There is no "from __future__ import" option that would allow Python 2x users the "raise... from" syntax that is provided with Python 3. This is especially unfortunate because even if the "raise... from" is included in a branch that is never executed, loading the code in 2.7 can cause a seemingly unrelated exception (in my case, an import error for StringIO). Since my code must work in 2.7, 3.3, and 3.4, this is a problem.

I have found a workaround. If I want to achieve the equivalent of "raise XXX from None", for instance, I can do it as follows:

    if hasattr(newExc, '__cause__'):
        newExc.__cause__ = None
    raise newExc

This works because the Exception class exists in all three Python versions, but only has a __cause__ attribute in Python 3. However, it's clumsier than I would like. Also, it's not obvious. 

I discovered that exception handling had changed when I saw that raising an exception when another one was active caused a "During handling of the above exception, another exception occurred" message on Python 3 (behavior that I didn't want) but was silently dropped in Python 2.7 (behavior that I did want). After I found a description of the "raise... from None" syntax in Python 3, I expected to find a "from __future__ import" statement that would handle this, but was disappointed. It took me a while longer to figure out the workaround above. I'd rather that other people didn't have to go through the process of figuring out the workaround.