In article <4df669ea$0$49182$e4fe5...@news.xs4all.nl>
Irmen de Jong <irmen.nos...@xs4all.nl> wrote:
>I've pasted my test code below. It works fine if 'substitute' is True,
>but as soon as it is set to False, it is supposed to call the original
>__reduce__ method of the base class. However, that seems to crash
>because of infinite recursion on Jython and IronPython and I don't
>know why. It works fine in CPython and Pypy.
In this particular case (no fancy inheritance going on), the base
__reduce__ method would be object.__reduce__. Perhaps in those
implementations, object.__reduce__ goes back to TestClass.__reduce__,
rather than being appropriately magic.
>I wonder if my understanding of __reduce__ is wrong, or that I've
>hit a bug in IronPython and Jython? Do I need to do something with
>__reduce_ex__ as well?
You should not *need* to; __reduce_ex__ is just there so that you
can do something different for different versions of the pickle
protocol (I believe).
Nonetheless, there is something at least slightly suspicious here:
>import pickle
>
>class Substitute(object):
> def __init__(self, name):
> self.name=name
> def getname(self):
> return self.name
>
>class TestClass(object):
> def __init__(self, name):
> self.name=name
> self.substitute=True
> def getname(self):
> return self.name
> def __reduce__(self):
> if self.substitute:
> return Substitute, ("SUBSTITUTED:"+self.name,)
> else:
> # call the original __reduce__ from the base class
> return super(TestClass, self).__reduce__() # crashes on
>ironpython/jython
[snip]
In general, the way __reduce__ is written in other class implementations
(as distributed with Python2.5 at least) boils down to the very
simple:
def __reduce__(self):
return self.__class__, (arg, um, ents)
For instance, consider a class with a piece that looks like this:
def __init__(self, name, value):
self.name = name
self.value = value
self.giant_cached_state = None
def make_parrot_move(self):
if self.giant_cached_state is None:
self._do_lots_of_computation()
return self._quickstuff_using_cache()
Here, the Full Internal State is fairly long but the part that
needs to be saved (or, for copy operations, copied -- but you can
override this with __copy__ and __deepcopy__ members, if copying
the cached state is a good idea) is quite short. Pickled instances
need only save the name and value, not any of the computed cached
stuff (if present). So:
def __reduce__(self):
return self.__class__, (name, value)
If you define this (and no __copy__ and no __deepcopy__), the
pickler will save the name and value and call __init__ with the
name and value arguments. The copy.copy and copy.deepcopy operations
will also call __init__ with these arguments (unless you add
__copy__(self) and __deepcopy__(self) functions).
So, it seems like in this case, you would want:
def __reduce__(self):
if self.substitute:
return Substitute, ("SUBSTITUTED:"+self.name,)
else:
return self.__class__, (self.name,)
or if you want to be paranoid and only do a Substitute if
self.__class__ is your own class:
if type(self) == TestClass and self.substitute:
return Substitute, ("SUBSTITUTED:"+self.name,)
else:
return self.__class__, (self.name,)
In CPython, if I import your code (saved in foo.py):
>>> x = foo.TestClass("janet")
>>> x
<foo.TestClass object at 0x66290>
>>> x.name
'janet'
>>> x.__reduce__()
(<class 'foo.Substitute'>, ('SUBSTITUTED:janet',))
>>> x.substitute=False
>>> x.__reduce__()
(<function _reconstructor at 0x70bf0>, (<class 'foo.TestClass'>, <type
'object'>, None), {'name': 'janet', 'substitute': False})
which is of course the same as:
>>> object.__reduce__(x)
(<function _reconstructor at 0x70bf0>, (<class 'foo.TestClass'>, <type
'object'>, None), {'name': 'janet', 'substitute': False})
which means that CPython's object.__reduce__() uses a "smart" fallback
reconstructor. Presumably IronPython and Jython lack this.
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
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