From https://faster-cpython.readthedocs.io/pypy.html https://faster-cpython.readthedocs.io/pypy.html (emphasis mine): > - Bad support of the Python C API: PyPy

From https://faster-cpython.readthedocs.io/pypy.html https://faster-cpython.readthedocs.io/pypy.html (emphasis mine): > - Bad support of the Python C API: PyPy was written from scratch and uses different memory structures for objects. The cpyext module emulates the Python C API but it’s slow. > - New features are first developed in CPython. In january 2016, PyPy only supports Python 2.7 and 3.2, whereas CPython is at the version 3.5. It’s hard to have a single code base for Python 2.7 and 3.2, Python 3.3 reintroduced u'...' syntax for example. > - Not all modules are compatible with PyPy: see PyPy Compatibility Wiki. For example, numpy is not compatible with PyPy, but there is a project under development: pypy/numy. PyGTK, PyQt, PySide and wxPython libraries are not compatible with PyPy; these libraries heavily depend on the Python C API. GUI applications (ex: gajim, meld) using these libraries don’t work on PyPy :-( Hopefully, a lot of popular modules are compatible with PyPy (ex: Django, Twisted). > - PyPy is slower than CPython on some use cases. Django: “PyPy runs the templating engine faster than CPython, but so far DB access is slower for some drivers.”