Here is a nice and simple way to do it (but on CENTOS), without braking the operating system.
yum install scl-utils
next
yum install centos-release-scl-rh
And lastly you install the version that you want, lets say python3.5
yum install rh-python35
And lastly:
scl enable rh-python35 bash
Since MAC-OS is a unix operating system, the way to do it it should be quite similar.