fix how derives are imported when reading output

pyro/compressible/derives.py

@@ -57,6 +57,9 @@ def derive_primitives(myd, varnames):
         elif var == "soundspeed":
             derived_vars.append(np.sqrt(gamma*p/dens))
 
+        elif var == "machnumber":
+            derived_vars.append(np.sqrt(u**2 + v**2) / np.sqrt(gamma*p/dens))
+
         elif var == "vorticity":
             derived_vars.append(vort)
 

pyro/util/io_pyro.py

@@ -127,13 +127,20 @@ def read(filename):
 
             sim.read_extras(f)
 
-        # check if there are derived variables
-        try:
-            derives = importlib.import_module(f"pyro.{solver_name}.derives")
-            sim.cc_data.add_derived(derives.derive_primitives)
-
-        except ModuleNotFoundError:
-            pass
+            # check if there are derived variables -- since we do a lot of
+            # inheritance, we want to start with the deepest class and
+            # work backwards through the inheritance until we find a derives
+            # module
+            for mod in [cls.__module__ for cls in type(sim).__mro__ if cls is not object]:
+                try:
+                    derives = importlib.import_module(f"{mod.replace('simulation', 'derives')}")
+                    sim.cc_data.add_derived(derives.derive_primitives)
+
+                except ModuleNotFoundError:
+                    continue
+
+                else:
+                    break
 
     if solver_name is not None:
         return sim