Throw an exception if the user tries to render a trace without stepping the simulation (#472)

Author: gaborszita

pyrtl/simulation.py

@@ -1755,6 +1755,9 @@ def render_trace(
             is not found in the map, the argument ``repr_func`` will be used instead.
         :param segment_size: Traces are broken in the segments of this number of cycles.
         """
+        if len(self) == 0:
+            msg = "You need to step the simulation at least once to render a trace."
+            raise PyrtlError(msg)
         if repr_per_name is None:
             repr_per_name = {}
         if _currently_in_jupyter_notebook():

tests/test_compilesim.py

@@ -1206,14 +1206,8 @@ def test_empty_trace_after_untraceable_removed(self):
         r.next <<= r + 1
         sim = self.sim()
         sim.step_multiple(provided_inputs={}, nsteps=10)
-        with self.assertRaises(pyrtl.PyrtlError) as ex:
+        with self.assertRaises(pyrtl.PyrtlError):
             sim.tracer.render_trace()
-        self.assertEqual(
-            str(ex.exception),
-            "Empty trace list. This may have occurred because "
-            "untraceable wires were removed prior to simulation, "
-            "if a CompiledSimulation was used.",
-        )
 
     def test_invalid_base(self):
         self.in1 = pyrtl.Input(8, "in1")