Respect parent-scoping rules for NamedExpr assignments

[charliermarsh]

Summary

Per PEP 572, a named expression assignment within a generator or comprehension actually binds to the parent scope. This is useful as it enables patterns like:

# Compute partial sums in a list comprehension
total = 0
partial_sums = [total := total + v for v in values]
print("Total:", total)

Previously, we bound the named expression within the generator or comprehension, which led to us flagging key as undefined here:

if any((key := x) for x in ["ok"]):
    print(key)

Closes #3997.

[github-actions]

PR Check Results

Ecosystem

✅ ecosystem check detected no changes.

Benchmark

Linux

group                                      main                                    pr
-----                                      ----                                    --
linter/all-rules/large/dataset.py          1.03     14.3±0.97ms     2.8 MB/sec     1.00     14.0±0.09ms     2.9 MB/sec
linter/all-rules/numpy/ctypeslib.py        1.00      3.4±0.01ms     5.0 MB/sec     1.00      3.3±0.01ms     5.0 MB/sec
linter/all-rules/numpy/globals.py          1.00    419.4±1.06µs     7.0 MB/sec     1.00    418.6±1.14µs     7.0 MB/sec
linter/all-rules/pydantic/types.py         1.00      5.8±0.01ms     4.4 MB/sec     1.00      5.8±0.02ms     4.4 MB/sec
linter/default-rules/large/dataset.py      1.00      6.9±0.01ms     5.9 MB/sec     1.00      6.9±0.03ms     5.9 MB/sec
linter/default-rules/numpy/ctypeslib.py    1.00   1478.9±2.42µs    11.3 MB/sec     1.00   1480.5±2.86µs    11.2 MB/sec
linter/default-rules/numpy/globals.py      1.01    166.8±0.35µs    17.7 MB/sec     1.00    166.0±0.21µs    17.8 MB/sec
linter/default-rules/pydantic/types.py     1.00      3.1±0.01ms     8.3 MB/sec     1.00      3.1±0.02ms     8.3 MB/sec
parser/large/dataset.py                    1.00      5.5±0.08ms     7.4 MB/sec     1.00      5.5±0.01ms     7.5 MB/sec
parser/numpy/ctypeslib.py                  1.15  1220.1±288.13µs    13.6 MB/sec    1.00   1065.2±2.45µs    15.6 MB/sec
parser/numpy/globals.py                    1.14   123.7±49.00µs    23.9 MB/sec     1.00    108.6±0.15µs    27.2 MB/sec
parser/pydantic/types.py                   1.15      2.7±0.72ms     9.6 MB/sec     1.00      2.3±0.04ms    11.0 MB/sec

Windows

group                                      main                                   pr
-----                                      ----                                   --
linter/all-rules/large/dataset.py          1.01     16.6±0.25ms     2.4 MB/sec    1.00     16.5±0.21ms     2.5 MB/sec
linter/all-rules/numpy/ctypeslib.py        1.00      4.2±0.06ms     4.0 MB/sec    1.00      4.1±0.06ms     4.0 MB/sec
linter/all-rules/numpy/globals.py          1.00   493.1±13.65µs     6.0 MB/sec    1.00   490.9±14.77µs     6.0 MB/sec
linter/all-rules/pydantic/types.py         1.00      7.0±0.09ms     3.7 MB/sec    1.00      6.9±0.08ms     3.7 MB/sec
linter/default-rules/large/dataset.py      1.01      8.4±0.09ms     4.9 MB/sec    1.00      8.3±0.08ms     4.9 MB/sec
linter/default-rules/numpy/ctypeslib.py    1.00  1777.5±18.24µs     9.4 MB/sec    1.00  1775.8±21.25µs     9.4 MB/sec
linter/default-rules/numpy/globals.py      1.00    198.9±5.61µs    14.8 MB/sec    1.00    198.3±4.32µs    14.9 MB/sec
linter/default-rules/pydantic/types.py     1.01      3.7±0.04ms     6.8 MB/sec    1.00      3.7±0.04ms     6.9 MB/sec
parser/large/dataset.py                    1.00      6.8±0.07ms     6.0 MB/sec    1.00      6.8±0.06ms     6.0 MB/sec
parser/numpy/ctypeslib.py                  1.00  1293.6±13.72µs    12.9 MB/sec    1.01  1301.9±18.06µs    12.8 MB/sec
parser/numpy/globals.py                    1.00    132.2±2.45µs    22.3 MB/sec    1.01    133.7±2.87µs    22.1 MB/sec
parser/pydantic/types.py                   1.00      2.9±0.04ms     8.8 MB/sec    1.00      2.9±0.03ms     8.8 MB/sec

[MichaReiser]

Nit: The documentation mentions that the comprehension binds to the parent scope but the find binds to the first non generator scope. Can we either update the documentation or the code.

Could this crash if we support linting individual expressions?

[MichaReiser]

Nit: Would it make sense to return the scope instead

Suggested change

	let scope_id = if binding.kind.is_named_expr_assignment() {
	let scope = if binding.kind.is_named_expr_assignment() {

It would avoid retrieving the id from the ancestor scope only to look it up again.

[charliermarsh]

Yeah, it has to be mut though. I'll try to get it working.