can't pickle instancemethod objects - pickle

Question

I met a problem of pickle, Code is that:
import cPickle
class A(object):
def __init__(self):
self.a = 1
def methoda(self):
print(self.a)
class B(object):
def __init__(self):
self.b = 2
a = A()
self.b_a = a.methoda
def methodb(self):
print(self.b)
if __name__ == '__main__':
b = B()
with open('best_model1.pkl', 'w') as f:
cPickle.dump(b, f)
Error is that:
File "/usr/lib/python2.7/copy_reg.py", line 70, in _reduce_ex
raise TypeError, "can't pickle %s objects" % base.name TypeError: can't pickle instancemethod objects

Answer 1

You can if you use dill instead of cPickle.
>>> import dill
>>>
>>> class A(object):
... def __init__(self):
... self.a = 1
... def methods(self):
... print(self.a)
...
>>>
>>> class B(object):
... def __init__(self):
... self.b = 2
... a = A()
... self.b_a = a.methods
... def methodb(self):
... print(self.b)
...
>>> b = B()
>>> b_ = dill.dumps(b)
>>> _b = dill.loads(b_)
>>> _b.methodb()
2
>>>
Also see:
Can't pickle <type 'instancemethod'> when using python's multiprocessing Pool.map()

Answer 2

Also, when dill is installed pickle will work but as usual not cPickle.
import cPickle, pickle
class A(object):
def __init__(self):
self.a = 1
def methoda(self):
print(self.a)
class B(object):
def __init__(self):
self.b = 2
a = A()
self.b_a = a.methoda
def methodb(self):
print(self.b)
# try using cPickle
try:
c = cPickle.dumps(b)
d = cPickle.loads(c)
except Exception as err:
print('Unable to use cPickle (%s)'%err)
else:
print('Using cPickle was successful')
print(b)
print(d)
# try using pickle
try:
c = pickle.dumps(b)
d = pickle.loads(c)
except Exception as err:
print('Unable to use pickle (%s)'%err)
else:
print('Using pickle was successful')
print(b)
print(d)
>>> Unable to use cPickle (can't pickle instancemethod objects)
>>> Using pickle was successful
>>> <__main__.B object at 0x10e9b84d0>
>>> <__main__.B object at 0x13df07190>
for whatever reason, cPickle is not simply a C version of pickle 100 times faster but there are some differences