joshuarli · Oct 17, 2026 · Oct 17, 2026 · Oct 17, 2026 · Oct 17, 2026
@@ -22,6 +22,7 @@
 import json
 import math
 import os
+import shlex
 import sys
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -161,8 +162,31 @@ def file_weights(files: list[str]) -> dict[str, float]:
     return weights
 
 
+def per_file_plan_applies(args: list[str], files: list[str]) -> bool:
+    """Whether planning by file gives each shard the tests pytest would run.
+
+    `-m`/`-k` filter after the plan, so most shards could end up with no
+    selected tests (e.g. `-m snuba_ci` in Snuba's CI). The sentry plugin
+    matches the planned paths relative to the rootdir, which
+    pytest_shard_plugin checks is the working directory.
+    """
+    every_arg = [*args, *shlex.split(os.environ.get("PYTEST_ADDOPTS", ""))]
+    if any(arg.startswith(("-m", "-k")) and not arg.startswith("--") for arg in every_arg):
+        return False
+    return not any(os.path.isabs(f) or f.startswith(os.pardir + os.sep) for f in files)
+
+
 def run_full_shard(args: list[str], total: int, group: int) -> int:
     files = list_test_files(args)
+    if not per_file_plan_applies(args, files):
+        # As before per-file sharding: every shard collects everything and the
+        # sentry plugin keeps its hash share of the items.
+        print(
+            "pytest-selected: -m/-k or paths outside the working directory, so "
+            "every shard collects everything and keeps its hash share",
+            file=sys.stderr,
+        )
+        return int(pytest.main(args))
     plan = plan_full_shards(file_weights(files), total) if files else None
     if plan is None:
         return int(pytest.main(args))

@@ -11,6 +11,7 @@
 import json
 import os
 from hashlib import sha256
+from pathlib import Path
 
 import pytest
 
@@ -19,6 +20,16 @@ def item_part(nodeid: str, parts: int) -> int:
     return int(sha256(nodeid.encode()).hexdigest(), 16) % parts
 
 
+def pytest_configure(config: pytest.Config) -> None:
+    # pytest-selected.py writes paths relative to the working directory, and
+    # the sentry plugin matches them relative to the rootdir.
+    if os.environ.get("PYTEST_SHARD_PARTS") and config.rootpath != Path.cwd().resolve():
+        raise pytest.UsageError(
+            f"pytest_shard_plugin: rootdir {config.rootpath} is not the working "
+            f"directory {Path.cwd()}; run pytest-selected.py from the rootdir"
+        )
+
+
 def pytest_collection_modifyitems(config: pytest.Config, items: list[pytest.Item]) -> None:
     parts_file = os.environ.get("PYTEST_SHARD_PARTS")
     if not parts_file:

@@ -42,12 +42,28 @@ export function parseFailures(files, core) {
       .filter(t => t.outcome === 'failed')
       .map(t => ({
         nodeid: t.nodeid ?? 'unknown',
-        longrepr: (t.call ?? t.setup ?? t.teardown ?? {}).longrepr ?? '',
+        longrepr:
+          failedSubtests(t) || ((t.call ?? t.setup ?? t.teardown ?? {}).longrepr ?? ''),
         artifactDir,
       }));
   });
 }
 
+// The parent's own longrepr only says "contains N failed subtests". Render
+// each failed subtest the way pytest's SUBFAILED line does.
+function failedSubtests(test) {
+  return (test.subtests ?? [])
+    .filter(s => s.outcome === 'failed')
+    .map(({context, longrepr}) => {
+      const kwargs = Object.entries(context?.kwargs ?? {}).map(([k, v]) => `${k}=${v}`);
+      const msg =
+        context?.msg === null || context?.msg === undefined ? [] : [`[${context.msg}]`];
+      const description = [...msg, ...(kwargs.length ? [`(${kwargs.join(', ')})`] : [])];
+      return `SUBFAILED ${description.join(' ') || '(<subtest>)'}\n${longrepr ?? ''}`;
+    })
+    .join('\n\n');
+}
+
 // Maps artifact directory names to GitHub Actions job log URLs.
 const JOB_MATCHERS = [
   {

@@ -238,6 +238,34 @@ describe('parseFailures', () => {
 
     assert.equal(parseFailures([file1, file2], mockCore()).length, 2);
   });
+
+  it('uses failed subtests instead of the parent summary', () => {
+    const file = createArtifactFile(
+      'pytest-results-backend-111-0',
+      'pytest.json',
+      makePytestJson([
+        {
+          nodeid: 'tests/a.py::test_a',
+          outcome: 'failed',
+          call: {outcome: 'failed', longrepr: 'contains 1 failed subtest'},
+          subtests: [
+            {outcome: 'passed', context: {msg: null, kwargs: {n: '1'}}},
+            {
+              outcome: 'failed',
+              longrepr: 'AssertionError: boom',
+              context: {msg: 'm', kwargs: {f: '<function f>', n: '2'}},
+            },
+          ],
+        },
+      ])
+    );
+
+    const [failure] = parseFailures([file], mockCore());
+    assert.equal(
+      failure.longrepr,
+      'SUBFAILED [m] (f=<function f>, n=2)\nAssertionError: boom'
+    );
+  });
 });
 
 describe('extractNodeids', () => {

@@ -2,12 +2,13 @@
 
 import importlib
 import json
+from pathlib import Path
 
 # Module has a hyphen in its name, so use importlib.
 _mod = importlib.import_module("backend-test-durations")
 
 
-def test_extract_sums_stages_per_file(tmp_path):
+def test_extract_sums_stages_per_file(tmp_path: Path) -> None:
     report = tmp_path / "pytest.json"
     report.write_text(
         json.dumps(
@@ -18,6 +19,9 @@ def test_extract_sums_stages_per_file(tmp_path):
                         "setup": {"duration": 1.0},
                         "call": {"duration": 2.0},
                         "teardown": {"duration": 0.5},
+                        # Subtests ran inside the call; reruns don't count.
+                        "subtests": [{"duration": 1.5}],
+                        "reruns": [{"call": {"duration": 9.0}}],
                     },
                     {"nodeid": "tests/a.py::test_2[x]", "setup": {"duration": 0.5}},
                     {"nodeid": "tests/b.py::test_3", "call": {"duration": 4.0}},
@@ -28,7 +32,7 @@ def test_extract_sums_stages_per_file(tmp_path):
     assert _mod.extract(report) == {"files": {"tests/a.py": 4.0, "tests/b.py": 4.0}, "tests": 3}
 
 
-def test_merge_adds_up_shards():
+def test_merge_adds_up_shards() -> None:
     shards = [
         {"files": {"tests/a.py": 1.04, "tests/b.py": 2.0}, "tests": 2},
         {"files": {"tests/a.py": 1.0}, "tests": 2},