I think the Python docs can sometimes be a little imprecise about what "sequence" means. E.g., the actual code that dir() uses just calls PySequence_List on the result of __dir__(). PySequence_List is another good example of imprecise use of "sequence"; it's perfectly happy to take in an iterable. So everything always works with Iterable at runtime.

Indeed. Another alternative is to fix the Python docs to match the implementation. Maybe this is something we should try first? In any case, I think it's important to be precise here, for cases, where someone uses __dir__ directly.

I can find one use of __dir__ in the stdlib where it returns a non-sequence:

It's a module-level use of __dir__ rather than a a method. (But then, ModuleType is a subclass of object, just like any other class. And the datamodel docs for __dir__ as a module-level function also state that it "should return a sequence".)

I suggest to move this issue to python/cpython and suggest to change the docs. Returning a set from __dir__ makes a lot of sense to me.

I've transferred this issue to CPython and made some light edits to a few comments by participants in the discussion, so that the issue thread will make sense to CPython triagers reading it for the first time.

My 2c:

• PySequence_List already says: Return a list object with the same contents as the sequence or iterable *o*, which I guess is fine
• I've updated __dir__ docs to mention iterable instead of sequence, because it is how runtime behaves (correctly)