This may not come as a surprise to experienced Python programmers, but objects in Python can reference or "contain" themselves and/or their parent objects.
A trivial example, the self-containing list:
>>> a = 
Kind of a cool repr, at least. How else could you flatly represent this infinitely nested object? What happens when you take two of these and smash them together with an equals comparator?
>>> b = 
>>> a == b
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: maximum recursion depth exceeded in cmp
Like two mirrors pointed at one another, they compare back and forth forever, or at least for...