TensorFlow and App Engine are both released by Google which prefers Py2 because of the FUD, although regarding TF hopefully it won't be for too long. The first

TensorFlow and App Engine are both released by Google which prefers Py2 because of the FUD, although regarding TF hopefully it won't be for too long. The first issue created on GitHub[1] is basically about adding Py3 support. Regarding GIL and JIT. Those problems are mostly performance related and not easy. As you mentioned GIL is available everywhere so it's not a good argument regarding py2 vs py3. 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't work). 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. For me, it's the other way around, I have not used PyPy primarily because it's compatible 2.7 and I don't use Python for performance, but if I could get performant Py3 implementation then why wouldn't I want to use it? [1] https://github.com/tensorflow/tensorflow/issues/1 https://github.com/tensorflow/tensorflow/issues/1