If I have a script that defines a class:
script = """
class myClass:
def __init__(self):
self.name = 'apple'
self.color = 'green'
"""
and then exec this script in its own namespace dict:
NS = {}
exec script in NS
and then create an instance of the class and pickle it:
a = NS['myClass']()
import pickle
save = pickle.dumps(a)
Now if I try to unpickle it:
load = pickle.loads(save)
I get the error
AttributeError: 'module' object has no attribute 'myClass'
I gather that this doesn't work because python doesn't know where to find myClass in order to rebuild the object. But myClass does exist in the NS dict. Is there a way to tell pickle where to find the class for the object it is loading?
You can actually go one step further, and have the object reconstruct itself into whatever type you want.
import pickle
import copy_reg
class myClass(object):
def __init__(self):
self.apple = 'banana'
class otherclass(object):
def __init__(self):
self.apple = 'existential woe'
def pickle_an_object(o):
print "pickling %s" % str(o)
return otherclass, (o.apple,)
copy_reg.pickle(myClass, pickle_an_object)
foo = myClass()
s = pickle.dumps(foo)
del myClass
del otherclass
class otherclass(object):
def __init__(self, appletype):
self.apple = 'not %s' % appletype
o2 = pickle.loads(s)
print o2.apple
The basic idea is that you pack your class into a "trojan horse" of sorts, where its reconstruction causes an instantiation of a different class from what it originally was.
It does not matter what the otherclass
on the pickling side contains. All that matters is that it exist at the same module path as the "destination" class - pickle
is just putting a string representation of the module name into the serialized stream.
So, to break down what's happening in the above code in detail:
myClass
. This can be done via copy_reg
or the __reduce_ex__
function. otherclass
" (which is a dummy. You do not need the "real" contents of otherclass
on the pickling side, because all that goes into the pickle is the module/class name). otherclass
exists. otherclass
is instantiated with the data from the tuple returned by the custom pickling function. Python can be pretty powerful!
I discovered a solution this. It seems the problem is executing code in a dict prevents python from figuring out where the class is defined. The solution is to create an empty module, execute the code in the module, and then add the module to sys.modules so python knows about it.
script = """
class myClass:
def __init__(self):
self.name = 'apple'
self.color = 'green'
"""
import imp, sys
moduleName = 'custom'
module = imp.new_module(moduleName)
exec script in module.__dict__
sys.modules[moduleName] = module
Now it is possible to pickle and unpickle an instance of the class:
import pickle
a = module.myClass()
s = pickle.dumps(a)
b = pickle.loads(s)
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