The link below defines __len__

https://docs.python.org/3/reference/datamodel.html?highlight=__len__#object.__len__

However, I was reading in the StackOverflow thread below that CPython does some validation to check that the return value of __len__ should be >= 0. Does this mean that len must return a value >= 0, in the RFC 2119 sense of the word?

https://stackoverflow.com/questions/42521449/how-does-python-ensure-the-return-value-of-len-is-an-integer-when-len-is-cal

There is probably validation code in len(), but the method itself can do whatever it wants if you call it directly. So the documentation is accurate in saying the method _should_ return >= 0 but that there's nothing strictly enforcing that in Python for that specific method when defined and called directly.

nosy: + brett.cannon
messages: + msg340851

resolution: not a bug
stage: resolved