As a more concrete extension to my last email, I propose the following doctrine:
""" No major documented feature should be removed or have changed semantics in Python 3000 or any other new version of Python until users have had a year (preferably MORE!) of upgrade time. Upgrade time entails the following parts:
1. the released Python version has a new recommended way to accomplish the task in a manner that will remain available in the "breakage version" e.g. a div() function that people can use for a few years while the semantics of "/" are in transition.
2. the mechanism/syntax that will be removed is formally deprecated. The documentation would say, e.g. "You should not use '/' for now. It is changing semantics in the future."
3. the released Python version sports a runtime warning to tell users that the mechanism/syntax is going away. "CompatibilityError: Future versions of Python will have different semantics for the '/' operator. Please use div() instead."
The actual "right" amount of upgrade time depends on the extent of the breakage and its ease of detection. """
I can PEP this if people agree. I think that the user community would appreciate our effort to promise not to break code suddenly and capriciously.
Go for it. I have little bandwidth to think about this deeply, but what you're proposing here sounds like a good approach. Certainly it will make it easier if I can point to this PEP when I get the next FUD email about "should I bother to learn Python 2.0 when Py3K is going to be all different?"... --Guido van Rossum (home page: http://www.python.org/~guido/)