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

Chris Angelico rosuav at gmail.com
Thu Apr 26 11:21:13 EDT 2018

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 COMP_DICTCOMP:
        /* With 'd[k] = v', v is evaluated before k, so we do
           the same. */
        VISIT(c, expr, val);
        VISIT(c, expr, elt);
        ADDOP_I(c, MAP_ADD, gen_index + 1);
        break;

So if it's a bug, it's still a deliberate bug. :)

ChrisA

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