[Python-Dev] PySequence_Check but no __len__

Terry Reedy tjreedy at udel.edu
Fri Jun 22 20:30:25 EDT 2018

On 6/22/2018 7:57 PM, Greg Ewing wrote:
> Terry Reedy wrote:
>> I am surprised that a C-API function calls something a 'sequence' 
>> without it having __len__.
> It's a bit strange that PySequence_Check exists at all.
> The principle of duck typing would suggest that one
> should be checking for the specific methods one needs.
> I suspect it's a holdover from very early Python, where
> the notion of a "sequence type" and a "mapping type"
> were more of a concrete thing. This is reflected in
> the existence of the tp_as_sequence and tp_as_mapping
> substructures. It was expected that a given type would
> either implement all the methods in one of those
> substructures or none of them, so shorcuts such as
> checking for just one method and assuming the others
> would exist made sense.
> But user-defined classes messed all that up, because
> it became possible to create a type that has __getitem__
> but not __len__, etc. It also made it impossible to
> distinguish reliably between a sequence and a mapping.
> So it seems to me that PySequence_Check and related
> functions are not very useful any more, since it's not
> possible for them to really do what they claim to do.

So one should not take them as defining what they appear to define.
In a sense, 'PySequence_Check' should be 'PySubscriptable_Check'.

Terry Jan Reedy

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