python - How can I get generators/iterators to evaluate as False when exhausted?

Question

Other empty objects in Python evaluate as False -- how can I get iterators/generators to do so as well?

Answer 1

Guido doesn't want generators and iterators to behave that way.

Objects are true by default. They can be false only if they define __len__ that returns zero or __nonzero__ that returns False (the latter is called __bool__ in Py3.x).

You can add one of those methods to a custom iterator, but it doesn't match Guido's intent. He rejected adding __len__ to iterators where the upcoming length is known. That is how we got __length_hint__ instead.

So, the only way to tell if an iterator is empty is to call next() on it and see if it raises StopIteration.

On ASPN, I believe there are some recipes using this technique for lookahead wrapper. If a value is fetched, it is saved-up the an upcoming next() call.