diff --git a/gprMax/subgrids/precursor_nodes.py b/gprMax/subgrids/precursor_nodes.py
index ab679638..718310c9 100644
--- a/gprMax/subgrids/precursor_nodes.py
+++ b/gprMax/subgrids/precursor_nodes.py
@@ -218,9 +218,9 @@ class PrecursorNodesBase:
 
     def update_previous_timestep_fields(self, field_names):
         for fn in field_names:
-            val = getattr(self, fn + '_1')
+            val = getattr(self, f'{fn}_1')
             val_c = np.copy(val)
-            setattr(self, fn + '_0', val_c)
+            setattr(self, f'{fn}_0', val_c)
 
     def interpolate_to_sub_grid(self, field, coords):
         x, z, x_sg, z_sg = coords
diff --git a/gprMax/subgrids/updates.py b/gprMax/subgrids/updates.py
index 07b1a9c5..b38e3cc3 100644
--- a/gprMax/subgrids/updates.py
+++ b/gprMax/subgrids/updates.py
@@ -36,7 +36,7 @@ def create_updates(G):
         elif sg_type == SubGridHSG and not sg.filter:
             precursors = PrecursorNodes(G, sg)
         else:
-            logger.exception(str(sg) + ' is not a subgrid type')
+            logger.exception(f'{str(sg)} is not a subgrid type')
             raise ValueError
 
         sgu = SubgridUpdater(sg, precursors, G)
diff --git a/gprMax/subgrids/user_objects.py b/gprMax/subgrids/user_objects.py
index 1136ea01..586b20b1 100644
--- a/gprMax/subgrids/user_objects.py
+++ b/gprMax/subgrids/user_objects.py
@@ -45,7 +45,7 @@ class SubGridBase(UserObjectMulti):
         elif isinstance(node, UserObjectGeometry):
             self.children_geometry.append(node)
         else:
-            logger.exception(str(node) + ' this Object can not be added to a sub grid')
+            logger.exception(f'{str(node)} this Object can not be added to a sub grid')
             raise ValueError
 
     def set_discretisation(self, sg, grid):
@@ -123,12 +123,16 @@ class SubGridBase(UserObjectMulti):
         self.subgrid = sg
 
         # Copy over built in materials
-        sg.materials = [copy(m) for m in grid.materials if m.numID in range(0, grid.n_built_in_materials + 1)]
+        sg.materials = [
+            copy(m)
+            for m in grid.materials
+            if m.numID in range(grid.n_built_in_materials + 1)
+        ]
 
         # Don't mix and match different subgrid types
         for sg_made in grid.subgrids:
             if type(sg) != type(sg_made):
-                logger.exception(self.__str__() + ' please only use one type of subgrid')
+                logger.exception(f'{self.__str__()} please only use one type of subgrid')
                 raise ValueError
 
         # Reference the subgrid under the main grid to which it belongs