From 3e726b9df721f2288f4ae9a685de9bdff0762c06 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Iv=C3=A1n=20de=20Prado?= <ivan.prado@gmail.com>
Date: Tue, 25 Aug 2020 11:22:05 +0100
Subject: [PATCH] Support for delegated methods as callbacks

It can be useful to structure the spiders code around some helper classes.
---
 scrapy/utils/reqser.py     | 29 ++++++++++++++---------------
 tests/test_utils_reqser.py | 12 ++++++++++++
 2 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/scrapy/utils/reqser.py b/scrapy/utils/reqser.py
index 5ea2aafb8..35a4fc72c 100644
--- a/scrapy/utils/reqser.py
+++ b/scrapy/utils/reqser.py
@@ -73,23 +73,22 @@ def request_from_dict(d, spider=None):
 def _find_method(obj, func):
     if obj:
         try:
-            func_self = func.__self__
-        except AttributeError:  # func has no __self__
+            func.__func__
+        except AttributeError:  # func is not a instance method. Not supported.
             pass
         else:
-            if func_self is obj:
-                members = inspect.getmembers(obj, predicate=inspect.ismethod)
-                for name, obj_func in members:
-                    # We need to use __func__ to access the original
-                    # function object because instance method objects
-                    # are generated each time attribute is retrieved from
-                    # instance.
-                    #
-                    # Reference: The standard type hierarchy
-                    # https://docs.python.org/3/reference/datamodel.html
-                    if obj_func.__func__ is func.__func__:
-                        return name
-    raise ValueError("Function %s is not a method of: %s" % (func, obj))
+            members = inspect.getmembers(obj, predicate=inspect.ismethod)
+            for name, obj_func in members:
+                # We need to use __func__ to access the original
+                # function object because instance method objects
+                # are generated each time attribute is retrieved from
+                # instance.
+                #
+                # Reference: The standard type hierarchy
+                # https://docs.python.org/3/reference/datamodel.html
+                if obj_func.__func__ is func.__func__:
+                    return name
+    raise ValueError("Function %s is not an instance method in: %s" % (func, obj))
 
 
 def _get_method(obj, name):
diff --git a/tests/test_utils_reqser.py b/tests/test_utils_reqser.py
index de94ec960..c8d1db138 100644
--- a/tests/test_utils_reqser.py
+++ b/tests/test_utils_reqser.py
@@ -102,6 +102,12 @@ class RequestSerializationTest(unittest.TestCase):
                     errback=self.spider.handle_error)
         self._assert_serializes_ok(r, spider=self.spider)
 
+    def test_delegated_callback_serialization(self):
+        r = Request("http://www.example.com",
+                    callback=self.spider.delegated_callback,
+                    errback=self.spider.handle_error)
+        self._assert_serializes_ok(r, spider=self.spider)
+
     def test_unserializable_callback1(self):
         r = Request("http://www.example.com", callback=lambda x: x)
         self.assertRaises(ValueError, request_to_dict, r)
@@ -131,6 +137,9 @@ class TestSpiderMixin:
     def __mixin_callback(self, response):
         pass
 
+class TestSpiderDelegation:
+    def delegated_callback(self, response):
+        pass
 
 def parse_item(response):
     pass
@@ -155,6 +164,9 @@ class TestSpider(Spider, TestSpiderMixin):
     __parse_item_reference = private_parse_item
     __handle_error_reference = private_handle_error
 
+    def __init__(self):
+        self.delegated_callback = TestSpiderDelegation().delegated_callback
+
     def parse_item(self, response):
         pass