Store failure cause and provide to _get_preference

src/resolvelib/resolvers.py

@@ -99,7 +99,7 @@ def __init__(self, round_count):
 
 
 # Resolution state in a round.
-State = collections.namedtuple("State", "mapping criteria")
+State = collections.namedtuple("State", "mapping criteria backtrack_causes")
 
 
 class Resolution(object):
@@ -131,6 +131,7 @@ def _push_new_state(self):
         state = State(
             mapping=base.mapping.copy(),
             criteria=base.criteria.copy(),
+            backtrack_causes=base.backtrack_causes[:],
         )
         self._states.append(state)
 
@@ -185,6 +186,7 @@ def _get_preference(self, name):
                 self.state.criteria,
                 operator.attrgetter("information"),
             ),
+            backtrack_causes=self.state.backtrack_causes,
         )
 
     def _is_current_pin_satisfying(self, name, criterion):
@@ -335,7 +337,13 @@ def resolve(self, requirements, max_rounds):
         self._r.starting()
 
         # Initialize the root state.
-        self._states = [State(mapping=collections.OrderedDict(), criteria={})]
+        self._states = [
+            State(
+                mapping=collections.OrderedDict(),
+                criteria={},
+                backtrack_causes=[],
+            )
+        ]
         for r in requirements:
             try:
                 self._add_to_criteria(self.state.criteria, r, parent=None)
@@ -369,11 +377,13 @@ def resolve(self, requirements, max_rounds):
                 # Backtrack if pinning fails. The backtrack process puts us in
                 # an unpinned state, so we can work on it in the next round.
                 success = self._backtrack()
+                self.state.backtrack_causes[:] = [
+                    i for c in failure_causes for i in c.information
+                ]
 
                 # Dead ends everywhere. Give up.
                 if not success:
-                    causes = [i for c in failure_causes for i in c.information]
-                    raise ResolutionImpossible(causes)
+                    raise ResolutionImpossible(self.state.backtrack_causes)
             else:
                 # Pinning was successful. Push a new state to do another pin.
                 self._push_new_state()

tests/functional/cocoapods/test_resolvers_cocoapods.py

@@ -105,7 +105,14 @@ def __init__(self, filename):
     def identify(self, requirement_or_candidate):
         return requirement_or_candidate.name
 
-    def get_preference(self, identifier, resolutions, candidates, information):
+    def get_preference(
+        self,
+        identifier,
+        resolutions,
+        candidates,
+        information,
+        backtrack_causes,
+    ):
         return sum(1 for _ in candidates[identifier])
 
     def _iter_matches(self, name, requirements, incompatibilities):

tests/functional/python/test_resolvers_python.py

@@ -72,7 +72,14 @@ def identify(self, requirement_or_candidate):
             return "{}[{}]".format(name, extras_str)
         return name
 
-    def get_preference(self, identifier, resolutions, candidates, information):
+    def get_preference(
+        self,
+        identifier,
+        resolutions,
+        candidates,
+        information,
+        backtrack_causes,
+    ):
         transitive = all(p is not None for _, p in information[identifier])
         return (transitive, identifier)
 

tests/functional/swift-package-manager/test_resolvers_swift.py

@@ -79,7 +79,14 @@ def __init__(self, filename):
     def identify(self, requirement_or_candidate):
         return requirement_or_candidate.container["identifier"]
 
-    def get_preference(self, identifier, resolutions, candidates, information):
+    def get_preference(
+        self,
+        identifier,
+        resolutions,
+        candidates,
+        information,
+        backtrack_causes,
+    ):
         return sum(1 for _ in candidates[identifier])
 
     def _iter_matches(self, identifier, requirements, incompatibilities):