[Python-Dev] Strange bug only happens with Python 2.2

Michael Hudson mwh@python.net
Fri, 27 Sep 2002 16:39:42 +0100 (BST)

On Fri, 27 Sep 2002, Gerhard H=E4ring wrote:

> This is somewhat off-topic, but I'm hoping maybe someone can give a hint
> why this only happens on Python 2.2.1.

Guessing, but the (Jeremy's?) changes I recently backported to=20
classobject.c on the release22-maint branch might relate to this.

Can you try with a 222 build?

> Ok, here's the story:
> I've had a bug report against our pyPgSQL database interface package that
> retrieving Large Objects doesn't work with Python 2.2.1. The reproducible
> traceback we get is:
> Traceback (most recent call last):
>   File "p.py", line 20, in ?
>     res =3D cs.fetchone()
>   File "pyPgSQL/PgSQL.py", line 2672, in fetchone
>     return self.__fetchOneRow()
>   File "pyPgSQL/PgSQL.py", line 2281, in __fetchOneRow
>     for _i in range(self.res.nfields):
> AttributeError: 'str' object has no attribute '__bases__'
> This traceback is quite obviously bogus, as self.res.nfields is a Python
> int and no strings are involved here whatsoever. After some debugging, I
> found that something very strange happens in a function call that
> happens in this for loop. Inside the for loop, a function typecast is
> called, which has this code within:
> if isinstance(value, PgBytea) or type(value) is PgLargeObjectType:
> This code is causing the problems which result in the bogus traceback
> later on.

So something's setting an exception and not letting the interpreter know.

> Now in my case, 'value' is of type PgLargeObjectType, which is a custom
> type from our extension module. PgBytea is a Python class.
> Now comes the first very strange observation: Swapping the checks, so
> that the 'type(value) is PgLargeObjectType' check comes first makes the
> problem go away. So my conclusion is that there's some problem with
> isinstance and my custom extension type.
> The second strange thing is that this only happens on Python 2.2.1
> (Linux, FreeBSD, Windows), but _not_ on Python 2.1.3 or Python 2.3-CVS.

This is no surprise.

> Oh, the problem isn't tied to isinstance(value, PgBytea). Any isinstance
> check causes it later on.


> Of course I'm suspecting that there's some problem with the extension
> type. Looks like some internal interpreter data gets corrupted. No idea
> how to debug that, too.
> Does anybody have any tips where to look or how to debug this further?

Try a release22-maint build?