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

[Donald Stufft]

On Sun, Aug 17, 2014, at 09:02 PM, Guido van Rossum wrote:
> On Sun, Aug 17, 2014 at 6:29 AM, Barry Warsaw <barry at python.org> wrote:
>> On Aug 16, 2014, at 07:43 PM, Guido van Rossum wrote:
>>  
>> 
>(Don't understand this to mean that we should never deprecate things.
>> 
>Deprecations will happen, they are necessary for the evolution of any
>> 
>programming language. But they won't ever hurt in the way that Python 3
>> 
>hurt.)
>>  
>> It would be useful to explore what causes the most pain in the 2->3
>> 
transition?  IMHO, it's not the deprecations or changes such as print ->
>> 
print().  It's the bytes/str split - a fundamental change to core and
common
>> 
data types.  The question then is whether you foresee any similar
looming
>> 
pervasive change? [*]
> 
> I'm unsure about what's the single biggest pain moving to Python 3. In the past I would have said that it's for sure the bytes/str split (which both the biggest pain and the biggest payoff).
>  
> But if I look carefully into the soul of teams that are still on 2.7 (I know a few... :-), I think the real reason is that Python 3 changes so many different things, you have to actually understand your code to port it (unlike with minor version transitions, where the changes usually spike in one specific area, and you can leave the rest to normal attrition and periodic maintenance).
>  

In my experience bytes/str is the single biggest change that causes the
most problems. Most of the other changes can be mechanically transformed
and/or papered over using helpers like six. The bytes/str change is the
main one that requires understanding code and where it requires a
serious untangling of things in code bases where str/bytes are freely
used intechangingbly. Often times this requires making a decision about
what *should* be bytes or str as well which requires having some deep
knowledge about the APIs in question too.