Switch Python Version for Vim & Syntastic

Easiest solution:

Add this to you .vimrc

 let g:syntastic_python_python_exec = 'python3'
 let g:syntastic_python_checkers = ['python']

This is the straightforward solution to switch to python3.

The below is no longer necessary, and might screw up if you're forced to work on a strictly python 2.x script.

The best option is to leave the Syntastic defaults alone, and to use conda to manage separate environments for python 3 and 2 (each with their own version-specific installs of flake8, pyflakes, etc), and to switch to the appropriate environment to edit each file. Syntastic will then call python/flake8/whatever else according to the paths set in the activated environment.

From the Syntastic repository README:

Q. The python checker complains about syntactically valid Python 3 constructs...

A. Configure the python checker to call a Python 3 interpreter rather than Python 2, e.g:

let g:syntastic_python_python_exec = '/path/to/python3'

Add that line to your .vimrc - that should fix your problem.

In spite of all the answers here, I still find the recommendation from the FAQ to be the best. I have added this to my .vimrc so that I can easily switch between python versions.

function Py2()
  let g:syntastic_python_python_exec = '/usr/local/bin/python2.7'
endfunction

function Py3()
  let g:syntastic_python_python_exec = '/usr/local/bin/python3.6'
endfunction

call Py3()   " default to Py3 because I try to use it when possible

With those functions installed, it's easy to switch python version right within vim with :call Py2() or :call Py3() depending on what I need at the moment. No need to exit vim and activate a different virtualenv as the popular answer would have you do.