The current Ubuntu long-term support release is Xenial, which comes with only Python 3 installed. If you're talking about Trusty, its support is ending fairly soon.

On Xenial, you have to separately install an optional package for Python 2. When you try to run /usr/bin/python on a fresh Xenial install, it'll tell you that you need to install a package such as python-minimal==2.7.11, whereas /usr/bin/python3 is already there.