[Python-Dev] PEP 4000 to explicitly declare we won't be doing a Py3k style compatibility break again?

Steven D'Aprano steve at pearwood.info
Sun Aug 17 04:39:02 CEST 2014

On Sun, Aug 17, 2014 at 11:28:48AM +1000, Nick Coghlan wrote:
> I've seen a few people on python-ideas express the assumption that
> there will be another Py3k style compatibility break for Python 4.0.

I used to refer to Python 4000 as the hypothetical compatibility break 
version. Now I refer to Python 5000.

> I've also had people express the concern that "you broke compatibility
> in a major way once, how do we know you won't do it again?".

Even languages with ISO standards behind them and release schedules 
measured in decades make backward-incompatible changes. For example, I 
see that Fortran 95 (despite being classified as a minor revision) 
deleted at least six language features. To expect Python to never break 
compatibility again is asking too much.

But I think it is fair to promise that Python won't make *so 
many* backwards incompatible changes all at once again, and has no 
concrete plans to make backwards incompatible changes to syntax in the 
foreseeable future. (That is, not before Python 5000 :-)

> If folks (most signficantly, Guido) are amenable to the idea, it
> shouldn't take long to put such a PEP together, and I think it could
> help reduce some of the confusions around the expectations for Python
> 4.0 and the evolution of 3.x in general.

I think it's a good idea, so long as there's no implied or explicit 
promise that Python language is now set in stone never to change.


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