refactor: simplify fillna within transmission dcline function

powersimdata/design/compare/transmission.py

@@ -53,17 +53,9 @@ def calculate_dcline_differences(grid1, grid2):
     dc_merge = dcline1.merge(
         dcline2, how="outer", right_index=True, left_index=True, suffixes=(None, "_2")
     )
-    # fillna for bus ids, since some lines in one frame won't be in the other frame
-    dc_merge.fillna(
-        {
-            "from_lat": dc_merge.from_lat_2,
-            "from_lon": dc_merge.from_lon_2,
-            "to_lat": dc_merge.to_lat_2,
-            "to_lon": dc_merge.to_lon_2,
-        },
-        inplace=True,
-    )
-    # Calculate differences in Pmax
     dc_merge["diff"] = dc_merge.Pmax_2.fillna(0) - dc_merge.Pmax.fillna(0)
+    # Ensure that lats & lons get filled in as necessary from grid2.dcline entries
+    for l in ["from_lat", "from_lon", "to_lat", "to_lon"]:
+        dc_merge[l].fillna(dc_merge[f"{l}_2"], inplace=True)
 
     return dc_merge