I have Python 2.x and 3.x on my machine (Mac OS X 10.6). For some things I want to use ver 2, but for others I want ver 3. I like the IDLE software for editing/running, but
If there are 2 or more Python versions installed on you computer but you can only open one Python IDLE (suppose IDLE using Python 2.7) and if you dont have IDLE for Python 3.5 installed on your computer (check the folder /usr/bin for idle or idle-python2.7 And if idle-python3.5 is not there then its not currently installed). Then you just have to install idle-python3.5 by executing following command through CLI.
sudo apt install idle-python3.5
Or you can just install it by Software Manager in Linux. Search for IDLE and select your preferred IDLE to install.