import __past__ as a fade-out mechanism?

[Huaiyu Zhu]

On Fri, 15 Feb 2002 15:05:00 +1300, Greg Ewing <greg at cosc.canterbury.ac.nz>
wrote: 
>Huaiyu Zhu wrote:
>> 
>>    from __future__ import zerolead_octal so that 041 will mean 33.
>
>Shouldn't that be from __past__ import zerolead_octal? :-)

Yes, but the timbot had ruled out __past__ in many other threads.  So I just
had to play with __future__.  :-)

Thinking about this a bit more, I think __past__ can be a good mechanism for
fade-out transition just as __future__ is a good mechanism for fade-in
transition, as long as it is made clear that anything in the __past__ is
doomed to disappear sometime in the future.

Here's an example.  Suppose a feature is being changed from A to B in
version 5.  Early birds can import __future__ to use B in version 3, while
old holdouts can import __past__ to hold on to A until version 6:

version     1   2   3   4   5   6   7   8
__future__  .   .   B   B   B   B   B   B
plain       A   A   A  (A)  B   B   B   B
__past__    A   A   A   A   A  (A) (.) (.)

[Parenthesis denotes warning.  Dot denotes unavailable.]

How about that?

Huaiyu