Implement Ken French factor download and cache fallback

factor_attribution.py

@@ -4,12 +4,32 @@ import io
 import zipfile
 from pathlib import Path
 from urllib.error import URLError
+from urllib.request import Request, urlopen
 
 import pandas as pd
 
+KEN_FRENCH_DAILY_FF5_ZIP_URL = (
+    "https://mba.tuck.dartmouth.edu/pages/faculty/ken.french/ftp/"
+    "F-F_Research_Data_5_Factors_2x3_daily_CSV.zip"
+)
+
+SOURCE_PARSE_EXCEPTIONS = (
+    zipfile.BadZipFile,
+    UnicodeDecodeError,
+    StopIteration,
+    KeyError,
+    ValueError,
+    pd.errors.ParserError,
+)
+
 
 def _download_kf_zip_bytes() -> bytes:
-    raise NotImplementedError
+    request = Request(
+        KEN_FRENCH_DAILY_FF5_ZIP_URL,
+        headers={"User-Agent": "quant-factor-attribution/0.1"},
+    )
+    with urlopen(request, timeout=30) as response:
+        return response.read()
 
 
 def _parse_kf_daily_csv(raw_bytes: bytes) -> pd.DataFrame:
@@ -46,6 +66,12 @@ def load_external_us_factors(cache_dir: Path | str = "data/factors") -> pd.DataF
             return pd.read_csv(cache_path, index_col=0, parse_dates=True)
         raise
 
+    try:
         factors = _parse_kf_daily_csv(raw_bytes)
+    except SOURCE_PARSE_EXCEPTIONS:
+        if cache_path.exists():
+            return pd.read_csv(cache_path, index_col=0, parse_dates=True)
+        raise
+
     factors.to_csv(cache_path)
     return factors

tests/test_factor_attribution.py

@@ -8,10 +8,28 @@ from unittest import mock
 
 import pandas as pd
 
-from factor_attribution import load_external_us_factors
+from factor_attribution import (
+    KEN_FRENCH_DAILY_FF5_ZIP_URL,
+    _download_kf_zip_bytes,
+    load_external_us_factors,
+)
 
 
 class ExternalFactorLoaderTests(unittest.TestCase):
+    def test_download_kf_zip_bytes_fetches_official_ken_french_zip(self):
+        response = mock.MagicMock()
+        response.read.return_value = b"zip-bytes"
+        response.__enter__.return_value = response
+        response.__exit__.return_value = False
+
+        with mock.patch("factor_attribution.urlopen", return_value=response) as mocked_urlopen:
+            raw_bytes = _download_kf_zip_bytes()
+
+        self.assertEqual(raw_bytes, b"zip-bytes")
+        request = mocked_urlopen.call_args.args[0]
+        self.assertEqual(request.full_url, KEN_FRENCH_DAILY_FF5_ZIP_URL)
+        self.assertEqual(mocked_urlopen.call_args.kwargs["timeout"], 30)
+
     def test_load_external_us_factors_parses_percent_values_and_dates_from_zip_payload(self):
         csv_text = (
             "This line is ignored\n"
@@ -65,7 +83,7 @@ class ExternalFactorLoaderTests(unittest.TestCase):
         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):
+    def test_load_external_us_factors_falls_back_to_cache_when_parse_fails(self):
         cached = pd.DataFrame(
             {
                 "MKT_RF": [0.01],
@@ -85,8 +103,31 @@ class ExternalFactorLoaderTests(unittest.TestCase):
                 "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)
+                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_surfaces_cache_write_failures(self):
+        csv_text = (
+            "This line is ignored\n"
+            ",Mkt-RF,SMB,HML,RMW,CMA,RF\n"
+            "20260102,1.00,0.50,-0.25,0.10,-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=zip_bytes,
+            ):
+                with mock.patch("pandas.DataFrame.to_csv", side_effect=OSError("disk full")):
+                    with self.assertRaises(OSError):
+                        load_external_us_factors(cache_dir=Path(tmpdir))
 
     def _make_zip_bytes(self, filename: str, contents: str) -> bytes:
         buffer = io.BytesIO()