Incompatibility between pickled node attributes py2<>py3

[tomkooij]

Issue #368 contains a lot of useful background info. Most of the problems reported there were fixed (upstream), but a related issue still remains:

When porting code from py2 to 3 that stores classes in node attributes, I ran into unpickling errors, caused by incompatibility between python 2 and 3 pickles. (Although they are suppossed to be compatible).

Most of these problems were fixed in numpy-1.9 (see also #368) but some classes still fail to be unpickled correctly on python 3. It is easiest demonstrated with datetime.datetime objects (see code below).

The problem is non-ascii characters in the pickled string. Pickle throws an UnicodeDecodeError on these malformed pickled strings in python 3, so it should be possible to fix. I have a fix that I need to test.

"""
write datetime object to node attribute on py2
read datetime from node attribute on py3
"""
import sys
import tables
from datetime import datetime

FILENAME = 'py2pickled.h5'

if sys.version_info[0] == 2:
    with tables.open_file(FILENAME, 'w') as data:
        data.set_node_attr('/', 'datetime', datetime.now())

if sys.version_info[0] > 2:
    with tables.open_file(FILENAME, 'r') as data:
        print('datetime = ', data.get_node_attr('/', 'datetime'))

[tomkooij]

Fixed in #561