Apply suggestions from code review

Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>

Author: timhoffm

doc/sphinxext/gallery_order.py

@@ -110,35 +110,33 @@ def read_gallery_order(src_dir: Path):
             front = lines[:placeholder_index]
             back = lines[placeholder_index+1:]
 
-        listed_examples = set(front + back)
-        existing_examples = set(
-            str(file.name) for file in src_dir.iterdir() if file.suffix == ".py"
-        )
+        listed_examples = {*front, *back}
+        existing_examples = {
+            file.name for file in src_dir.iterdir() if file.suffix == ".py"
+        }
 
-        non_exiting_examples = listed_examples - existing_examples
+        non_existing_examples = listed_examples - existing_examples
         missing_examples = existing_examples - listed_examples
-        print(f"non_exiting_examples: {non_exiting_examples}")
+        print(f"non_existing_examples: {non_existing_examples}")
         print(f"missing_examples: {missing_examples}")
 
         if non_exiting_examples:
             raise ValueError(
                 f"The following examples listed in {gallery_order_txt} do not exist: "
                 f"{', '.join(non_exiting_examples)}"
-            )
+        if non_existing_examples:
+            raise ValueError(
+                f"The following examples listed in {gallery_order_txt} do not exist: "
+                f"{', '.join(non_existing_examples)}"
         if placeholder_index is None and missing_examples:
             raise ValueError(
                 f"The following examples are not listed in {gallery_order_txt}. "
-                f"Either include them or add a '*' to indicate where not listed"
+                f"Either include them or add a '*' to indicate where not listed "
                 f"examples should be placed: "
                 f"{', '.join(missing_examples)}"
             )
 
-        mid = list(
-            sorted(
-                str(file.name) for file in src_dir.iterdir()
-                if file.suffix == ".py" and str(file.name) not in listed_examples
-            )
-        )
+        mid = list(sorted(missing_examples))
         return front + mid + back
 
     def __call__(self, item):