Constant folding is skipped in named expressions

[ncoghlan]

BPO	42282
Nosy	@ncoghlan
PRs	bpo-42282: Fold constants inside named expressions #23190

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = 'https://github.com/ncoghlan'
closed_at = <Date 2020-11-07.12:36:30.609>
created_at = <Date 2020-11-07.06:59:44.846>
labels = ['3.10', 'performance']
title = 'Constant folding is skipped in named expressions'
updated_at = <Date 2020-11-07.12:36:30.609>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2020-11-07.12:36:30.609>
actor = 'ncoghlan'
assignee = 'ncoghlan'
closed = True
closed_date = <Date 2020-11-07.12:36:30.609>
closer = 'ncoghlan'
components = []
creation = <Date 2020-11-07.06:59:44.846>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 42282
keywords = ['patch']
message_count = 3.0
messages = ['380494', '380505', '380506']
nosy_count = 1.0
nosy_names = ['ncoghlan']
pr_nums = ['23190']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue42282'
versions = ['Python 3.10']

[ncoghlan]

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

[ncoghlan]

New changeset 8805a4d by Nick Coghlan in branch 'master':
bpo-42282: Fold constants inside named expressions (GH-23190)
8805a4d

[ncoghlan]

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