add runtime error if no X(...) found in plotfile

Author: zhichen3

Exec/science/xrb_spherical/analysis/slice.py

@@ -26,7 +26,15 @@ def _ash(field, data):
     if ("boxlib", "X(ash)") in field_list:
         return data["boxlib", "X(ash)"] * data["boxlib", "density"]
 
-    # If we cannot find X(ash) as a available  field then compute manually
+    # If we cannot find X(ash) as a available field then compute manually.
+    # First check if the plotfile has massfractions -- i.e. using plt not smallplt
+    has_species = any(f[1].startswith("X(") for f in field_list)
+    if not has_species:
+        raise RuntimeError(
+            "Derived field 'ash' requires species mass fractions X(nuc), "
+            "but no X(...) fields were found in this plotfile."
+        )
+
     rho = data["boxlib", "density"]
     rhoAsh = rho * 0.0
     for f in field_list:
@@ -320,9 +328,10 @@ def slice(fnames:list[str], fields:list[str],
                                              show_full_star=show_full_star)
 
         #add rhoX_ash as a derived field
-        ds.add_field(("gas", "ash"), function=_ash,
-                     display_name=r"\rho X\left(ash\right)",
-                     units="auto", sampling_type="cell")
+        if "ash" in fields:
+            ds.add_field(("gas", "ash"), function=_ash,
+                         display_name=r"\rho X\left(ash\right)",
+                         units="auto", sampling_type="cell")
 
         for i, field in enumerate(fields):
             # Plot each field parameter