How can I check for Python version in a program that uses new language features?

If I have a Python script that requires at least a particular
version of Python, what is the correct way to fail gracefully
when an earlier version of Python is used to launch the script?

How do I get control early enough to issue an error message
and exit?

For example, I have a program that uses the ternery operator (new in 2.5) and “with” blocks
(new in 2.6). I wrote a simple little interpreter-version
checker routine which is the first thing the script would
call … except it doesn’t get that far. Instead, the
script fails during python compilation, before my routines
are even called. Thus the user of the script sees some very
obscure synax error tracebacks – which pretty much require
an expert to deduce that it is simply the case of running
the wrong version of Python.

I know how to check the version of Python. The issue is that some syntax is illegal in older versions of Python. Consider this program:

import sys
if sys.version_info < (2, 4):
    raise "must use python 2.5 or greater"
else:
    # syntax error in 2.4, ok in 2.5
    x = 1 if True else 2
    print x

When run under 2.4, I want this result

$ ~/bin/python2.4 tern.py 
must use python 2.5 or greater

and not this result:

$ ~/bin/python2.4 tern.py 
  File "tern.py", line 5
    x = 1 if True else 2
           ^
SyntaxError: invalid syntax

(Channeling for a coworker.)