On 02.03.2017 11:22, Stephan Houben wrote: > In cases like this I would recommend creating the sentinel yourself: > > NoDefault = object() > > def get(store, key, default=NoDefault): > if default is NoDefault: > # do something > > You can arrange to not export NoDefault so that the client code cannot even > access > the sentinel value.
Yes, I know... I've been using the mxTools NotGiven since 1998. > This is strictly preferable over having yet another global > value meaning "no value", since that just moves the goal posts: > clients will complain they cannot pass in a default=NoDefault and get back > NoDefault. Not really. NoDefault would mean: no value provided, not that you don't want a value. As a result, passing NoDefault would not be allowed, since then you'd be providing a value :-) > Stephan > > > 2017-03-02 11:04 GMT+01:00 M.-A. Lemburg <m...@egenix.com>: > >> On 02.03.2017 10:06, Serhiy Storchaka wrote: >>> On 02.03.17 10:36, M.-A. Lemburg wrote: >>>> Why a new syntax ? Can't we just have a pre-defined sentinel >>>> singleton NoDefault and use that throughout the code (and also >>>> special case it in argument parsing/handling)? >>>> >>>> def get(store, key, default=NoDefault): >>>> if store.exists(key): >>>> return store.retrieve(key) >>>> ... >>> >>> This means adding a new syntax. NoDefault should be a keyword (we can >>> reuse existing keyword couldn't be used in expression), and it should be >>> accepted only in the specific context of declaring function parameter. >> >> This is not new syntax, nor is it a keyword. It's only a >> new singleton and it is well usable outside of function >> declarations as well, e.g. for class attributes which are >> not yet initialized (and which can accept None as value). >> >> The only special casing would be in function call >> parameter parsing to signal errors when the parameter >> is used as keyword parameter. >> >> -- >> Marc-Andre Lemburg >> eGenix.com >> >> Professional Python Services directly from the Experts (#1, Mar 02 2017) >>>>> Python Projects, Coaching and Consulting ... http://www.egenix.com/ >>>>> Python Database Interfaces ... http://products.egenix.com/ >>>>> Plone/Zope Database Interfaces ... http://zope.egenix.com/ >> ________________________________________________________________________ >> >> ::: We implement business ideas - efficiently in both time and costs ::: >> >> eGenix.com Software, Skills and Services GmbH Pastor-Loeh-Str.48 >> D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg >> Registered at Amtsgericht Duesseldorf: HRB 46611 >> http://www.egenix.com/company/contact/ >> http://www.malemburg.com/ >> >> _______________________________________________ >> Python-ideas mailing list >> Python-ideas@python.org >> https://mail.python.org/mailman/listinfo/python-ideas >> Code of Conduct: http://python.org/psf/codeofconduct/ >> > > > > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ > -- Marc-Andre Lemburg eGenix.com Professional Python Services directly from the Experts (#1, Mar 02 2017) >>> Python Projects, Coaching and Consulting ... http://www.egenix.com/ >>> Python Database Interfaces ... http://products.egenix.com/ >>> Plone/Zope Database Interfaces ... http://zope.egenix.com/ ________________________________________________________________________ ::: We implement business ideas - efficiently in both time and costs ::: eGenix.com Software, Skills and Services GmbH Pastor-Loeh-Str.48 D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg Registered at Amtsgericht Duesseldorf: HRB 46611 http://www.egenix.com/company/contact/ http://www.malemburg.com/ _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
