Python 3 is killing Python

[Terry Reedy]

On 5/29/2014 12:03 AM, Steven D'Aprano wrote:
> On Wed, 28 May 2014 20:41:53 -0400, Terry Reedy wrote:
>
>> Claim: Another great strength of Python 2 was that programs written in
>> it would almost always run on the next version of Python without much
>> alteration.
>>
>> True.
>
> True, but only because of the weasel-words "almost always", and "without
> much alteration".
>
> And for the record, for many (although not all) programs written in
> Python 2.7, it is still true that they will often run in Python 3 with
> little or no modification.
>
>
>> Changes and removals of deprecated features (like old style
>> classes) were put off until 3.0 (at the request of some of the noiser
>> users).

I am specifically thinking of int / int -> float, which Guido initially 
proposed for 2.5 after warnings in 2.3, 2.4. The idea of 3.0 with more 
breakage and 2to3 emerged during that discussion.

> That's a little unfair. Noisy users or not, Python Dev has always taken
> backwards compatibility seriously.
>
> Nevertheless, there have been some big changes to Python 2.x that
> *didn't* wait for 3.x to break backwards compatibility. A few examples
> that come to mind:
>
> - removing string exceptions for good in 2.6;
> - changes to the treatment in hex() of negative numbers;
> - the repr() of floats;
> - changes to the sequence of random numbers generated by the
>    random number module (e.g. while random.random() is
>    guaranteed to return the same sequence of values,
>    random.choice is not);
> - removal of obsolete modules like bastion and rotor.
>
> Any and all of these things can break code that relies on them.


-- 
Terry Jan Reedy