Python version on Linux

[whois1230]

Hello, I'm running Linux Mint Cinnamon 18.3 64-bit and I want to learn Python 3. When I write

python --version

in the Terminal I get

user@user-Lenovo-ideapad-110-17IKB ~ $ python --version
Python 2.7.12

When I write

sudo apt-get install python3.5

in the Terminal I get

user@user-Lenovo-ideapad-110-17IKB ~ $ sudo apt-get install python3.5
Reading package lists... Done
Building dependency tree       
Reading state information... Done
python3.5 is already the newest version (3.5.2-2ubuntu0~16.04.9).
The following packages were automatically installed and are no longer required:
  codeblocks-common libcodeblocks0 libwxbase3.0-0v5 libwxgtk3.0-0v5
Use 'sudo apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 36 not upgraded.

How do I know which version I'm running?