sys.version_info and sys.version contain the version of Python that is being run. sys.executable contains the path to the specific interpreter running.
Python3:
>>> import sys
>>> sys.version_info
sys.version_info(major=3, minor=4, micro=3, releaselevel='final', serial=0)
>>> sys.version
'3.4.3 (default, Nov 17 2016, 01:08:31) \n[GCC 4.8.4]'
>>> sys.executable
'/usr/bin/python3'
Python2:
>>> import sys
>>> sys.version_info
sys.version_info(major=2, minor=7, micro=6, releaselevel='final', serial=0)
>>> sys.version
'2.7.6 (default, Oct 26 2016, 20:30:19) \n[GCC 4.8.4]'
>>> sys.executable
'/usr/bin/python2'
The problem seems to be that your registry editor has set a different version set to run "on-click" for Python executables. You can fix that by running the Python installer for the version you want, and setting it to repair, or modifying "HKEY_CLASSES_ROOT\Python.File\Shell\open\command" to run the correct python executable (Should be "C:\Windows\py.exe"). See this image for where to find it.
If you are already using py.exe, adding a hashbang to the top of the file (#!Python<version>, or to work with Unix executables, #!/usr/bin/env python<version>) should help py.exe pick the correct executable to run.
To install using pip for a specific executable, run Path\To\Executable -m pip install <module>.
To use modules from a different site-path, add the directory to the PYTHONPATH environment variable. Using import <file> will check for modules in directories in the PYTHONPATH.
python --versionwill give you the default version of Python in your system. You can use a specific version of python by typingpython2.7orpython3.5in the terminal. This will give you terminal access to the specific python. Different python versions present in your system will be available in the path:/usr/lib/python2.X.