["python","import","circular-dependency"]
Why do circular imports seemingly work further up in the call stack but then raise an ImportError further down?

I'm getting this error Traceback (most recent call last): File "/Users/alex/dev/runswift/utils/sim2014/simulator.py", line 3, in <module> from world import World File "/Users/alex/dev/runswift/utils/sim2014/world.py", line 2, in <module> from entities.field import Field File "/Users/alex/dev/runswift/utils/sim2014/entities/field.py", line 2, in <module> from entities.goal import Goal File "/Users/alex/dev/runswift/utils/sim2014/entities/goal.py", line 2, in <module> from entities.post import Post File "/Users/alex/dev/runswift/utils/sim2014/entities/post.py", line 4, in <module> from physics import PostBody File "/Users/alex/dev/runswift/utils/sim2014/physics.py", line 21, in <module> from entities.post import Post ImportError: cannot import name Post and you can see that I use the same import statement further up and it works. Is there some unwritten rule about circular importing? How do I use the same class further down the call stack? See also What happens when using mutual or circular (cyclic) imports in Python? for a general overview of what is allowed and what causes a problem WRT circular imports. See What can I do about "ImportError: Cannot import name X" or "AttributeError: ... (most likely due to a circular import)"? for techniques for resolving and avoiding circular dependencies.
I think the answer by jpmc26, while by no means wrong, comes down too heavily on circular imports. They can work just fine, if you set them up correctly. The easiest way to do so is to use import my_module syntax, rather than from my_module import some_object. The former will almost always work, even if my_module included imports us back. The latter only works if my_object is already defined in my_module, which in a circular import may not be the case. To be specific to your case: Try changing entities/post.py to do import physics and then refer to physics.PostBody rather than just PostBody directly. Similarly, change physics.py to do import entities.post and then use entities.post.Post rather than just Post.