How to write exception reraising code that's compatible with both Python 2 and Python 3?

Question

I'm trying to make my WSGI server implementation compatible with both Python 2 and Python 3. I had this code:

def start_response(status, response_headers, exc_info = None):     if exc_info:         try:             if headers_sent:                 # Re-raise original exception if headers sent.                 raise exc_info[0], exc_info[1], exc_info[2]         finally:             # Avoid dangling circular ref.             exc_info = None     elif headers_set:         raise AssertionError("Headers already set!")      headers_set[:] = [status, response_headers]     return write 

...with the relevant part being:

# Re-raise original exception if headers sent. raise exc_info[0], exc_info[1], exc_info[2] 

Python 3 doesn't support that syntax anymore so it must be translated to:

raise exc_info[0].with_traceback(exc_info[1], exc_info[2]) 

Problem: the Python 2 syntax generates a parse error in Python 3. How do I write code that can be parsed by both Python 2 and Python 3? I've tried the following, but that doesn't work:

if sys.version_info[0] >= 3:     raise exc_info[0].with_traceback(exc_info[1], exc_info[2]) else:     eval("raise exc_info[0], exc_info[1], exc_info[2]; 1", None, { 'exc_info': exc_info }) 

Answer 1

Can you use six? It exists to solve this very problem.

import six, sys six.reraise(*sys.exc_info()) 

See: https://six.readthedocs.io/index.html#six.reraise