[WIP] Minimize provider calls

tests/test_injection.py

@@ -29,6 +29,7 @@
     NonCallableProviderError,
     UndeclaredProvidedTypeError,
 )
+from scrapy_poet.page_input_providers import ItemProvider
 
 
 def get_provider(classes, content=None):
@@ -270,6 +271,62 @@ def callback(
             "d": ClsNoProviderRequired,
         }
 
+    @inlineCallbacks
+    def test_build_callback_dependencies_minimize_provider_calls(self):
+        """Test that build_callback_dependencies does not call any given
+        provider more times than it needs when one provided class is requested
+        directly while another is a page object dependency requested through
+        an item."""
+
+        class ExpensiveDependency1:
+            pass
+
+        class ExpensiveDependency2:
+            pass
+
+        class ExpensiveProvider(PageObjectInputProvider):
+            provided_classes = {ExpensiveDependency1, ExpensiveDependency2}
+
+            def __call__(self, to_provide):
+                if to_provide != self.provided_classes:
+                    raise RuntimeError(
+                        "The expensive dependency provider has been called "
+                        "with a subset of the classes that it provides and "
+                        "that are required for the callback in this test."
+                    )
+                return [cls() for cls in to_provide]
+
+        @attr.define
+        class MyItem:
+            pass
+
+        @attr.define
+        class MyPage(ItemPage[MyItem]):
+            expensive: ExpensiveDependency2
+
+        def callback(
+            expensive: ExpensiveDependency1,
+            item: MyItem,
+        ):
+            pass
+
+        providers = {
+            ItemProvider: 1,
+            ExpensiveProvider: 2,
+        }
+        injector = get_injector_for_testing(providers)
+        injector.registry.add_rule(ApplyRule("", use=MyPage, to_return=MyItem))
+        response = get_response_for_testing(callback)
+
+        # This would raise RuntimeError if expectations are not met.
+        kwargs = yield from injector.build_callback_dependencies(
+            response.request, response
+        )
+
+        # Make sure the test does not simply pass because some dependencies were
+        # not injected at all.
+        assert set(kwargs.keys()) == {"expensive", "item"}
+
 
 class Html(Injectable):
     url = "http://example.com"