Check the setting values before overriding.

scrapy_zyte_api/addon.py

@@ -1,10 +1,36 @@
+from typing import Optional
+
 from scrapy.settings import BaseSettings
 
 from scrapy_zyte_api import ScrapyZyteAPIDownloaderMiddleware
 
 
 class ScrapyZyteAPIAddon:
+    @staticmethod
+    def _check_settings(settings: BaseSettings) -> Optional[str]:
+        if (
+            settings.getwithbase("DOWNLOAD_HANDLERS")["http"]
+            != "scrapy.core.downloader.handlers.http.HTTPDownloadHandler"
+        ):
+            return "'http' value in the 'DOWNLOAD_HANDLERS'"
+        if (
+            settings.getwithbase("DOWNLOAD_HANDLERS")["https"]
+            != "scrapy.core.downloader.handlers.http.HTTPDownloadHandler"
+        ):
+            return "'https' value in the 'DOWNLOAD_HANDLERS'"
+        if (
+            settings.get("REQUEST_FINGERPRINTER_CLASS")
+            != "scrapy.utils.request.RequestFingerprinter"
+        ):
+            return "'REQUEST_FINGERPRINTER_CLASS'"
+        return None
+
     def update_settings(self, settings: BaseSettings) -> None:
+        nondefault_msg = self._check_settings(settings)
+        if nondefault_msg:
+            raise ValueError(
+                f"The {nondefault_msg} setting is set to a custom value, refusing to override it"
+            )
         settings["DOWNLOAD_HANDLERS"][
             "http"
         ] = "scrapy_zyte_api.ScrapyZyteAPIDownloadHandler"

tests/test_handler.py

@@ -18,7 +18,14 @@
 
 from scrapy_zyte_api.handler import ScrapyZyteAPIDownloadHandler
 
-from . import DEFAULT_CLIENT_CONCURRENCY, SETTINGS, UNSET, make_handler, set_env
+from . import (
+    DEFAULT_CLIENT_CONCURRENCY,
+    SETTINGS,
+    SETTINGS_ADDON,
+    UNSET,
+    make_handler,
+    set_env,
+)
 
 
 @pytest.mark.parametrize(
@@ -439,3 +446,18 @@ async def test_addon(mockserver):
         request = Request("https://example.com", meta=meta)
         await handler.download_request(request, None)
         assert handler._stats.get_value("scrapy-zyte-api/success") == 1
+
+
+@ensureDeferred
+async def test_addon_check_settings():
+    settings: Dict[str, Any]
+    with pytest.raises(
+        ValueError, match="The 'http' value in the 'DOWNLOAD_HANDLERS' setting is set"
+    ):
+        settings = {**SETTINGS_ADDON, "DOWNLOAD_HANDLERS": {"http": "foo"}}
+        get_crawler(settings_dict=settings)
+    with pytest.raises(
+        ValueError, match="The 'REQUEST_FINGERPRINTER_CLASS' setting is set"
+    ):
+        settings = {**SETTINGS_ADDON, "REQUEST_FINGERPRINTER_CLASS": "foo"}
+        get_crawler(settings_dict=settings)