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Beware of Cyclic Data Structures

We actually encountered this concept in a prior exercise: if a collection object contains a reference to itself, it’s called a cyclic object. Python prints a [...] whenever it detects a cycle in the object, rather than getting stuck in an infinite loop:

>>> L = ['grail']                # Append reference to same object
>>> L.append(L)                  # Generates cycle in object: [...]
>>> L
['grail', [...]]

Besides understanding that the three dots in square brackets represent a cycle in the object, this case is worth knowing about because it can lead to gotchas—cyclic structures may cause code of your own to fall into unexpected loops if you don’t anticipate them. For instance, some programs keep a list or dictionary of already visited items and check it to determine whether they’re in a cycle. See the solutions to the in Appendix B for more on this problem, and check out the reloadall.py program in Chapter 24 for a solution.

Don’t use cyclic references unless you really need to. There are good reasons to create cycles, but unless you have code that knows how to handle them, you probably won’t want to make your objects reference themselves very often in practice.