How do I fix TypeError: object is not iterable with my Python tree iterator?

[Timothy_R]

I'm trying to understand how to properly implement a custom iterator for a tree structure in Python, but I keep getting a `TypeError` saying the object is not iterable. My `__next__` method seems correct, but I think the issue might be with how I'm resetting the traversal state each time the iteration is supposed to start fresh.

[OliverBJ]

That TypeError almost always means Python didn't get an actual iterator object. __next__ alone does not make something iterable. You need __iter__ that returns an iterator. If you want fresh traversal every time, have __iter__ return a new iterator object instead of returning self and reusing internal state. The clean way is to put the traversal logic in a small separate class and return that from __iter__.

[Justin22]

I tried a similar thing last month. I kept the stack on the tree object and tried to reset it in __iter__, but then a second for would fail. I ended up moving the traversal into its own iterator class that stores its own stack and takes the root when created. Then tree.__iter__ just creates that and returns it.

[Jack10]

I wonder if the real issue is that you are mutating the tree during iteration. If you reset the state in __iter__ but someone holds a reference and calls next later, you might end up with surprises. Maybe print statements helped you see when the stack gets cleared.

[Paisley_J]

One quick check you can do: when you run for x in tree, print the type you get from iter(tree) and inspect its attributes. If __iter__ returns self, that might be the problem. Are you sure __iter__ is being called during the for loop?