Run python3 in a python2 code.

py3in2.py

import tempfile, subprocess
def py3in2(func_text, arg):
    # escape char in arg should be escaped twice. 
    payload = ['#/usr/bin/env python3', 'def _func(arg):']
    for line in func_text.split('\n'):
        payload.append('    ' + line)
    payload.append('with open("/dev/fd/1", "w") as f:\n    f.write(_func(\''+arg+'\'))\n')
    payload = '\n'.join(payload)

    with tempfile.NamedTemporaryFile() as tf:
        tf.file.write(payload)
        tf.file.close()
        output = subprocess.check_output(["python3", tf.name], stderr=subprocess.PIPE)
        return output

ret = py3in2('''
import sys
return sys.version
''', '')

print ret