Here's the steps (my non-computer-savvy girlfriend had to figure this one out for me, but unlike all the far more complicated processes one can find online, this one works)

1. Open Control Panel
2. Click "Uninstall a Program"
3. Scroll down to Python and click uninstall for each version you don't want anymore.

This works on Windows 7 out of the box, no additional programs or scripts required.

Do not uninstall your system's Python interpreter (Python 2.7 most probably). You might consider uninstalling the other version (Python 3.5 most probably), but I do not think you really need to do that (it may not be a bad idea to keep a system-wide Python 3 interpreter... who knows!).

If you want a clean state I would recommend you to use virtual environments for now on. You have two options:

• Use virtualenv and pip to setup your virtual environments and packages. However, using pip means you will have to compile the packages that need compilation (numpy, matplotlib and many other scientific Python packages that you may use for your "post-processing").
• Use Conda (or Miniconda). This way you will be able to handle virtual environments but without having to compile Python packages yourself. Conda also allows you to handle different Python interpreters without the need of having them installed in your system (it will download them for you).

Also, you say you are feeling MATLAB is inefficient for plotting millions of points. I do not know your actual needs/constraints, but I find Matplotlib to be very inefficient for plotting large data and/or real-time data.

Just as a suggestion, consider using PyQtGraph. If you still feel that is not fast enough, consider using VisPy (probably less functional/convenient at the moment, but more efficient).

Install all the Python X.Y versions you want, but include Python 3.3 or later (last is best, or reinstall it after all the others) to get the Python Launcher that @abarnert mentioned in his comments. It is also available as a standalone installer.

Earlier versions than 3.3 should not register extensions, so the launcher installed by 3.3 or later will remain the default handler for .py files.

From the command line:

py -2        # launch latest 2.x version installed.
py -3        # latest 3.x
py -3.2      # run exact version
py -3.2-32   # run 32-bit version on 64-bit system.

So you can even have mixed 32-bit and 64-bit installations.

The environment variable PY_PYTHON can be set to specify the default Python to run.

In scripts, add a comment of the following forms below to use that version of Python when the script is double-clicked or run from command line via py script.py:

#!python2
#!python3
#!python3.2
#!python3.2-32

See PEP 397 for further details.

Actually, there are only 3 versions of Python in Ubuntu 14.04 repository: 2.7, 3.3, 3.4

• Almost all applications use v2.7 , so I could say it: noway to remove it.
• v3.3 not used (only if you are a developer), could be removed no harm.
• v3.4 few applications start using it as alternative (they keep 2.7 compatibility), could be removed but check if it will suggest removing other packages.

It is possible to check their reverse dependencies, using apt-cache to get better idea:

apt-cache rdepends python2.7
apt-cache rdepends python3.3
apt-cache rdepends python3.4