python 3.4 is installed on the stable release of Ubuntu 14.04. You need to use python3 to use python 3.4. For example, to execute a script file.py, use:

python3 file.py

This will use python 3.4 to interpret your program or you can use the shebang to make it executable. The first line of your program should be:

#!/usr/bin/env python3

and then use chmod +x file.py to assign executable permissions and then run your python script as ./file.py which would use python3 to execute.

If you want python3 to be used when you type python on the terminal, you can use an alias. To add a new alias, open your ~/.bash_aliases file using gedit ~/.bash_aliases and type the following:

alias python=python3

and then save and exit and type

source ~/.bash_aliases

and then you can type

python file.py

to use python3 as your default python interpreter.

No, you don't need IDLE just to use python3 to interpret your programs.

As you can see from http://packages.ubuntu.com/search?keywords=python3.4&searchon=names&suite=xenial&section=all, there is no package for python3.4 for Ubuntu 16.04. You can compile and install Python 3.4 from source, but if you are not familiar with doing it or prefer to install packages using the APT package manager, I would recommend installing it from a well-known PPA that provides a variety of versions of Python for many Ubuntu versions.

1. Add the deadsnakes PPA (read more about it at https://launchpad.net/~deadsnakes/+archive/ubuntu/ppa):

   sudo add-apt-repository ppa:deadsnakes/ppa
   

2. Install python3.4:

   sudo apt-get update
   sudo apt-get install python3.4
   

According to https://askubuntu.com/a/682875/15003,/usr/bin/python3 should still be symlinked to /usr/bin/python3.5. Therefore, if you want to call Python 3.4, you would need to type the full path to it, which is /usr/bin/python3.4. To avoid accidentally breaking other programs, I strongly recommend you to not change the symlink that /usr/bin/python3 points to and instead just use /usr/bin/python3.4 whenever you need to call Python 3.4.

Alternatively, a popular method to manage multiple versions of Python, which I personally recommend, is to use virtualenv. You can read more about it at https://virtualenv.pypa.io/en/stable/; further elaboration of it in this post seems too far off the intent of the question.