Is there a way to prevent a python 3 script from being called in python 2?

Question

I have to turn in an assignment, and I'm extremely concerned that, since the single TA has many projects to run, it will be called using python, which will call python 2.7, when the program is written for python3.2 and should be called that way. This would result in a syntax error, and I would loose points. I know when working on side projects, this happens a lot, and if the TA encountered this I don't think he would follow up.

I'm going to submit a readme, but I was wondering if there was a way to catch this in my code without a lot of fuss, and print a statement that said to rerun the project as python3.2 project.py. I could to a try: print "Rerun project…" except:pass, but is there a better way?

Answer 1

This is actually a harder problem to implement well that you might at first think.

Suppose you have the following code:

import platform
import sys

if platform.python_version().startswith('2'):
    # This NEVER will be executed no matter the version of Python
    # because of the two syntax errors below...
    sys.stdout.write("You're using python 2.x! Python 3.2+ required!!!")
    sys.exit()     
else:
    # big program or def main(): and calling main() .. whatever
    # later in that file/module:
    x, *y=(1,2,3)      # syntax error on Python 2...
    # or
    print 'test'       # syntax error on Python 3...

One of the two syntax errors under the else clause is generated BEFORE the if is actually executed no matter the version of Python used to run it. Therefore, the program will not gracefully exit as you might expect; it will fail with a syntax error no matter what.

The workaround is to put your actual program in an external file/module and wrap in a try/except this way:

try:
    import Py3program    # make sure it has syntax guaranteed to fail on 
                         # Python 2 like    x, *y=1,2,3
except SyntaxError:
    sys.stdout.write(error message)
    sys.exit()

# rest of the Python 3 program...

If your TA will execute the file with a sheebang, that would be a better approach still. Perhaps ask the TA how he will run your script?

Answer 2

How about starting the program like so:

#!/usr/bin/env python
# -*- coding: utf8 -*-

import sys

if sys.version_info < (3,0,0):
    print(__file__ + ' requires Python 3, while Python ' + str(sys.version[0] + ' was detected. Terminating. '))
    sys.exit(1)