Bug in version solver when using a hashable object to represent a package, not just a string.

[Greedquest]

I believe it's this line:

mixology/mixology/version_solver.py

Line 130 in 067c5ae

 changed.add(str(self._propagate_incompatibility(root_cause))) 

self._propagate_incompatibility will return the _conflict type which is the hashable type used to represent a package (in my usecase this is not a string). The line converts it to str which is a problem since then it is no longer a valid key in the self._incompatibilities dictionary (line 113 of the same function).

mixology/mixology/version_solver.py

Lines 98 to 133 in 067c5ae

     def _propagate(self, package):  # type: (Hashable) -> None 
         """ 
         Performs unit propagation on incompatibilities transitively 
         related to package to derive new assignments for _solution. 
         """ 
         changed = set() 
         changed.add(package) 
  
         while changed: 
             package = changed.pop() 
  
             # Iterate in reverse because conflict resolution tends to produce more 
             # general incompatibilities as time goes on. If we look at those first, 
             # we can derive stronger assignments sooner and more eagerly find 
             # conflicts. 
             for incompatibility in reversed(self._incompatibilities[package]): 
                 result = self._propagate_incompatibility(incompatibility) 
  
                 if result is _conflict: 
                     # If the incompatibility is satisfied by the solution, we use 
                     # _resolve_conflict() to determine the root cause of the conflict as a 
                     # new incompatibility. 
                     # 
                     # It also backjumps to a point in the solution 
                     # where that incompatibility will allow us to derive new assignments 
                     # that avoid the conflict. 
                     root_cause = self._resolve_conflict(incompatibility) 
  
                     # Back jumping erases all the assignments we did at the previous 
                     # decision level, so we clear [changed] and refill it with the 
                     # newly-propagated assignment. 
                     changed.clear() 
                     changed.add(str(self._propagate_incompatibility(root_cause))) 
                     break 
                 elif result is not None: 
                     changed.add(result)