Move stratified_survey DGP test to benchmarks/ (CI isolated-install fix)

The R13 P1 regression test ``test_stratified_survey_dgp_post_periods_cover_full_post_tail``
imports ``benchmarks.python.coverage_sdid`` directly to exercise the
private ``_stratified_survey_dgp`` helper. CI's isolated-install job
deliberately copies only ``tests/``, not ``benchmarks/``, so the
module import failed with ``ModuleNotFoundError`` on CI runners that
install the package into a fresh site-packages and then run the test
suite against that install.

The target is a benchmarking harness helper, not shipped package
code, so the natural home is ``benchmarks/python/``. Moving it there
keeps the test runnable locally (developer invokes explicitly before
regenerating the coverage MC artifact) and out of CI's collection
(``pyproject.toml testpaths = ["tests"]`` scopes default discovery to
``tests/`` only, so the new file never interferes).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

benchmarks/python/test_coverage_sdid.py

@@ -0,0 +1,75 @@
+"""Harness-level sanity tests for ``coverage_sdid.py``.
+
+Lives under ``benchmarks/`` rather than ``tests/`` because the targets
+here (the private DGP helpers inside ``coverage_sdid.py``) are part of
+the MC harness, not the shipped ``diff_diff`` package. CI's isolated-
+install job deliberately copies only ``tests/``; running the pytest
+collection here would just fail with ``ModuleNotFoundError`` on the
+``benchmarks`` import.
+
+Invoke manually when about to regenerate the coverage MC artifact::
+
+    pytest benchmarks/python/test_coverage_sdid.py -v
+
+Or from the repo root::
+
+    python -m pytest benchmarks/python/test_coverage_sdid.py -v
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+import sys
+
+# Make the top-level ``benchmarks.python.coverage_sdid`` import resolvable
+# when pytest is invoked from the repo root. The harness itself does a
+# similar sys.path insert at module load; this mirror is only needed for
+# test collection from this file.
+_REPO_ROOT = Path(__file__).resolve().parent.parent.parent
+if str(_REPO_ROOT) not in sys.path:
+    sys.path.insert(0, str(_REPO_ROOT))
+
+from benchmarks.python import coverage_sdid  # noqa: E402
+
+
+def test_stratified_survey_dgp_post_periods_cover_full_post_tail():
+    """The ``stratified_survey`` coverage DGP must not drop any post period.
+
+    Regression against PR #355 R13 P1: the harness previously hard-
+    coded ``range(7, 12)`` as ``post_periods`` even though
+    ``generate_survey_did_data`` is 1-indexed and emits periods
+    1..n_periods, so period 12 was silently included in the pre window.
+    That contaminated the ``stratified_survey × bootstrap`` calibration
+    row and every downstream REGISTRY claim that transcribes from the
+    artifact. The fix derives ``post_periods`` from
+    ``df["period"].max()``; this test fails fast if a future refactor
+    reintroduces the off-by-one.
+    """
+    df, post_periods = coverage_sdid._stratified_survey_dgp(seed=0)
+
+    # Contract: post_periods covers the full tail from cohort onset
+    # through df["period"].max(), with no gaps (unique + sorted +
+    # contiguous + maxed at df["period"].max()).
+    assert len(post_periods) == len(set(post_periods)), (
+        f"post_periods has duplicates: {post_periods}"
+    )
+    assert post_periods == sorted(post_periods), (
+        f"post_periods not sorted: {post_periods}"
+    )
+    gaps = [
+        (a, b) for a, b in zip(post_periods, post_periods[1:]) if b - a != 1
+    ]
+    assert not gaps, (
+        f"post_periods has gaps: {gaps} in {post_periods}"
+    )
+    assert post_periods[-1] == int(df["period"].max()), (
+        f"post_periods max {post_periods[-1]} != df[period].max() "
+        f"{int(df['period'].max())} — DGP drops the last post period "
+        "(off-by-one on 1-indexed generate_survey_did_data)."
+    )
+    # Strong form: cohort onset is 7, n_periods=12 → [7,8,9,10,11,12].
+    assert post_periods == [7, 8, 9, 10, 11, 12], (
+        f"post_periods={post_periods} must equal [7,8,9,10,11,12] for "
+        "the documented 6-pre/6-post survey DGP; any other slice "
+        "changes the calibration interpretation in REGISTRY."
+    )

tests/test_methodology_sdid.py

@@ -3519,46 +3519,3 @@ def test_coverage_artifacts_present(self):
             "(strata/PSU/FPC raises NotImplementedError at fit-time)"
         )
 
-    def test_stratified_survey_dgp_post_periods_cover_full_post_tail(self):
-        """The ``stratified_survey`` coverage DGP must not drop any post period.
-
-        Regression against PR #355 R13 P1: the harness previously hard-
-        coded ``range(7, 12)`` as ``post_periods`` even though
-        ``generate_survey_did_data`` is 1-indexed and emits periods
-        1..n_periods, so period 12 was silently included in the pre
-        window. That contaminated the ``stratified_survey × bootstrap``
-        calibration row and every downstream REGISTRY claim that
-        transcribes from the artifact. The fix derives
-        ``post_periods`` from ``df["period"].max()``; this test fails
-        fast if a future refactor reintroduces the off-by-one.
-        """
-        import importlib
-        coverage_sdid = importlib.import_module("benchmarks.python.coverage_sdid")
-        df, post_periods = coverage_sdid._stratified_survey_dgp(seed=0)
-
-        # Contract: post_periods covers the full tail from cohort onset
-        # through df["period"].max(), with no gaps (unique + sorted +
-        # contiguous + maxed at df["period"].max()).
-        assert len(post_periods) == len(set(post_periods)), (
-            f"post_periods has duplicates: {post_periods}"
-        )
-        assert post_periods == sorted(post_periods), (
-            f"post_periods not sorted: {post_periods}"
-        )
-        gaps = [
-            (a, b) for a, b in zip(post_periods, post_periods[1:]) if b - a != 1
-        ]
-        assert not gaps, (
-            f"post_periods has gaps: {gaps} in {post_periods}"
-        )
-        assert post_periods[-1] == int(df["period"].max()), (
-            f"post_periods max {post_periods[-1]} != df[period].max() "
-            f"{int(df['period'].max())} — DGP drops the last post period "
-            "(off-by-one on 1-indexed generate_survey_did_data)."
-        )
-        # Strong form: cohort onset is 7, n_periods=12 → [7,8,9,10,11,12].
-        assert post_periods == [7, 8, 9, 10, 11, 12], (
-            f"post_periods={post_periods} must equal [7,8,9,10,11,12] for "
-            "the documented 6-pre/6-post survey DGP; any other slice "
-            "changes the calibration interpretation in REGISTRY."
-        )