copy corrupts BTrees.Length.Length instances

[Albertas Agejevas]

Copying BTrees.Length.Length fails when the stored length value is 0:

>>> from BTrees.Length import Length
>>> import copy
>>> l = Length()
>>> print l()
0
>>> c = copy.deepcopy(l)
>>> print c()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/alga/src/eggs/ZODB3-3.9.4-py2.5-linux-i686.egg/BTrees/Length.py", line 58, in __call__
    return self.value
AttributeError: 'Length' object has no attribute 'value'

This failure occurs because Length returns its stored value as the result of __getstate__, and the copy module's _reconstruct function uses the state value as a condition. It is a bug in copy (pickling/unpickling works fine), but it makes sense to add a class attribute value = 0 as a workaround until the proper fix is propagated in Python.

[Albertas Agejevas (alga)]

This bug is reported on the Python bugtracker as http://bugs.python.org/issue7848 .

[Marius Gedminas (mgedmin)]

The Official Python Documentation states

   Note: For new-style classes, if __getstate__() returns a false value, the __setstate__() method will not be called.

  -- http://docs.python.org/library/pickle.html#object.__setstate__

It would appear that copy.deepcopy() is working as per spec, and it's just a happy accident that pickling BTrees.Length.Length() instances with the value of 0 works.

The fix should be trivial: add a class attribute 'value = 0' to BTrees.Length.Length.

[Albertas Agejevas (alga)]

Fix committed in trunk rev 108750.

[Tres Seaver (tseaver)]

Fix released in ZODB 3.10.0a2 (not noted in changelog at time of release).