Quoting Raymond Hettinger <python@rcn.com>:
To help with get a comprehensive view when I look at this more closely tomorrow, can you try out variations on the theme with other mutables:
myset.update deque.extend dict.update dict.fromkeys array.extend
Returning to Tim's original infinite loop, the behaviour is interestingly variable. List and array go into the infinite loop. Deque and dictionary both detect that the loop variable has been mutated and throw a specific exception. Set throws the same exception as dictionary does (presumably, the main container inside 'set' is a dictionary) Details of behaviour: Python 2.4a3 (#16, Sep 21 2004, 17:33:57) [GCC 3.4.1 20040702 (Red Hat Linux 3.4.1-2)] on linux2 Type "help", "copyright", "credits" or "license" for more information. X>> x = [1] X>> x.extend(-y for y in x) Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 1, in <generator expression> KeyboardInterrupt X>> len(x) 73727215 X>> x = set([1]) X>> x set([1]) X>> x.update(-y for y in x) Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 1, in <generator expression> RuntimeError: dictionary changed size during iteration X>> x set([1, -1]) X>> from collections import deque X>> x = deque([1]) X>> x.extend(-y for y in x) Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 1, in <generator expression> RuntimeError: deque changed size during iteration X>> x deque([1, -1]) X>> from array import array X>> x = array('b', '1') X>> x.extend(-y for y in x) Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 1, in <generator expression> KeyboardInterrupt X>> len(x) 6327343 X>> x = dict.fromkeys([1]) X>> x {1: None} X>> x.update((-y, None) for y in x) Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 1, in <generator expression> RuntimeError: dictionary changed size during iteration X>> x {1: None, -1: None} X>> x.fromkeys(-y for y in x) {-1: None}