Allow passing classes directly in Settings

Closes: #1215, #1032, #3870
Thanks

docs/topics/settings.rst

@@ -16,6 +16,33 @@ project (in case you have many).
 
 For a list of available built-in settings see: :ref:`topics-settings-ref`.
 
+.. versionadded:: 2.0
+
+When a setting references an object to be imported by Scrapy, until now
+this had to be a string of an absolute (or project-wide) class path
+(e.g. ``'myproject.extensions.TelnetConsole'``).
+
+Now it is also possible to pass a class object (``TelnetConsole``) directly.
+Passing a classname — to be instanced by Scrapy — is mostly useful in cases
+without a Scrapy project environment, when writing Scrapy Crawlers in a
+single Python source file, or in projects using Scrapy as a library.
+
+Example::
+
+   from mybot.pipelines.validate import ValidateMyItem
+   ITEM_PIPELINES = {
+       # passing the classname...
+       ValidateMyItem: 300,
+       # ...equals passing the class path
+       'mybot.pipelines.validate.ValidateMyItem': 300,
+   }
+
+Passing of already instanced objects is not supported.
+
+.. note::
+    Directly passing a Scrapy class will circumvent Scrapy's
+    deprecation checks for that object.
+
 .. _topics-settings-module-envvar:
 
 Designating the settings

scrapy/utils/deprecate.py

@@ -131,7 +131,7 @@ def _clspath(cls, forced=None):
 def update_classpath(path):
     """Update a deprecated path from an object with its new location"""
     for prefix, replacement in DEPRECATION_RULES:
-        if path.startswith(prefix):
+        if isinstance(path, str) and path.startswith(prefix):
             new_path = path.replace(prefix, replacement, 1)
             warnings.warn("`{}` class is deprecated, use `{}` instead".format(path, new_path),
                           ScrapyDeprecationWarning)

scrapy/utils/misc.py

@@ -37,10 +37,19 @@ def arg_to_iter(arg):
 def load_object(path):
     """Load an object given its absolute object path, and return it.
 
-    object can be the import path of a class, function, variable or an
-    instance, e.g. 'scrapy.downloadermiddlewares.redirect.RedirectMiddleware'
+    The object can be the import path of a class, function, variable or an
+    instance, e.g. 'scrapy.downloadermiddlewares.redirect.RedirectMiddleware'.
+
+    If ``path`` is not a string, but a callable() object, then return it as is.
     """
 
+    if not isinstance(path, str):
+        if callable(path):
+            return path
+        else:
+            raise TypeError("Unexpected argument type, expected string "
+                            "or object, got: %s" % type(path))
+
     try:
         dot = path.rindex('.')
     except ValueError:

tests/test_settings/__init__.py

@@ -382,6 +382,39 @@ def test_getdict_autodegrade_basesettings(self):
         self.assertIn('key', mydict)
         self.assertEqual(mydict['key'], 'val')
 
+    def test_passing_objects_as_values(self):
+        from scrapy.core.downloader.handlers.file import FileDownloadHandler
+        from scrapy.utils.misc import create_instance
+        from scrapy.utils.test import get_crawler
+
+        class TestPipeline():
+            def process_item(self, i, s):
+                return i
+
+        settings = Settings({
+            'ITEM_PIPELINES': {
+                 TestPipeline: 800,
+            },
+            'DOWNLOAD_HANDLERS': {
+                'ftp': FileDownloadHandler,
+            },
+        })
+
+        self.assertIn('ITEM_PIPELINES', settings.attributes)
+
+        mypipeline, priority = settings.getdict('ITEM_PIPELINES').popitem()
+        self.assertEqual(priority, 800)
+        self.assertEqual(mypipeline, TestPipeline)
+        self.assertIsInstance(mypipeline(), TestPipeline)
+        self.assertEqual(mypipeline().process_item('item', None), 'item')
+
+        myhandler = settings.getdict('DOWNLOAD_HANDLERS').pop('ftp')
+        self.assertEqual(myhandler, FileDownloadHandler)
+        myhandler_instance = create_instance(myhandler, None, get_crawler())
+        self.assertIsInstance(myhandler_instance, FileDownloadHandler)
+        self.assertTrue(hasattr(myhandler_instance, 'download_request'))
+
+
 
 if __name__ == "__main__":
     unittest.main()

tests/test_utils_misc/__init__.py

@@ -13,10 +13,13 @@
 class UtilsMiscTestCase(unittest.TestCase):
 
     def test_load_object(self):
+        obj = load_object(load_object)
+        self.assertIs(obj, load_object)
         obj = load_object('scrapy.utils.misc.load_object')
-        assert obj is load_object
+        self.assertIs(obj, load_object)
         self.assertRaises(ImportError, load_object, 'nomodule999.mod.function')
         self.assertRaises(NameError, load_object, 'scrapy.utils.misc.load_object999')
+        self.assertRaises(TypeError, load_object, dict())
 
     def test_walk_modules(self):
         mods = walk_modules('tests.test_utils_misc.test_walk_modules')