Constant folding is skipped in named expressions #86448

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1. ncoghlan commented on Nov 7, 2020

   

   ncoghlan

   on Nov 7, 2020

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   While working on the PEP-642 reference implementation I removed the "default:" case from the switch statement in astfold_expr as part of making sure the new SkippedBinding node was being handled everywhere it needed to be.

   This change picked up that NamedExpr_kind wasn't being handled either, and a check with the dis module confirmed that using the walrus operator turns off constant folding:

   [ncoghlan@thechalk cpython]$ ./python
   Python 3.10.0a2+ (heads/pep-642-constraint-patterns-dirty:4db2fbd609, Nov  7 2020, 16:42:19) 
   [GCC 10.2.1 20201016 (Red Hat 10.2.1-6)] on linux
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import dis
   >>> dis.dis("1 + 1")
     1           0 LOAD_CONST               0 (2)
                 2 RETURN_VALUE
   >>> dis.dis("(x := 1 + 1)")
     1           0 LOAD_CONST               0 (1)
                 2 LOAD_CONST               0 (1)
                 4 BINARY_ADD
                 6 DUP_TOP
                 8 STORE_NAME               0 (x)
                10 RETURN_VALUE
   >>>
   

   The missing switch statement entry is just:

       case NamedExpr_kind:
           CALL(astfold_expr, expr_ty, node_->v.NamedExpr.value);
           break;
   

   Which gives the expected result:

   [ncoghlan@thechalk cpython]$ ./python -c "import dis; dis.dis('(x := 1 +1)')"
     1           0 LOAD_CONST               0 (2)
                 2 DUP_TOP
                 4 STORE_NAME               0 (x)
                 6 RETURN_VALUE
   
2. ncoghlan

   added

   3.10 (EOL)end of lifeend of life

   performancePerformance or resource usagePerformance or resource usage

   on Nov 7, 2020
3. ncoghlan

   assigned

   ncoghlan

   on Nov 7, 2020
4. ncoghlan commented on Nov 7, 2020

   

   ncoghlan

   on Nov 7, 2020

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   New changeset 8805a4d by Nick Coghlan in branch 'master':
   bpo-42282: Fold constants inside named expressions (GH-23190)
   8805a4d
5. ncoghlan commented on Nov 7, 2020

   

   ncoghlan

   on Nov 7, 2020

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   Since this was only a performance issue, I'm not planning to backport it to earlier releases.
6. ncoghlan

   closed this as completedon Nov 7, 2020
7. ncoghlan

   closed this as completedon Nov 7, 2020
8. ezio-melotti

   transferred this issue fromon Apr 10, 2022