Testing for presence of arguments

Dan Sommers me at privacy.net
Wed Aug 17 17:41:26 CEST 2005


On Wed, 17 Aug 2005 11:13:03 -0400,
Madhusudan Singh <spammers-go-here at spam.invalid> wrote:

> I know how to set optional arguments in the function definition. Is
> there an intrinsic function that determines if a certain argument was
> actually passed ? Like the fortran 95 present() logical intrinsic ?

    def f(**kw):
        if kw.has_key('required_argument'):
            print "require_argument was present"
        else:
            print "require_argument was not present"

> My required functionality depends on whether a certain argument is
> specified at all. (Setting default values is *not* good enough.).

You can very nearly achieve this with carefully planned default
arguments.  Put this into a module:

    class _SemiPrivateClass:
        pass

    def f(required_argument=_SemiPrivateClass):
        if required_argument == _SemiPrivateClass:
            print "required_argument was probably not present"
        else:
            print "required_argument was present"

It's not impossible fool f, but an external module has to try very hard
to do so.

(All code untested.)

Regards,
Dan

-- 
Dan Sommers
<http://www.tombstonezero.net/dan/>



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