[Python-3000] PEP3102 Keyword-Only Arguments (original) (raw)

Guido van Rossum guido at python.org
Mon Aug 14 20:04:19 CEST 2006

On 8/14/06, Steven Bethard <steven.bethard at gmail.com> wrote:

On 8/14/06, Guido van Rossum <guido at python.org> wrote: > I believe the PEP doesn't address the opposite use case: positional > arguments that should not be specified as keyword arguments. For > example, I might want to write > > def foo(a, b): ... > > but I don't want callers to be able to call it as foo(b=1, a=2) or > even foo(a=2, b=1).

Another use case is when you want to accept the arguments of another callable, but you have your own positional arguments:: >>> class Wrapper(object): ... def init(self, func): ... self.func = func ... def call(self, *args, **kwargs): ... print 'calling wrapped function' ... return self.func(*args, **kwargs) ... >>> @Wrapper ... def func(self, other): ... return self, other ... >>> func(other=1, self=2) Traceback (most recent call last): File "", line 1, in ? TypeError: call() got multiple values for keyword argument 'self' It would be really nice in the example above to mark self in _call_ as a positional only argument.

But this is a rather unusual use case isn't it? It's due to the bound methods machinery. Do you have other use cases? I would assume that normally such wrappers take their own control arguments in the form of keyword-only arguments (that are unlikely to conflict with arguments of the wrapped method).

> Perhaps we can use ** without following identifier to signal this? > It's not entirely analogous to * without following identifier, but at > least somewhat similar.

I'm certainly not opposed to going this way, but I don't think it would solve the problem above since you still need to take keyword arguments.

Can you elaborate?

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

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