Error subclassing datetime.date and pickling

[kyosohma at gmail.com]

On Aug 2, 11:02 am, Mike Rooney <m... at qvii.com> wrote:
> Hi everyone, this is my first post to this list. I am trying to create a
> subclass of datetime.date and pickle it, but I get errors on loading it
> back. I have created a VERY simple demo of this:
>
> import datetime
>
> class MyDate(datetime.date):
>     """
>     This should be pickleable.
>
>     >>> md = MyDate(2007, 1, 6)
>     >>> import pickle
>     >>> pickle.dump(md, open("test.pickle", 'w'))
>     >>> mdp = pickle.load(open("test.pickle"))
>     >>> import os; os.remove("test.pickle")
>     """
>     def __init__(self, y, m, d):
>         datetime.date.__init__(self, y, m, d)
>
> if __name__ == "__main__":
>     import doctest
>     doctest.testmod()
>
> The traceback that occurs is:
>
>     Traceback (most recent call last):
>       File "C:\Python25\lib\doctest.py", line 1212, in __run
>         compileflags, 1) in test.globs
>       File "<doctest __main__.MyDate[3]>", line 1, in <module>
>         mdp = pickle.load(open("test.pickle"))
>       File "C:\Python25\lib\pickle.py", line 1370, in load
>         return Unpickler(file).load()
>       File "C:\Python25\lib\pickle.py", line 858, in load
>         dispatch[key](self)
>       File "C:\Python25\lib\pickle.py", line 1133, in load_red
>         value = func(*args)
>     TypeError: __init__() takes exactly 4 arguments (2 given)
>
> I have found a few relating issues:
>  -https://sourceforge.net/tracker/?func=detail&atid=105470&aid=952807&g...
>  -http://sourceforge.net/tracker/index.php?func=detail&aid=720908&group...
>
> But these are both rather old and are marked as fixed. I am running
> Python 2.5.1 on Windows XP SP2. Any help here would be greatly appreciated!
>
> - Mike

I've never used pickle or shelve in my applications...yet. I tried
separating out the class into its own module and importing it, as
pickling classes is supposed to be tricky. I looked through all my
Python books and what few examples I found looked just like your code.

I did find this article which might be helpful to you more than it was
to me: http://www.ibm.com/developerworks/library/l-pypers.html

>From what I can tell, when your object gets unpickled it only passes 2
arguments to the class instead of the 4 that were supposed to be
pickled. If you change the number of arguments that the class accepts
and re-pickle it, you'll see that the unpickle object traceback
corresponds.

It's very weird. Sorry I didn't have some slam-bang awesome idea.

Mike