# The problem may be intractable, given the current gc+weakref design.

import weakref
import gc

gc.set_debug(gc.DEBUG_STATS)

class C(object):
    def __init__(self, i):
        self.i = i
        self.loop = self

c0 = C(0)
# Move all the above into generation 2.
gc.collect()

c1 = C(1)
c1.keep_c0_alive = c0
del c0.loop # now only c1 keeps c0 alive

c2 = C(2)
c2wr = weakref.ref(c2) # no callback!

ouch = []
def callback(ignored):
    ouch[:] = [c2wr()]

# The callback gets associated with a wr on an object in generation 2.
c0wr = weakref.ref(c0, callback)

print "id(c0)", id(c0)
print "id(c1)", id(c1)
print "id(c2)", id(c2)

c0 = c1 = c2 = None

# What we've set up:  c0, c1, and c2 are all trash now.  c0 is in generation
# 2.  The only thing keeping it alive is that c1 points to it.  c1 and c2
# are in generation 0, and are in self-loops.  There's a global weakref
# to c2 (c2wr), but that weakref has no callback.  There's also a global
# weakref to c0 (c0wr), and that does have a callback, and that callback
# references c2 via c2wr().
#
#               c0 has a wr with callback, which references c2wr
#               ^
#               |
#               |     Generation 2 above dots
#. . . . . . . .|. . . . . . . . . . . . . . . . . . . . . . . .
#               |     Generation 0 below dots
#               |
#               |
#            ^->c1   ^->c2 has a wr but no callback
#            |  |    |  |
#            <--v    <--v
#
# So this is the nightmare:  when generation 0 gets collected, we see that
# c2 has a callback-free weakref, and c1 doesn't even have a weakref.
# Collecting generation 0 doesn't see c0 at all, abd c0 is the only object
# that has a weakref with a callback.  gc clears c1 and c2.  Clearing c1 has
# the side effect of dropping the refcount on c0 to 0, so c0 goes away
# (despite that it's in an older generation) and c2's wr callback triggers.
# That in turn materializes a reference to c2 via c2wr(), but c2 gets cleared
# anyway by gc.

i = 0
junk = []
stillhappy = True
while stillhappy:
    del ouch[:]
    junk.append([])  # this will eventually trigger gc
    i += 1
    for x in ouch:
        if x is not None:
            assert isinstance(x, C)
            if x.__dict__ == {}:
                stillhappy = False
                print
                print "*" * 20, "C instance with id", id(x), "hosed on iteration", i
                print

# Typical output:
#
# gc: collecting generation 2...
# gc: objects in each generation: 324 1825 0
# gc: done.
# id(c0) 10327312
# id(c1) 10327344
# id(c2) 10327376
# gc: collecting generation 0...
# gc: objects in each generation: 714 0 2149
# gc: done, 4 unreachable, 0 uncollectable.
#
# ******************** C instance with id 10327376 hosed on iteration 705
#
# gc: collecting generation 2...
# gc: objects in each generation: 0 712 2141
# gc: done.