[Python-Dev] Bug in inspect module

[Ewan Mellor]

All,

I have a problem being reported (by Xen users) where inspect.stack() is
throwing IndexError.  I think that this is a bug in inspect.py -- findsource
generally throws IOError when it can't find a particular source file, but in
the case where it finds a source file, but that file is shorter than expected,
then findsource throws IndexError, and this is propagated all the way out of
inspect.stack().

I'm not sure why inspect.py is finding source files that don't match the code
being executed -- it seems to be dependent upon the install environment,
because it's not affecting most people.  That said, I think that it's
reasonable to cope with a too-short source file in the same way as we cope
with missing source files -- by throwing IOError from findsource and handling
that exception later.

Here's a patch.

Cheers,

Ewan Mellor, XenSource.

--- inspect.py.orig 2007-03-17 17:01:56.410399391 +
+++ inspect.py  2007-03-17 17:16:36.026005726 +
@@ -490,6 +490,8 @@
 if not hasattr(object, 'co_firstlineno'):
 raise IOError('could not find function definition')
 lnum = object.co_firstlineno - 1
+if len(lines) - 1 < lnum:
+raise IOError('source file is too short')
 pat = re.compile(r'^(\s*def\s)|(.*(? 0:
 if pat.match(lines[lnum]): break
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Re: [Python-Dev] Bug in inspect module

[Ewan Mellor]

On Sat, Mar 17, 2007 at 08:09:06PM -0500, Collin Winter wrote:

> On 3/17/07, Ewan Mellor <[EMAIL PROTECTED]> wrote:
> >I have a problem being reported (by Xen users) where inspect.stack() is
> >throwing IndexError.  I think that this is a bug in inspect.py -- 
> >findsource
> >generally throws IOError when it can't find a particular source file, but 
> >in
> >the case where it finds a source file, but that file is shorter than 
> >expected,
> >then findsource throws IndexError, and this is propagated all the way out 
> >of
> >inspect.stack().
> >
> >I'm not sure why inspect.py is finding source files that don't match the 
> >code
> >being executed -- it seems to be dependent upon the install environment,
> >because it's not affecting most people.  That said, I think that it's
> >reasonable to cope with a too-short source file in the same way as we cope
> >with missing source files -- by throwing IOError from findsource and 
> >handling
> >that exception later.
> 
> FYI: this has been reported at least once before: see bug #1628987
> (http://python.org/sf/1628987). The problem (in the bug report, at
> least) is that the source file changes between compilation and when
> findsource() is called, and findsource() picks up the modified
> version.

Thanks for that.

It's certainly a mismatch between the compiled code and the source file
subsequently found.  That said, I don't think that it only affects the case
when people are editing source files and not recompiling -- this isn't the
sort of thing that my users would be doing at this point.  I suspect that the
source file lookup is broken in some way, and so the wrong file ends up in the
line cache.  It's not affecting me, unfortunately, so I can't easily
investigate.

I see that you closed this bug as "Won't Fix".  My two line patch seems like a
reasonable workaround, and certainly more simple than the other proposals --
do you think it's worth dropping in?

Cheers,

Ewan.
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