diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index a7731bef37b..99f2ca5a8bd 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -866,7 +866,13 @@ def on_model(model):
 
             # print any unknown functions in the model
             for sym in best_model:
-                if sym.name not in ("attr", "error", "opt_criterion", "condition", "condition_cause"):
+                if sym.name not in (
+                    "attr",
+                    "error",
+                    "opt_criterion",
+                    "condition",
+                    "condition_cause",
+                ):
                     tty.debug(
                         "UNKNOWN SYMBOL: %s(%s)" % (sym.name, ", ".join(stringify(sym.arguments)))
                     )