Harden factor loader zip parsing and fallback

tests/test_factor_attribution.py

@@ -1,6 +1,9 @@
+import io
 import tempfile
 import unittest
+import zipfile
 from pathlib import Path
+from urllib.error import URLError
 from unittest import mock
 
 import pandas as pd
@@ -9,7 +12,7 @@ from factor_attribution import load_external_us_factors
 
 
 class ExternalFactorLoaderTests(unittest.TestCase):
-    def test_load_external_us_factors_parses_percent_values_and_dates(self):
+    def test_load_external_us_factors_parses_percent_values_and_dates_from_zip_payload(self):
         csv_text = (
             "This line is ignored\n"
             ",Mkt-RF,SMB,HML,RMW,CMA,RF\n"
@@ -17,11 +20,15 @@ class ExternalFactorLoaderTests(unittest.TestCase):
             "20260105,-0.20,0.10,0.30,-0.15,0.05,0.02\n"
             "\n"
         )
+        zip_bytes = self._make_zip_bytes(
+            "F-F_Research_Data_5_Factors_2x3_daily.csv",
+            csv_text,
+        )
 
         with tempfile.TemporaryDirectory() as tmpdir:
             with mock.patch(
                 "factor_attribution._download_kf_zip_bytes",
-                return_value=csv_text.encode("utf-8"),
+                return_value=zip_bytes,
             ):
                 factors = load_external_us_factors(cache_dir=Path(tmpdir))
 
@@ -51,9 +58,38 @@ class ExternalFactorLoaderTests(unittest.TestCase):
             cached.to_csv(cache_dir / "ff5_us_daily.csv")
             with mock.patch(
                 "factor_attribution._download_kf_zip_bytes",
-                side_effect=RuntimeError("boom"),
+                side_effect=URLError("boom"),
             ):
                 factors = load_external_us_factors(cache_dir=cache_dir)
 
         self.assertEqual(len(factors), 1)
         self.assertAlmostEqual(factors.iloc[0]["MKT_RF"], 0.01)
+
+    def test_load_external_us_factors_raises_parse_errors_instead_of_using_cache(self):
+        cached = pd.DataFrame(
+            {
+                "MKT_RF": [0.01],
+                "SMB": [0.0],
+                "HML": [0.0],
+                "RMW": [0.0],
+                "CMA": [0.0],
+                "RF": [0.0001],
+            },
+            index=pd.to_datetime(["2026-01-02"]),
+        )
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            cache_dir = Path(tmpdir)
+            cached.to_csv(cache_dir / "ff5_us_daily.csv")
+            with mock.patch(
+                "factor_attribution._download_kf_zip_bytes",
+                return_value=b"not-a-zip-file",
+            ):
+                with self.assertRaises(zipfile.BadZipFile):
+                    load_external_us_factors(cache_dir=cache_dir)
+
+    def _make_zip_bytes(self, filename: str, contents: str) -> bytes:
+        buffer = io.BytesIO()
+        with zipfile.ZipFile(buffer, mode="w") as archive:
+            archive.writestr(filename, contents)
+        return buffer.getvalue()