Switching between Python 3 and 2 in a runtime

switch.py

#!/usr/bin/env python

import sys, os

print(sys.version_info[:])

if sys.version_info[0] != 2:
  sys.argv.insert(0, "python2")
  os.execl("/usr/bin/env", "/usr/bin/env", *sys.argv)

try:
  eval('print sys.version_info[:]')
except SyntaxError:
  pass