Merge pull request #1187 from Curita/relax-spiderloader-check

Relax SpiderLoader interface check

scrapy/crawler.py

@@ -3,7 +3,7 @@ import signal
 import warnings
 
 from twisted.internet import reactor, defer
-from zope.interface.verify import verifyClass
+from zope.interface.verify import verifyClass, DoesNotImplement
 
 from scrapy.core.engine import ExecutionEngine
 from scrapy.resolver import CachingThreadedResolver
@@ -195,5 +195,13 @@ def _get_spider_loader(settings):
     cls_path = settings.get('SPIDER_MANAGER_CLASS',
                             settings.get('SPIDER_LOADER_CLASS'))
     loader_cls = load_object(cls_path)
-    verifyClass(ISpiderLoader, loader_cls)
+    try:
+        verifyClass(ISpiderLoader, loader_cls)
+    except DoesNotImplement:
+        warnings.warn(
+            'SPIDER_LOADER_CLASS (previously named SPIDER_MANAGER_CLASS) does '
+            'not fully implement scrapy.interfaces.ISpiderLoader interface. '
+            'Please add all missing methods to avoid unexpected runtime errors.',
+            category=ScrapyDeprecationWarning, stacklevel=2
+        )
     return loader_cls.from_settings(settings.frozencopy())

tests/test_crawler.py

@@ -1,8 +1,6 @@
 import warnings
 import unittest
 
-from zope.interface.verify import DoesNotImplement
-
 from scrapy.crawler import Crawler, CrawlerRunner
 from scrapy.settings import Settings, default_settings
 from scrapy.spiderloader import SpiderLoader
@@ -71,8 +69,12 @@ class CrawlerRunnerTestCase(unittest.TestCase):
         settings = Settings({
             'SPIDER_LOADER_CLASS': 'tests.test_crawler.SpiderLoaderWithWrongInterface'
         })
-        with self.assertRaises(DoesNotImplement):
+        with warnings.catch_warnings(record=True) as w, \
+                self.assertRaises(AttributeError):
             CrawlerRunner(settings)
+            self.assertEqual(len(w), 1)
+            self.assertIn("SPIDER_LOADER_CLASS", str(w[0].message))
+            self.assertIn("scrapy.interfaces.ISpiderLoader", str(w[0].message))
 
     def test_crawler_runner_accepts_dict(self):
         runner = CrawlerRunner({'foo': 'bar'})