create a weak_cache in Injector

docs/providers.rst

@@ -127,6 +127,14 @@ which Scrapy has. Although they are quite similar in its intended purpose,
 could be anything that could stretch beyond Scrapy's ``Responses`` `(e.g. Network
 Database queries, API Calls, AWS S3 files, etc)`.
 
+.. note::
+
+   The :class:`scrapy_poet.injection.Injector` maintains a ``.weak_cache`` which
+   stores the instances created by the providers as long as the corresponding
+   :class:`scrapy.Request <scrapy.http.Request>` instance exists. This means that
+   the instances created by earlier providers can be accessed and reused by latter
+   providers. This is turned on by default and the instances are stored in memory.
+
 
 Configuring providers
 =====================

scrapy_poet/injection.py

@@ -5,6 +5,7 @@
 import pprint
 import warnings
 from typing import Any, Callable, Dict, List, Mapping, Optional, Set, Type, cast
+from weakref import WeakKeyDictionary
 
 import andi
 from andi.typeutils import issubclass_safe
@@ -95,6 +96,11 @@ def init_cache(self):  # noqa: D102
                 f"Cache enabled. Folder: {cache_path!r}. Caching errors: {self.caching_errors}"
             )
 
+        # This is different from the cache above as it only stores instances as long
+        # as the request exists. This is useful for latter providers to re-use the
+        # already built instances by earlier providers.
+        self.weak_cache: WeakKeyDictionary[Request, Dict] = WeakKeyDictionary()
+
     def available_dependencies_for_providers(
         self, request: Request, response: Response
     ):  # noqa: D102
@@ -294,6 +300,11 @@ def build_instances_from_providers(
                 )
             instances.update(objs_by_type)
 
+            if self.weak_cache.get(request):
+                self.weak_cache[request].update(objs_by_type)
+            else:
+                self.weak_cache[request] = objs_by_type
+
             if self.cache and not cache_hit:
                 # Save the results in the cache
                 self.cache[fingerprint] = serialize(objs)

tests/test_injection.py

@@ -203,6 +203,7 @@ def callback(
             ClsReqResponse: ClsReqResponse(),
             ClsNoProviderRequired: ClsNoProviderRequired(),
         }
+        assert injector.weak_cache.get(request).keys() == {ClsReqResponse, Cls1, Cls2}
 
         instances = yield from injector.build_instances_from_providers(
             request, response, plan
@@ -212,6 +213,7 @@ def callback(
             Cls2: Cls2(),
             ClsReqResponse: ClsReqResponse(),
         }
+        assert injector.weak_cache.get(request).keys() == {ClsReqResponse, Cls1, Cls2}
 
     @inlineCallbacks
     def test_build_instances_from_providers_unexpected_return(self):
@@ -230,6 +232,7 @@ def callback(response: DummyResponse, a: Cls1):
             yield from injector.build_instances_from_providers(
                 response.request, response, plan
             )
+        assert injector.weak_cache.get(response.request) is None
 
         assert "Provider" in str(exinf.value)
         assert "Cls2" in str(exinf.value)
@@ -256,6 +259,7 @@ def callback(response: DummyResponse, arg: str):
         instances = yield from injector.build_instances_from_providers(
             response.request, response, plan
         )
+        assert injector.weak_cache.get(response.request).keys() == {str}
 
         assert instances[str] == min(str_list)
 
@@ -628,6 +632,7 @@ def callback_factory():
             if name.startswith(prefix)
         }
         assert set(poet_stats) == expected
+        assert injector.weak_cache.get(response.request) is None
 
     @inlineCallbacks
     def test_po_provided_via_item(self):
@@ -642,6 +647,7 @@ def callback(response: DummyResponse, item: TestItem):
         _ = yield from injector.build_callback_dependencies(response.request, response)
         key = "poet/injector/tests.test_injection.TestItemPage"
         assert key in set(injector.crawler.stats.get_stats())
+        assert injector.weak_cache.get(response.request) is None
 
 
 class TestInjectorOverrides:
@@ -787,6 +793,7 @@ def callback(response: DummyResponse, arg_price: Price, arg_name: Name):
         response.request, response, plan
     )
     assert cache.exists()
+    assert injector.weak_cache.get(response.request).keys() == {Price, Name}
 
     validate_instances(instances)
 
@@ -799,6 +806,7 @@ def callback(response: DummyResponse, arg_price: Price, arg_name: Name):
         instances = yield from injector.build_instances_from_providers(
             response.request, response, plan
         )
+    assert injector.weak_cache.get(response.request) is None
 
     # Different providers. They return a different result, but the cache data should prevail.
     providers = {
@@ -812,6 +820,7 @@ def callback(response: DummyResponse, arg_price: Price, arg_name: Name):
     instances = yield from injector.build_instances_from_providers(
         response.request, response, plan
     )
+    assert injector.weak_cache.get(response.request).keys() == {Price, Name}
 
     validate_instances(instances)
 
@@ -823,3 +832,4 @@ def callback(response: DummyResponse, arg_price: Price, arg_name: Name):
         instances = yield from injector.build_instances_from_providers(
             response.request, response, plan
         )
+    assert injector.weak_cache.get(response.request) is None