Robert, please look at this line: https://github.com/cython/cython/blob/0.22.x/Cython/Utility/TypeConversion.c... You are still using a PyLong internal detail to test for signedness. I'm getting a strange error with PyPy3 that might be related to this, see this traceback: https://travis-ci.org/mpi4py/mpi4py/jobs/53793609#L5650 The code generating this traceback is linked below, the__setitem__method of the variable "m" is coded to only accept "usigned char" values. https://bitbucket.org/mpi4py/mpi4py/src/86e4235ee2ec31832031ce020421810c51d4... Any suggestions about how to check signedness in Python 3 (CPython and PyPy) without using Py_SIZE() ? -- Lisandro Dalcin ============ Research Scientist Computer, Electrical and Mathematical Sciences & Engineering (CEMSE) Numerical Porous Media Center (NumPor) King Abdullah University of Science and Technology (KAUST) http://numpor.kaust.edu.sa/ 4700 King Abdullah University of Science and Technology al-Khawarizmi Bldg (Bldg 1), Office # 4332 Thuwal 23955-6900, Kingdom of Saudi Arabia http://www.kaust.edu.sa Office Phone: +966 12 808-0459