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.
Generally you can pickle any object if you can pickle every attribute of that object. Classes, functions, and methods cannot be pickled -- if you pickle an object, the object's class is not pickled, just a string that identifies what class it belongs to.
The pickle module also consists of dumps() function that returns a string representation of pickled data.
Pickle in Python is primarily used in serializing and deserializing a Python object structure. In other words, it's the process of converting a Python object into a byte stream to store it in a file/database, maintain program state across sessions, or transport data over the network.
load() – This function is used to read a pickled object representation from the open file object file and return the reconstituted object hierarchy specified.
You could file a bug against Python for not including more helpful error messages. In the meantime, modify the _reduce_ex()
function in copy_reg.py
.
if base is self.__class__: print self # new raise TypeError, "can't pickle %s objects" % base.__name__
Output:
<bound method ?.test_func of <__main__.Test object at 0xb7f4230c>> Traceback (most recent call last): File "nopickle.py", line 14, in ? pickle.dumps(test) File "/usr/lib/python2.4/copy_reg.py", line 69, in _reduce_ex raise TypeError, "can't pickle %s objects" % base.__name__ TypeError: can't pickle instancemethod objects
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