Hi Brett, because you did not understand me, I must have had a fundamental misunderstanding. So I started a self-analysis and came to the conclusion that this was my error since maybe a decade:
When iterators and generators came into existence, I somehow fell into the trap to think that there are now sequences with undetermined or infinite length. They would be exactly those sequences which have no __len__ attribute. I understand now that sequences are always of fixed length and adjusted myself. ----------------------------------------- My problem is to find out how to deal with a class which has __getitem__ but no __len__. The documentation suggests that the length of a sequence can always be obtained by len(). https://docs.python.org/3/reference/datamodel.html But the existence of __len__ is not guaranteed or enforced. And if you look at the definition of PySequence_Fast(), you find that a sequence can be turned into a list with iteration only and no __len__. So, is a sequence valid without __len__, if iteration is supported, instead? There is the whole chapter about sequence protocol https://docs.python.org/3/c-api/sequence.html?highlight=sequence but I cannot find out an exact definition what makes up a sequence? Sorry if I'm again the only one who misunderstands the obvious :) Best -- Chris On 21.06.18 18:29, Brett Cannon wrote: > Sorry, I don't quite follow. > > On 2018年6月21日 at 08:50 Christian Tismer <[email protected] > <mailto:[email protected]>> wrote: > > Hi friends, > > there is a case in the Python API where I am not sure what to do: > > If an object defines __getitem__() only but no __len__(), > then PySequence_Check() already is true and does not care. > > > Which matches > https://docs.python.org/3/c-api/sequence.html#c.PySequence_Check . > > From Objects/abstract.c: > > int > PySequence_Check(PyObject *s) > { > if (PyDict_Check(s)) > return 0; > return s != NULL && s->ob_type->tp_as_sequence && > s->ob_type->tp_as_sequence->sq_item != NULL; > } > > > > > So if I define no __len__, it simply fails. Is this intended? > > > What is "it" in this case that is failing? It isn't PySequence_Check() > so I'm not sure what the issue is. > > -Brett > > > > I was mislead and thought this was the unlimited case, but > it seems still to be true that sequences are always finite. > > Can someone please enlighten me? > -- > Christian Tismer-Sperling :^) [email protected] > <mailto:[email protected]> > Software Consulting : http://www.stackless.com/ > Karl-Liebknecht-Str. 121 : http://pyside.org > 14482 Potsdam : GPG key -> 0xE7301150FB7BEE0E > phone +49 173 24 18 776 <tel:+49%20173%202418776> fax +49 (30) > 700143-0023 <tel:+49%2030%207001430023> > > _______________________________________________ > Python-Dev mailing list > [email protected] <mailto:[email protected]> > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: > https://mail.python.org/mailman/options/python-dev/brett%40python.org > > > > _______________________________________________ > Python-Dev mailing list > [email protected] > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: > https://mail.python.org/mailman/options/python-dev/tismer%40stackless.com > -- Christian Tismer-Sperling :^) [email protected] Software Consulting : http://www.stackless.com/ Karl-Liebknecht-Str. 121 : http://pyside.org 14482 Potsdam : GPG key -> 0xE7301150FB7BEE0E phone +49 173 24 18 776 fax +49 (30) 700143-0023
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