This comes (indirectly) from a user of my doctest.py, who noticed that sometimes tempfiles created by his docstring tests got cleaned up (via __del__), but other times not. Here's a hard-won self-contained program illustrating the true cause: class Critical: count = 0 def __init__(self): Critical.count = Critical.count + 1 self.id = Critical.count print "acquiring Critical", self.id def __del__(self): print "releasing Critical", self.id good = "temp = Critical()\n" bad = "def f(): pass\n" + good basedict = {"Critical": Critical} for test in good, bad, good: print "\nStarting test case:" print test exec compile(test, "<string>", "exec") in basedict.copy() And here's output: D:\Python>python misc\doccyc.py Starting test case: temp = Critical() acquiring Critical 1 releasing Critical 1 Starting test case: def f(): pass temp = Critical() acquiring Critical 2 Starting test case: temp = Critical() acquiring Critical 3 releasing Critical 3 D:\Python> That is, in the "bad" case, which differs from the "good" case merely in defining an unreferenced function, temp.__del__ not only doesn't get executed "when expected", it never gets executed at all. This appears to be due to a cycle between the function object and the anonymous dict passed to exec, causing the entire dict to become immortal, thus making "temp" immortal too. I can fiddle the doctest framework to manually nuke the temp dict it creates for execution context; the same kind of leak likely occurs in any exec'ed string that contains a function defn. For future reference, note that the finalizer in question belongs to an object not itself in a cycle, it's an object reachable only from a dead cycle. the-users-don't-stand-a-chance<wink>-ly y'rs - tim