification for python decorators

The syntax specification for function definitions:

funcdef        ::=  [decorators] "def" funcname "(" [parameter_list] ")" ["->" expression] ":" suite
decorators     ::=  decorator+
decorator      ::=  "@" dotted_name ["(" [parameter_list [","]] ")"] NEWLINE
dotted_name    ::=  identifier ("." identifier)*
parameter_list ::=  (defparameter ",")*
                    | "*" [parameter] ("," defparameter)* ["," "**" parameter]
                    | "**" parameter
                    | defparameter [","] )
parameter      ::=  identifier [":" expression]
defparameter   ::=  parameter ["=" expression]
funcname       ::=  identifier

Seems to suggest the following is syntactically valid:

@some.dotted.name(*what : "is this")
def my_func(x):
    pass

Yet it is not accepted by the interpreter. Am I misreading the grammar, or is the grammar incorrect?

Solution

  • Looks like a documentation bug to me.

    It was

    decorator      ::=  "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE

    until v3.2, when argument_list changed to parameter_list. That cannot be right, although whoever submitted the one-line patch evidently thought otherwise.

    The grammar itself was not modified. It still says:

    decorator: '@' dotted_name [ '(' [arglist] ')' ] NEWLINE

    And since that is:

    the full Python grammar, as it is read by the parser generator and used to parse Python source files

    I think we can conclude that Python decorators still have the syntax we all expect.