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According to 
https://bugs.edge.launchpad.net/ubuntu/+source/boost1.38/+bug/457688 , 
Python 2.6.3 stopped working for something that Python 2.6.2 worked for, 
involving Boost.
Andrew Mitchell looked at the Python 2.6.3 release notes, saw 
http://bugs.python.org/issue5890 which was fixed by 
http://svn.python.org/view/python/branches/release26-
maint/Objects/descrobject.c?r1=71756&r2=72302&pathrev=72302 . Could this 
patch be causing this problem?

I think this is caused by the fix for bug 5890 and isn't a regression.

Apparently this affects more than just Boost. lifeless (Robert Collins)
tells me that "__doc__ becoming readonly buggered some code of ours in
bzr too, IIRC".
He thinks it was in pure Python (i.e. not extension code). Does anybody
have a pure Python reproducible example?

Robert indicated later that this probably did /not/ affect Bazaar.

I added some tests in test_property in my checkout and the __doc__
property is not, as far as I can tell, set read-only by the issue 5890 fix.
The error message referenced in the report is:
AttributeError: 'Boost.Python.StaticProperty' object attribute '__doc__'
is read-only
If Boost.Python.StaticProperty is a subclass of Property, which seems
likely, then the 5890 fix could well be triggering this error, since if
a property created with a subclass is given a docstring, the fix creates
a __doc__ attribute in the subclass's dict. If the subclass dict is
read only, the above error would be triggered.
I don't know enough about extension types to know if the property code
should be special casing extension types, but if it should, then IMO
that would qualify 5890 as a regression. Otherwise I don't think it is,
though we could still consider whether making a change to reduce user
pain is worthwhile. (To be worthwhile it would have to be likely to
reach the users before the Boost fix does...apparently Boos has a fix in
their SVN for this problem.)
I'm guessing that this is a problem only with extension types, not with
pure Python code.

That should have been 'instance's dict', not the subclass dict.

Thanks David, that's what I suspect too (that's its a problem with
extension types). Unless we get more information, I'm not inclined to
hold up the 2.6.4 release for this.

I'm not sure I understand, how do you create a "read-only instance dict"?

I'm not sure you do. I have no idea how Boost works, but the fact that
removing a Boost 'read-only' declaration circumvents the problem in at
least some cases argues that Boost is setting _something_ read-only.

After discussion on python-dev, this will not block 2.6.4

See my comment on issue #5890. I attached a patch to solve the
Boost.Python issue.

Since boost has changed their code and no one else has reported a problem and 2.6 is now in bug fix only mode, I'm going to close this as out of date (sorry I overlooked it for 2.6.5). 
If anyone disagrees, let me know what we should change and why in 2.7.