How to pickle a namedtuple instance correctly

Create the named tuple outside of the function:

from collections import namedtupleimport pickleP = 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.

After I added my question as a comment to the main answer I found a way to solve the problem of making a dynamically created namedtuple pickle-able. This is required in my case because I'm figuring out its fields only at runtime (after a DB query).

All I do is monkey patch the namedtuple by effectively moving it to the __main__ module:

def _CreateNamedOnMain(*args):    import __main__    namedtupleClass = collections.namedtuple(*args)    setattr(__main__, namedtupleClass.__name__, namedtupleClass)    namedtupleClass.__module__ = "__main__"    return namedtupleClass

Mind that the namedtuple name (which is provided by args) might overwrite another member in __main__ if you're not careful.

I found this answer in another thread. This is all about the naming of the named tuple. This worked for me:

group_t =            namedtuple('group_t', 'field1, field2')  # this will workmismatched_group_t = namedtuple('group_t', 'field1, field2')  # this will throw the error