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

You could do something creative.

Have a check at the start of your code – your constructor or whatever, check what version of python you are using, since your normal version checker is not working, try this instead:

try:
  eval('a python 3 expression') # something that only works in python3+
  python_version = 3
except:
  python_version = 2

Then the rest of your code can easily just reference this to know what to use.

As for the parse errors, you can use exec in a function, like so:

def what_to_run():
    if python_version = 3:
        return 'raise exc_info[0].with_traceback(exc_info[1], exc_info[2])'
    else:
        return 'raise exc_info[0], exc_info[1], exc_info[2]'

In your function you would write this:

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

    headers_set[:] = [status, response_headers]
    return write

A bit messy, untested, but it should work, at least you understand the idea.