[Python-Dev] Gradual migration

[Guido van Rossum]

> 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/)