Ok I see this is nothing for any 3.x release. 
I imagine this now either ‚clean‘ for users with compatibility break or just leave things as they are. 
So, if at all, maybe something for Python 4 :)

Coincidence I watched yesterday Armin Ronachers talk related to seeing compatibility as the holy cow - interesting watch...


Steven D'Aprano <steve@pearwood.info> schrieb am So. 1. Apr. 2018 um 03:49:
On Sun, Apr 01, 2018 at 02:20:16AM +0100, Rob Cliffe via Python-ideas wrote:

> >New unordered 'd' and 'D' prefixes, for 'dedent', applied to multiline
> >strings only, would multiply the number of alternatives by about 5 and
> >would require another rewrite of all code (Python or not) that parses
> >Python code (such as in syntax colorizers).
> I think you're exaggerating the difficulty somewhat.  Multiplying the
> number of alternatives by 5 is not the same thing as increasing the
> complexity of code to parse it by 5.

Terry didn't say that it would increase the complexity of the code by a
factor of five. He said it would multiply the number of alternatives by
"about 5". There would be a significant increase in the complexity of
the code too, but I wouldn't want to guess how much.

Starting with r and f prefixes, in both upper and lower case, we have:

4 single letter prefixes
(plus 2 more, u and U, that don't combine with others)
8 double letter prefixes

making 14 in total. Adding one more prefix, d|D, increases it to:

6 single letter prefixes
(plus 2 more, u and U)
24 double letter prefixes
48 triple letter prefixes

making 80 prefixes in total. Terry actually underestimated the explosion
in prefixes: it is closer to six times more than five (but who is
counting? apart from me *wink*)

[Aside: if we add a fourth, the total becomes 634 prefixes.]

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