[Python-Dev] PySequence_Check but no len (original) (raw)

Christian Tismer tismer at stackless.com
Fri Jun 22 07:17:11 EDT 2018

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 Thu, 21 Jun 2018 at 08:50 Christian Tismer <tismer at stackless.com_ _<mailto:tismer at stackless.com>> 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 PySequenceCheck() already is true and does not care.

Which matches https://docs.python.org/3/c-api/sequence.html#c.PySequenceCheck . From Objects/abstract.c: int PySequenceCheck(PyObject *s) { if (PyDictCheck(s)) return 0; return s != NULL && s->obtype->tpassequence && s->obtype->tpassequence->sqitem != 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 PySequenceCheck() 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    :^)   tismer at stackless.com <mailto:tismer at stackless.com> 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

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