added comments

Author: RemDelaporteMathurin

src/festim/exports/vtx.py

@@ -250,29 +250,35 @@ def set_dolfinx_expression(
             temperature: The temperature field to use in the expression
             time: The time to use in the expression
         """
-        # check
+        # check if we are in a mixed domain/discontinuous case
+        mixed_domain = any(
+            spe.subdomain_to_post_processing_solution
+            for spe in self.species_dependent_value.values()
+        ) or (self.subdomain.sub_T if self.subdomain else None)
+
+        # get the arguments of the user-provided expression
         arguments = inspect.signature(self.expression).parameters
+
+        # create a dictionary mapping the arguments to the appropriate values
         kwargs = {}
         if "t" in arguments:
             kwargs["t"] = time
         if "x" in arguments:
             x = ufl.SpatialCoordinate(time._ufl_domain)
             kwargs["x"] = x
         if "T" in arguments:
-            if (
-                isinstance(temperature, fem.Function)
-                and self.subdomain.sub_T is not None
-            ):
-                kwargs["T"] = (
-                    self.subdomain.sub_T
-                )  # NOTE I'm not sure that sub_T is updated at every time step
+            if isinstance(temperature, fem.Function) and mixed_domain:
+                # fem.Function in mixed domain/discontinuous case, use sub_T
+                # NOTE I'm not sure that sub_T is updated at every time step
+                kwargs["T"] = self.subdomain.sub_T
             else:
+                # else use the provided temperature
                 kwargs["T"] = temperature
         # check if there are other arguments and if they are in species_dependent_value
         for arg in arguments:
             if arg in self.species_dependent_value:
                 spe = self.species_dependent_value[arg]
-                if spe.subdomain_to_post_processing_solution:
+                if mixed_domain:
                     kwargs[arg] = spe.subdomain_to_post_processing_solution[
                         self.subdomain
                     ]
@@ -281,6 +287,9 @@ def set_dolfinx_expression(
             assert kwargs[arg] is not None, (
                 f"Argument {arg} not found in species_dependent_value"
             )
+
+        # evaluate the user-provided expression with the appropriate arguments and create a
+        # dolfinx.fem.Expression
         self.dolfinx_expression = fem.Expression(
             self.expression(**kwargs),
             get_interpolation_points(self.function.function_space.element),