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)
with open('abe.pickle', 'wb') as f:
pickle.dump(abe, f)
pickle_test()
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)
with open('abe.pickle', 'wb') as f:
pickle.dump(abe, f)
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.
Note that pickle stores just the module and the class name, as taken from the class's __name__ attribute. Make sure that the first argument of the namedtuple() call matches the global variable you are assigning to; P.__name__ must be "P"!
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.