How to pickle a namedtuple instance correctly

Question

I'm learning how to use pickle. I've created a namedtuple object, appended it to a list, and tried to pickle that list. However, I get the following error:

pickle.PicklingError: Can't pickle <class '__main__.P'>: it's not found as __main__.P

I found that if I ran the code without wrapping it inside a function, it works perfectly. Is there an extra step required to pickle an object when wrapped inside a function?

Here is my code:

from collections import namedtuple
import pickle

def pickle_test():
    P = namedtuple("P", "one two three four")
    my_list = []
    abe = P("abraham", "lincoln", "vampire", "hunter")
    my_list.append(abe)
    f = open('abe.pickle', 'w')
    pickle.dump(abe, f)
    f.close()

pickle_test()

Answer 1

Create the named tuple outside of the function:

from collections import namedtuple
import pickle

P = namedtuple("P", "one two three four")

def pickle_test():
    my_list = []
    abe = P("abraham", "lincoln", "vampire", "hunter")
    my_list.append(abe)
    f = open('abe.pickle', 'w')
    pickle.dump(abe, f)
    f.close()

pickle_test()

Now pickle can find it; it is a module global now. When unpickling, all the pickle module has to do is locate __main__.P again. In your version, P is a local, to the pickle_test() function, and that is not introspectable or importable.

It is important to remember that namedtuple() is a class factory; you give it parameters and it returns a class object for you to create instances from. pickle only stores the data contained in the instances, plus a string reference to the original class to reconstruct the instances again.