Avoid calling fails_species_constraints twice on rejection

We called fails_species_constraints in the `if` condition
and then again to retrieve the `reason` string for the exception
message. Since the function already returns either False or the reason
string, store the result once and branch on it. This halves the
constraint-checking work on the rejection path (the expensive path,
where a failing check actually runs) with no change in behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rwest

rmgpy/data/kinetics/family.py

@@ -1655,8 +1655,8 @@ def _generate_product_structures(self, reactant_structures, maps, forward, relab
         for struct in product_structures:
             if self.is_molecule_forbidden(struct):
                 raise ForbiddenStructureException()
-            if fails_species_constraints(struct):
-                reason = fails_species_constraints(struct)
+            reason = fails_species_constraints(struct)
+            if reason:
                 raise ForbiddenStructureException(
                     "Species constraints forbids product species {0}. Please "
                     "reformulate constraints, or explicitly "

rmgpy/rmg/main.py

@@ -734,11 +734,11 @@ def initialize(self, **kwargs):
                         "Input species {0} is globally forbidden. You may explicitly "
                         "allow it by adding 'input species' to the `generatedSpeciesConstraints` `allowed` list.".format(spec.label)
                     )
-            if fails_species_constraints(spec):
+            reason = fails_species_constraints(spec)
+            if reason:
                 if "allowed" in self.species_constraints and "input species" in self.species_constraints["allowed"]:
                     self.species_constraints["explicitlyAllowedMolecules"].append(spec.molecule[0])
                 else:
-                    reason = fails_species_constraints(spec)
                     raise ForbiddenStructureException(
                         "Species constraints forbids input species {0}. Please "
                         "reformulate constraints, remove the species, or explicitly "

rmgpy/rmg/model.py

@@ -1739,11 +1739,11 @@ def add_seed_mechanism_to_core(self, seed_mechanism, react=False, requires_rms=F
                         "found in a seed mechanism or reaction "
                         "library.".format(spec.label, seed_mechanism.label)
                     )
-            if fails_species_constraints(spec):
+            reason = fails_species_constraints(spec)
+            if reason:
                 if "allowed" in rmg.species_constraints and "seed mechanisms" in rmg.species_constraints["allowed"]:
                     rmg.species_constraints["explicitlyAllowedMolecules"].extend(spec.molecule)
                 else:
-                    reason = fails_species_constraints(spec)
                     raise ForbiddenStructureException(
                         "Species constraints forbids species {0} from seed mechanism {1}."
                         " Please reformulate constraints, remove the species, or"
@@ -1867,11 +1867,11 @@ def add_reaction_library_to_edge(self, reaction_library, requires_rms=False):
                             "inert unless found in a seed mechanism or reaction "
                             "library.".format(spec.label, reaction_library.label)
                         )
-            if fails_species_constraints(spec):
+            reason = fails_species_constraints(spec)
+            if reason:
                 if "allowed" in rmg.species_constraints and "reaction libraries" in rmg.species_constraints["allowed"]:
                     rmg.species_constraints["explicitlyAllowedMolecules"].extend(spec.molecule)
                 else:
-                    reason = fails_species_constraints(spec)
                     raise ForbiddenStructureException(
                         "Species constraints forbids species {0} from reaction library "
                         "{1}. Please reformulate constraints, remove the species, or "