fix: keep the modules cache when the downloaded JSON is malformed or has no modules

lean/models/__init__.py

@@ -30,10 +30,16 @@ def _download_modules_json() -> None:
     from requests import get
     res = get(url, timeout=5)
     if res.ok:
-        from json import dump
+        from json import dumps
+        # Decoded and checked before the file is opened, since opening it for writing empties it: a response that
+        # isn't a modules file would replace a usable one with one the CLI can't start from
+        data = res.json()
+        if not isinstance(data, dict) or "modules" not in data:
+            raise ValueError(f"{url} returned no modules")
+        content = dumps(data, ensure_ascii=False, indent=4)
         file_path.parent.mkdir(parents=True, exist_ok=True)
         with open(file_path, 'w', encoding='utf-8') as f:
-            dump(res.json(), f, ensure_ascii=False, indent=4)
+            f.write(content)
     else:
         res.raise_for_status()
 

tests/models/test_download_modules_json.py

@@ -0,0 +1,54 @@
+# QUANTCONNECT.COM - Democratizing Finance, Empowering Individuals.
+# Lean CLI v1.0. Copyright 2021 QuantConnect Corporation.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import json
+from pathlib import Path
+
+import pytest
+from responses import RequestsMock
+
+import lean.models as models
+
+
+def _write_cache(path: Path) -> str:
+    content = json.dumps({"modules": {"SavedModule": {"version": "1.0"}}})
+    path.write_text(content, encoding="utf-8")
+    return content
+
+
+@pytest.mark.parametrize("body", ["not json", "", "{}", "[]", "null"])
+def test_download_modules_json_keeps_cache_when_response_has_no_modules(requests_mock: RequestsMock,
+                                                                        monkeypatch: pytest.MonkeyPatch,
+                                                                        body: str) -> None:
+    cache = Path.cwd() / "modules.json"
+    content = _write_cache(cache)
+    monkeypatch.setattr(models, "file_path", cache)
+    requests_mock.add(requests_mock.GET, models.url, body=body, status=200)
+
+    with pytest.raises(ValueError):
+        models._download_modules_json()
+
+    assert cache.read_text(encoding="utf-8") == content
+
+
+def test_download_modules_json_replaces_cache_with_valid_response(requests_mock: RequestsMock,
+                                                                  monkeypatch: pytest.MonkeyPatch) -> None:
+    cache = Path.cwd() / "modules.json"
+    _write_cache(cache)
+    monkeypatch.setattr(models, "file_path", cache)
+    new_modules = {"modules": {"NewModule": {"version": "2.0"}}}
+    requests_mock.add(requests_mock.GET, models.url, json=new_modules, status=200)
+
+    models._download_modules_json()
+
+    assert json.loads(cache.read_text(encoding="utf-8")) == new_modules