diff --git a/pyro/pyro_sim.py b/pyro/pyro_sim.py
index bdaf6efa9..afe0ae949 100755
--- a/pyro/pyro_sim.py
+++ b/pyro/pyro_sim.py
@@ -128,7 +128,7 @@ def initialize_problem(self, problem_name, *, inputs_file=None, inputs_dict=None
             self.problem_source = None
 
         else:
-            problem = importlib.import_module("pyro.{}.problems.{}".format(self.solver_name, problem_name))
+            problem = importlib.import_module(f"pyro.{self.solver_name}.problems.{problem_name}")
             self.problem_name = problem_name
             self.problem_func = problem.init_data
             self.problem_params = problem.PROBLEM_PARAMS