New submission from Alexander Belopolsky <belopol...@users.sourceforge.net>:
Attached script, cycle.py 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:
pass
c = C()
cycle = set([c])
c.foo = 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 cycle.py
FAIILURE
..
$ python2 cycle.py
Traceback (most recent call last):
..
File ".../pickle.py", line 244, in memoize
assert id(obj) not in self.memo
AssertionError
If you run cycle.py 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 cycle.py dict
SUCCESS
..
After reporting success or failure, cycle.py, prints a disassembly of the
pickle stream which makes it clear what happens:
In case of dict, we see the following:
$ python3 cycle.py dict
SUCCESS
..
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 cycle.py
FAIILURE
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 c.foo
72: R REDUCE
.. another set object is constructed containing c
As a result, we have
cycle = {c}
c.foo = {c}
Instead of
c.foo = cycle
----------
assignee: belopolsky
components: Interpreter Core
files: cycle.py
messages: 110397
nosy: belopolsky
priority: normal
severity: normal
stage: needs patch
status: open
title: Cannot pickle self-referencing sets
type: behavior
versions: Python 2.7, Python 3.2
Added file: http://bugs.python.org/file18020/cycle.py
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue9269>
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