Find wppm from the checkout, not from luck

changelog_files.py imported wppm and CI answered ModuleNotFoundError. Running a script puts the script's own directory on sys.path -- here .github/scripts -- and not the checkout root, so the import fell through to whatever else was reachable. On a developer machine that is the installed wppm and everything looks fine; a runner installs none, so the step died. The workflow job would have died the same way, not just the tests. The path is now derived from __file__ and inserted first, so the wppm imported is the one being released. The tests missed it for the same reason CI caught it, so they now run the script by absolute path from an unrelated directory, with a decoy wppm on PYTHONPATH. The insert puts the checkout ahead of that decoy; without it the decoy wins and the test fails, on any machine. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01MBk5k7WdpPvx4SUFyEk3U3
stonebig · Sep 6, 2026 · Sep 6, 2026 · Sep 6, 2026 · Sep 6, 2026 · dab14efd1bd26c85a1def2454c140ab6c64f83cf
commit dab14efd1bd26c85a1def2454c140ab6c64f83cf
diff --git a/.github/scripts/changelog_files.py b/.github/scripts/changelog_files.py
@@ -13,16 +13,21 @@
 only a checkout of the repository has that to compare against. Ordering is
 `wppm.diff`'s job; this decides what to hand it.
 
-Run from the checkout root, so `from wppm import diff` finds the wppm being
-released rather than an installed one.
 """
 import re
 import shutil
 import sys
 from pathlib import Path
 
-from wppm import diff
-from wppm.diff import version  # packaging, or pip's vendored copy
+# Running a script puts the script's own directory on sys.path, not the
+# checkout root, so wppm has to be found deliberately: this file is
+# .github/scripts/changelog_files.py, hence two levels up. Without it the
+# import finds whatever wppm happens to be installed, or -- as on a CI runner,
+# which installs none -- nothing at all.
+sys.path.insert(0, str(Path(__file__).resolve().parents[2]))
+
+from wppm import diff  # noqa: E402  the path above has to be set first
+from wppm.diff import version  # noqa: E402  packaging, or pip's vendored copy
 
 # WinPythonslim-64bit-3.15.0.5b1.md -- the flavor may be empty, and the version
 # may carry a release level, which is why the parser decides and not the regex

diff --git a/tests/test_changelog_files.py b/tests/test_changelog_files.py
@@ -11,6 +11,7 @@
 only the repository has.
 """
 import importlib.util
+import os
 import shutil
 import subprocess
 import sys
@@ -87,6 +88,19 @@ def test_directories_are_skipped(self, changelog_files, tmp_path):
         ]
 
 
+def run_script(*args, cwd, env=None):
+    """The script by absolute path, from an unrelated directory.
+
+    It must not need to be run from the checkout: the workflow's own step and
+    these tests both invoke it as a path, which puts .github/scripts on
+    sys.path rather than the repository root.
+    """
+    return subprocess.run(
+        [sys.executable, str(SCRIPT), *map(str, args)],
+        cwd=str(cwd), capture_output=True, text=True, env=env,
+    )
+
+
 @needs_script
 class TestEndToEnd:
     """Run the script the way the workflow runs it."""
@@ -111,12 +125,30 @@ def cycle(self, tmp_path):
         (source / "hashes_3.15.0.5slimb1.md").write_text("x", encoding="utf-8")
         return source, changelogs
 
-    def test_files_the_three_and_writes_the_history(self, cycle):
+    def test_wppm_comes_from_the_checkout(self, cycle, tmp_path):
+        """A decoy wppm on PYTHONPATH must lose to the one being released.
+
+        Running a script puts the script's directory on sys.path, not the
+        checkout root, so without a deliberate insert the import falls through
+        to whatever else is reachable. On a machine with wppm installed that
+        looks fine -- which is how it once slipped past a green local run --
+        and on a CI runner, which installs none, it is ModuleNotFoundError.
+        The decoy makes the difference visible either way.
+        """
         source, changelogs = cycle
-        proc = subprocess.run(
-            [sys.executable, ".github/scripts/changelog_files.py", str(source), str(changelogs)],
-            cwd=REPO, capture_output=True, text=True,
+        decoy = tmp_path / "decoy"
+        (decoy / "wppm").mkdir(parents=True)
+        (decoy / "wppm" / "__init__.py").write_text(
+            "raise RuntimeError('decoy wppm imported')", encoding="utf-8"
         )
+        env = {**os.environ, "PYTHONPATH": str(decoy)}
+        proc = run_script(source, changelogs, cwd=tmp_path, env=env)
+        assert proc.returncode == 0, proc.stderr
+        assert "decoy" not in proc.stderr
+
+    def test_files_the_three_and_writes_the_history(self, cycle, tmp_path):
+        source, changelogs = cycle
+        proc = run_script(source, changelogs, cwd=tmp_path)
         assert proc.returncode == 0, proc.stderr
         landed = sorted(p.name for p in changelogs.iterdir())
         assert landed == [
@@ -127,12 +159,10 @@ def test_files_the_three_and_writes_the_history(self, cycle):
             "requir.64-3_15_0_5slimb1.txt",
         ]
 
-    def test_the_history_names_the_release_it_compares_against(self, cycle):
+    def test_the_history_names_the_release_it_compares_against(self, cycle, tmp_path):
         source, changelogs = cycle
-        subprocess.run(
-            [sys.executable, ".github/scripts/changelog_files.py", str(source), str(changelogs)],
-            cwd=REPO, capture_output=True, text=True, check=True,
-        )
+        proc = run_script(source, changelogs, cwd=tmp_path)
+        assert proc.returncode == 0, proc.stderr
         history = (changelogs / "WinPythonslim-64bit-3.15.0.5b1_History.md").read_text(
             encoding="utf-8"
         )
@@ -144,18 +174,11 @@ def test_an_empty_metadata_directory_is_an_error(self, tmp_path):
         source, changelogs = tmp_path / "src", tmp_path / "changelogs"
         source.mkdir()
         changelogs.mkdir()
-        proc = subprocess.run(
-            [sys.executable, ".github/scripts/changelog_files.py", str(source), str(changelogs)],
-            cwd=REPO, capture_output=True, text=True,
-        )
+        proc = run_script(source, changelogs, cwd=tmp_path)
         assert proc.returncode != 0
         assert "held no changelog" in proc.stdout + proc.stderr
 
     def test_a_missing_directory_is_an_error(self, tmp_path):
-        proc = subprocess.run(
-            [sys.executable, ".github/scripts/changelog_files.py",
-             str(tmp_path / "nope"), str(tmp_path)],
-            cwd=REPO, capture_output=True, text=True,
-        )
+        proc = run_script(tmp_path / "nope", tmp_path, cwd=tmp_path)
         assert proc.returncode != 0
         assert "no such directory" in proc.stdout + proc.stderr