[Python-Dev] PySequence_Check but no __len__

[Christian Tismer]

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 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    :^)   tismer at stackless.com
>     <mailto:tismer at stackless.com>
>     Software Consulting          :     http://www.stackless.com/
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> 
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-- 
Christian Tismer-Sperling    :^)   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  fax +49 (30) 700143-0023

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