Request: remove restriction about errback without callback

scrapy/http/request/__init__.py

@@ -32,7 +32,6 @@ def __init__(self, url, callback=None, method='GET', headers=None, body=None,
             raise TypeError('callback must be a callable, got %s' % type(callback).__name__)
         if errback is not None and not callable(errback):
             raise TypeError('errback must be a callable, got %s' % type(errback).__name__)
-        assert callback or not errback, "Cannot use errback without a callback"
         self.callback = callback
         self.errback = errback
 

tests/test_http_request.py

@@ -246,25 +246,41 @@ def test_immutable_attributes(self):
         self.assertRaises(AttributeError, setattr, r, 'url', 'http://example2.com')
         self.assertRaises(AttributeError, setattr, r, 'body', 'xxx')
 
-    def test_callback_is_callable(self):
+    def test_callback_and_errback(self):
         def a_function():
             pass
-        r = self.request_class('http://example.com')
-        self.assertIsNone(r.callback)
-        r = self.request_class('http://example.com', a_function)
-        self.assertIs(r.callback, a_function)
-        with self.assertRaises(TypeError):
-            self.request_class('http://example.com', 'a_function')
 
-    def test_errback_is_callable(self):
-        def a_function():
-            pass
-        r = self.request_class('http://example.com')
-        self.assertIsNone(r.errback)
-        r = self.request_class('http://example.com', a_function, errback=a_function)
-        self.assertIs(r.errback, a_function)
+        r1 = self.request_class('http://example.com')
+        self.assertIsNone(r1.callback)
+        self.assertIsNone(r1.errback)
+
+        r2 = self.request_class('http://example.com', callback=a_function)
+        self.assertIs(r2.callback, a_function)
+        self.assertIsNone(r2.errback)
+
+        r3 = self.request_class('http://example.com', errback=a_function)
+        self.assertIsNone(r3.callback)
+        self.assertIs(r3.errback, a_function)
+
+        r4 = self.request_class(
+            url='http://example.com',
+            callback=a_function,
+            errback=a_function,
+        )
+        self.assertIs(r4.callback, a_function)
+        self.assertIs(r4.errback, a_function)
+
+    def test_callback_and_errback_type(self):
+        with self.assertRaises(TypeError):
+            self.request_class('http://example.com', callback='a_function')
         with self.assertRaises(TypeError):
-            self.request_class('http://example.com', a_function, errback='a_function')
+            self.request_class('http://example.com', errback='a_function')
+        with self.assertRaises(TypeError):
+            self.request_class(
+                url='http://example.com',
+                callback='a_function',
+                errback='a_function',
+            )
 
     def test_from_curl(self):
         # Note: more curated tests regarding curl conversion are in