We have implemented the context manager API for connections and cursors in our mxODBC module and this works fine in Python 2.6.

In Python 2.7 we get the following error:

Traceback (most recent call last):
  File "context-manager.py", line 6, in <module>
    with db.cursor() as cursor:
AttributeError: __exit__

Here's the code snippet:

import mx.ODBC.unixODBC as ODBC
connectionString = '...'
db = ODBC.DriverConnect(connectionString)
with db.cursor() as cursor:
    print cursor

The mxODBC cursor is not an instance, it's implemented as Python type in C. It implements the tp_getattr slot, but not the tp_desc_get slot.

Looking at the apparently new API _PyObject_LookupSpecial(), this does not appear to support the tp_getattr slot and goes straight for the tp_desc_get slot.

After some experiments, it turned out that by simply filling in the tp_methods slot, the problem went away.

Still, the change to use _PyObject_LookupSpecial() appears to have missed the (older) use case where you don't define tp_members, but instead implement method lookup as part of the tp_getattr slot.

Not sure whether this is worth fixing. I just filed this report so that others running into the same problem can find it.

The change should also be documented here, http://docs.python.org/2.7/whatsnew/2.7.html#porting-to-python-2-7

On 21.02.2013 17:36, Marc-Andre Lemburg wrote:
> After some experiments, it turned out that by simply filling in the tp_methods slot, the problem went away.
> 
> Still, the change to use _PyObject_LookupSpecial() appears to have missed the (older) use case where you don't define tp_members, but instead implement method lookup as part of the tp_getattr slot.

Sorry: *tp_methods*, not tp_members.

This is a property of all special methods not just __enter__ and __exit__.

As 2.7 has reached EOL, I'm going to go ahead and close the issue.