With both python and python3 packages installed, my Debian stretch systems seem to use python2.7 when /usr/bin/python is executed.
I prefer python3 as a default, as python2 is IMHO somewhat deprecated and I try to avoid it whenever I can.

Therefore, I was surprised that update-alternatives --config python resulted in an error message:
update-alternatives: error: no alternatives for python

However, this is easily fixed by manually installing alternative entries: update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2

A higher number specifies a higher priority, which in this case results in python3 being the default from now on.