On 02.03.2017 09:45, Ivan Levkivskyi wrote: > On 2 March 2017 at 09:36, M.-A. Lemburg <m...@egenix.com> wrote: > >> On 02.03.2017 09:03, Serhiy Storchaka wrote: >>> Function implemented in Python can have optional parameters with default >> [...] >> > 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)? >> > > I think for the sane reason that we didn't add Undefined to PEP 484 > and PEP 526: > Having "another kind of None" will cause code everywhere to expect it.
But that's exactly the point :-) Code should be made aware of such a special value and act accordingly. I had introduced NotGiven in our code to be able to differentiate between having a parameter provided to a method/function or not, and I needed a new singleton, because None was in fact a permitted value for the parameters, but I still had to detect whether this parameter was passed in or not. Example: >>> import mx.Tools >>> mx.Tools.NotGiven NotGiven >>> def f(x=mx.Tools.NotGiven): pass ... >>> help(f) Help on function f in module __main__: f(x=NotGiven) Because it's a singleton, you can use "is" for test which is very fast. BTW: NotGiven was named after NotImplemented, another singleton we have in Python. I had introduced this a long time ago to implement better coercion logic: http://web.archive.org/web/20011222024710/http://www.lemburg.com/files/python/CoercionProposal.html -- 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/
