[Python-Dev] The new and improved PEP 572, same great taste with 75% less complexity! (original) (raw)

Guido van Rossum gvanrossum at gmail.com
Thu Apr 26 12:02:58 EDT 2018

Maybe the order for d[k] = v should also be reconsidered?

On Thu, Apr 26, 2018, 08:23 Chris Angelico <rosuav at gmail.com> wrote:

On Fri, Apr 27, 2018 at 1:07 AM, Guido van Rossum <guido at python.org> wrote: > Just as I feared. While currently fixing this is just fixing a bug (nobody's > code is going to break except perhaps some tests), with PEP 572 fixing this > would be mandatory. > > On Thu, Apr 26, 2018 at 6:57 AM, Nick Coghlan <ncoghlan at gmail.com> wrote: >> >> On 25 April 2018 at 18:23, Chris Angelico <rosuav at gmail.com> wrote: >> >>>> x={print(2): print(1) for in [1]} >> > 1 >> > 2 >> > >> > Hmmmmmmmmm. One of these is not like the others... >> >> Huh, it looks like we missed checking dict comprehensions when we >> fixed dict displays to evaluate keys before the corresponding values. >> That would qualify as a reasonable request for improvement in Python >> 3.8 :)

It's deliberate code in compile.c: case COMPDICTCOMP: /* With 'd[k] = v', v is evaluated before k, so we do the same. */ VISIT(c, expr, val); VISIT(c, expr, elt); ADDOPI(c, MAPADD, genindex + 1); break; So if it's a bug, it's still a deliberate bug. :) ChrisA

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