Wednesday, September 23, 2015

Loopy references

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 = []
>>> a.append(a)
>>> 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 = []
>>> b.append(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...

>>> sys.getrecursionlimit()

... loops.