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From: Serhiy Storchaka <storchaka@gmail.com>
Subject: Re: Why has python3 been created as a seperate language where there is still python2.7 ?
Date: Wed, 27 Jun 2012 22:08:07 +0300
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Xref: csiph.com comp.lang.python:24533

On 27.06.12 14:22, Stefan Behnel wrote:
> For comparison, the revival of the "u" string prefix in Py3.3 is a simple
> change in the parser grammar that's easy to maintain but that has a huge
> impact on the Py3 compatibility of code that accepts to drop support for
> Py2.5 and earlier (as well as Py3.[012]) but wants to keep working in
> Py2.[67] (which supports the opposite "b" prefix).

And even this simple change has caused unexpected issues (see issues 
#15054 and #15096), which were not predicted by the preceding stormy 
discussion. IMHO, the negative consequences of this change are undervalued.