[Python-Dev] funcdef grammar production (original) (raw)

Guido van Rossum guido at python.org
Mon Mar 15 15:10:32 EST 2004

PEP 318 is likely to change the BNF productions for funcdef, so I was taking a look at what is there right now.

funcdef ::= "def" funcname "(" [parameterlist] ")" ":" suite parameterlist ::= (defparameter ",")* ("*" identifier [, "**" identifier] | "**" identifier | defparameter [","]) defparameter ::= parameter ["=" expression] sublist ::= parameter ("," parameter)* [","] parameter ::= identifier | "(" sublist ")" funcname ::= identifier But the text points out that no purely positional (without a default value) arguments can occur after any keyword arguments. Is there a reason not to include this in the BNF? (Assuming the other productions are unchanged,) this seems to work: parameterlist ::= starparameter | defparameter ("," defparameter)* ["," [starparameter]] | parameter ("," parameter)* ("," defparameter)* ["," [starparameter]] starparameter ::= "*" identifier [, "**" identifier] | "**" identifier defparameter ::= parameter "=" expression

This can't work (at least not in the parser -- I don't care what's put in the docs) because parameter and defparameter have the same set of initial tokens.

starparameter was separated out because a trailing comma is permitted after a positional or default parameter, but not after *args or **kwargs.

Is there some technical reason not to do this?

Not to do what? Allow a comma after *args?

For instance, is the doco autogenerated from the actual code?

You should know by now that there are various tools for that, although again maybe you're asking the question in a different context.

Is it somehow cheaper to check for an optional first parameter than for three possibilities?

Who cares?

--Guido van Rossum (home page: http://www.python.org/~guido/)

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