When you pickle an object that has some attributes which cannot be pickled it will fail with a generic error message like:
PicklingError: Can't pickle <type 'instancemethod'>: attribute lookup __builtin__.instancemethod failed
Is there any way to tell which attribute caused the exception? I am using Python 2.5.2.
Even though I understand in principle the root cause of the problem (e.g. in the above example having an instance method) it can still be very hard to exactly pinpoint it. In my case I already defined a custom __getstate__
method, but forgot about a critical attribute. This happened in a complicated structure of nested objects, so it took me a while to identify the bad attribute.
As requested, here is one simple example were pickle intentionally fails:
import cPickle as pickle
import new
class Test(object):
pass
def test_func(self):
pass
test = Test()
pickle.dumps(test)
print "now with instancemethod..."
test.test_meth = new.instancemethod(test_func, test)
pickle.dumps(test)
This is the output:
now with instancemethod...
Traceback (most recent call last):
File "/home/wilbert/develop/workspace/Playground/src/misc/picklefail.py", line 15, in <module>
pickle.dumps(test)
File "/home/wilbert/lib/python2.5/copy_reg.py", line 69, in _reduce_ex
raise TypeError, "can't pickle %s objects" % base.__name__
TypeError: can't pickle instancemethod objects
Unfortunately there is no hint that the attribute test_meth
causes the problem.
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