[QGIS-Developer] tricontour crashing in QGIS3

[Chris Crook]

I'm looking for help debugging or fixing a problem in matplotlib tricontour.  I am using this in a QGIS plugin (www.qgis.org).  The call to tricontour causes QGIS to crash.   Running python3 on ubuntu 16.04 (matplotlib 1.5.1) and 17.10 (matplotlib 2.0.0).

Running in the debugger gives the following result

Thread 1 "qgis.bin" received signal SIGSEGV, Segmentation fault.
0x00007fff4f7ca098 in qh_initstatistics ()
   from /usr/lib/python3/dist-packages/matplotlib/_qhull.cpython-35m-x86_64-linux-gnu.so
#0  0x00007fff4f7ca098 in qh_initstatistics ()
   from /usr/lib/python3/dist-packages/matplotlib/_qhull.cpython-35m-x86_64-linux-gnu.so
#1  0x00007fff4f7c6f82 in qh_initqhull_start ()
   from /usr/lib/python3/dist-packages/matplotlib/_qhull.cpython-35m-x86_64-linux-gnu.so
#2  0x00007fff4f7d12bc in qh_new_qhull ()
   from /usr/lib/python3/dist-packages/matplotlib/_qhull.cpython-35m-x86_64-linux-gnu.so
#3  0x00007fff4f7a3821 in ?? ()
   from /usr/lib/python3/dist-packages/matplotlib/_qhull.cpython-35m-x86_64-linux-gnu.so
#4  0x00007fff5d977039 in PyCFunction_Call ()
   from /usr/lib/x86_64-linux-gnu/libpython3.5m.so.1.0
#5  0x00007fff5da831b5 in PyEval_EvalFrameEx ()
   from /usr/lib/x86_64-linux-gnu/libpython3.5m.so.1.0
#6  0x00007fff5db13cac in ?? ()
   from /usr/lib/x86_64-linux-gnu/libpython3.5m.so.1.0
#7  0x00007fff5db13d83 in PyEval_EvalCodeEx ()
   from /usr/lib/x86_64-linux-gnu/libpython3.5m.so.1.0
#8  0x00007fff5d99bad8 in ?? ()
   from /usr/lib/x86_64-linux-gnu/libpython3.5m.so.1.0
....

The python code is

                np.save('/home/chris/temp/data.x',x)
                np.save('/home/chris/temp/data.y',y)
                np.save('/home/chris/temp/data.z',z)
                np.save('/home/chris/temp/data.l',levels)
                print("Extend",repr(extend))
                cs = plt.tricontour(x, y, z, levels, extend=extend)

I've called matplotlib from a simple python script using the saved data (np.save) in a simply python script and it runs without problem, so the problem appears to arise running within the QGIS3 python environment.

Can you suggest any reason why it might be different running from within QGIS and how I can work around/fix this!

Thanks
Chris Crook

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