TensorFlow and App Engine are both released by Google which prefers Py2 because of the FUD, although regarding TF hopefully it won&#x27;t be for too long. The first issue created on GitHub[1] is basically about adding Py3 support.<p>Regarding GIL and JIT. Those problems are mostly performance related and not easy. As you mentioned GIL is available everywhere so it&#x27;s not a good argument regarding py2 vs py3.<p>As for JIT you get it with PyPy which by itself is not 100% compatible with CPython (for example as of now majority of python modules written in C simply won&#x27;t work).<p>Although they do have PyPy compatible with Python 3 although it is not as stable, but since Py3 is picking up hopefully it will be becoming more popular.<p>For me, it&#x27;s the other way around, I have not used PyPy primarily because it&#x27;s compatible 2.7 and I don&#x27;t use Python for performance, but if I could get performant Py3 implementation then why wouldn&#x27;t I want to use it?<p>[1] <a href="https:&#x2F;&#x2F;github.com&#x2F;tensorflow&#x2F;tensorflow&#x2F;issues&#x2F;1" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;tensorflow&#x2F;tensorflow&#x2F;issues&#x2F;1</a>