[Cython] Cython code producing different, incorrect results under Python 2.7 (not 3.x) under Cython 0.19

[Stefan Behnel]

Hi,

Josh Warner, 24.04.2013 08:06:
> Over in scikit-image we have traced an odd problem with a particular Cython
> file to the 0.19 update of Cython. From at least Cython 0.15.1 (probably
> earlier) through 0.18, `_mcp.pyx` in `skimage.graph` compiled and executed
> correctly, passing all package tests on both Python 2.7 and Python 3.
> 
> After 0.19 was released and the Travis builds began using it, we began
> getting 100% repeatable errors from the previously clean master branch
> (example of an otherwise clean Python
> 2.7<https://travis-ci.org/scikit-image/scikit-image/jobs/6545505>Travis
> build; the
> Python 3 build passed<https://travis-ci.org/scikit-image/scikit-image/builds/6545504>all
> tests). All of these errors/failures trace back to this Cython file.
> Oddly, the errors only happen on Python 2.7; our Python 3 Travis build
> passes.
> 
> We are discussing this issue in scikit-image Github Issue
> #534<https://github.com/scikit-image/scikit-image/issues/534>;
> feel free to join the discussion there.
> 
> The .pyx Cython file is located
> here<https://github.com/scikit-image/scikit-image/blob/master/skimage/graph/_mcp.pyx>and
> it has
> an associated .pxd file
> here<https://github.com/scikit-image/scikit-image/blob/master/skimage/graph/_mcp.pxd>.
> It should be noted the file compiles and executes without errors, but its
> output is now incorrect in Python 2.x.
> 
> In case the compiled results might be relevant, for your diffing pleasure here
> is the compiled .c file from Cython
> 0.18<https://gist.github.com/JDWarner/af4f8ea85dce356ce95c>which
> passes all tests on both Python 2.7 and Python 3.x, while here
> is the compiled .c file from Cython
> 0.19<https://gist.github.com/JDWarner/56d15b7a7527b8d4314e>which
> produces different, incorrect results in Python 2.7.
> 
> In the short term we are temporarily forcing Travis to use the 0.18 release
> of Cython, but that isn't a viable long term solution.
> 
> It's possible the error is on our end, but seeing as it worked with prior
> Cython releases we'd appreciate you taking a look.

Thanks for bringing this up. You could make it a little easier for us by
pointing us at the code that produces the incorrect results you are
experiencing. The set of failing tests seems to be quite small, but before
we start digging through your code, I'm sure you can provide pointers to
the relevant code snippets for a couple of these tests (i.e. the test code
itself and the major code parts that produce the results) much more quickly.

Stefan