As I understand it, by default, every Python object is “truthy”, except for a few, select cases:
- None
- False
- Zero
- Empty containers (including strings, tuples, sets, lists, dicts, …)
- Objects of custom-built classes, if they were explicitly built to override that default
I’m not familiar with any such custom classes. I suspect they’re fairly rare.
Iterators are not among the exceptions, so they’re “truthy”.
Reference: Truth Value Testing
Edit: To be fair, it isn’t necessarily a bad idea. C++ has the concept of “empty ranges”, which test false when the range is exhausted. This method of stopping is handy in situations where Python’s exception-based approach (raise StopIteration when done) is too slow or bulky.
But there are situations where determining “emptiness” can be expensive. For example, an iterator backed by a network socket, and a buffer. If the buffer is empty, then the iterator might be “empty”; but that could change in a microsecond, as data arrives from across the network. In this case, Python’s iterator model, which is more general, looks to me like a better fit.
By now, there’s on the order of a billion lines of Python code that work under the above rules. Changing the rules now is probably not an option.