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Author belopolsky
Recipients belopolsky
Date 2010-07-15.23:01:38
SpamBayes Score 8.11767e-12
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Message-id <>
Attached script, demonstrates a simplification of the behavior reported by mike bayer in msg77200.

Essentially, the script attempts to pickle a set that contains a class instance that has an attribute referring back to the set:

class C:
c = C()
cycle = set([c]) = cycle

An attempt to pickle the *cycle* object triggers an assertion error in 2.7 or in 3.2 with disabled _pickle acceleration and produces a broken cycle in 3.2 or if cPickle is used instead of pickle in 2.7.

$ python3

$ python2
Traceback (most recent call last):
  File ".../", line 244, in memoize
    assert id(obj) not in self.memo

If you run with an argument, it uses a dict instead of set to create the cycle and shows that the cycles with dict can be pickled correctly:

$ python3 dict

After reporting success or failure,, prints a disassembly of the pickle stream which makes it clear what happens:

In case of dict, we see the following:

$ python3 dict
    2: }    EMPTY_DICT
    3: q    BINPUT     0
   26: X    BINUNICODE 'foo'
   36: h    BINGET     0
   38: s    SETITEM
   40: N    NONE
   41: s    SETITEM

An empty dict is created and saved in the memo.  Then a C object is built with foo attribute is set to the dict retrieved from the memo. Finally, the same dict is updated with (C object, None) key-value pair.  The result is the cycle identical to the one we built in python code.

The sets, however, are built differently.  There is no pickle opcode to add items to a set, so all set items must exist by the time set is built.  So here is what we see:  

$ python3
    2: c    GLOBAL     'builtins set'
   16: q    BINPUT     0
.. instead of empty set the constructor is saved in memo
   42: X    BINUNICODE 'foo'
   52: h    BINGET     0
   63: R    REDUCE
.. a set object containing c is constructed 
   66: s    SETITEM  
.. and assigned to
   72: R    REDUCE
.. another set object is constructed containing c
As a result, we have

cycle = {c} = {c}

Instead of = cycle
Date User Action Args
2010-07-15 23:01:43belopolskysetrecipients: + belopolsky
2010-07-15 23:01:42belopolskysetmessageid: <>
2010-07-15 23:01:40belopolskylinkissue9269 messages
2010-07-15 23:01:39belopolskycreate