Python Language Reference And Function Definitions

Question

As I understand it, the python language reference is written in extended BNF format. While looking at the documentation for function definitions I noticed that the spec doesn't seem to support the usage of trailing *args or **kwargs parameters.

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

See: https://docs.python.org/3/reference/compound_stmts.html#grammar-token-defparameter

And yet when I do a function def like so:

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

The python interpreter considers it to be legal.

What am I missing? From the spec it appears that you must have the * at the beginning of the function definition before any other parameters.

I don't understand your question. It says `"*" [parameter]` and `"**" parameter` right there. — BrenBarn, Mar 05 '15 at 19:54
right but doesn't the parameter_list spec mean that a parameter list is either a list of parameters OR a * followed by a list of parameters, OR ** parameter, OR a single parameter. In which case there is no where that a parameter list may be parameters followed by *identifier or parameters followed by **kwargs etc. — limik, Mar 05 '15 at 19:57
There's a note there that says (about something different) "... this is a syntactic restriction that is not expressed by the grammar." It would seem that in your case this is a syntactic flexibility/allowance that s not expressed by the grammar. — jedwards, Mar 05 '15 at 20:08

score 1 · Accepted Answer · answered Mar 05 '15 at 20:10

It appears to be a documentation bug for which an issue has been raised.

I think the definitions in the documentation are slightly simplified in order to be easier to read. The "real" definition is given in the full grammar specification:

funcdef: 'def' NAME parameters ['->' test] ':' suite
parameters: '(' [typedargslist] ')'
typedargslist: (tfpdef ['=' test] (',' tfpdef ['=' test])* [','
       ['*' [tfpdef] (',' tfpdef ['=' test])* [',' '**' tfpdef] | '**' tfpdef]]
     |  '*' [tfpdef] (',' tfpdef ['=' test])* [',' '**' tfpdef] | '**' tfpdef)
tfpdef: NAME [':' test]

This is more difficult to read but appears to correctly allow varargs with or without ordinary arguments.

Thank you for the help. I didn't know about the issue tracker! — limik, Mar 05 '15 at 20:19

Python Language Reference And Function Definitions

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