fix(books): improve find_by_author to use casefold and partial matching

- Replace .lower() with .casefold() for correct Unicode handling
  (e.g., Straße now matches strasse)
- Change exact match (==) to substring match (in) so partial
  author names like 'Orwell' match 'George Orwell'
- Pre-compute normalized query outside loop to avoid redundant calls
- Add 12 new tests covering partial name, case variations, and
  edge cases (empty string, single character, multiple matches)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ishizuka-nkc

samples/book-app-project/books.py

@@ -70,8 +70,9 @@ def remove_book(self, title: str) -> bool:
         return False
 
     def find_by_author(self, author: str) -> List[Book]:
-        """Find all books by a given author."""
-        return [b for b in self.books if b.author.lower() == author.lower()]
+        """Find all books where author contains the given string (case-insensitive partial match)."""
+        normalized = author.casefold()
+        return [b for b in self.books if normalized in b.author.casefold()]
 
     def search_books(self, query: str) -> List[Book]:
         """Find books where query matches (partial, case-insensitive) title or author."""

samples/book-app-project/tests/test_books.py

@@ -197,6 +197,61 @@ def test_find_returns_all_books_by_author(self, collection):
         results = collection.find_by_author("George Orwell")
         assert len(results) == 2
 
+    # --- Partial match ---
+
+    def test_partial_last_name_returns_match(self, populated_collection):
+        results = populated_collection.find_by_author("Orwell")
+        assert len(results) == 1
+        assert results[0].title == "1984"
+
+    def test_partial_first_name_returns_match(self, populated_collection):
+        results = populated_collection.find_by_author("George")
+        assert len(results) == 1
+        assert results[0].author == "George Orwell"
+
+    def test_partial_name_matches_multiple_authors(self, collection):
+        collection.add_book("Book A", "George Orwell", 1945)
+        collection.add_book("Book B", "George Bernard Shaw", 1900)
+        collection.add_book("Book C", "Frank Herbert", 1965)
+        results = collection.find_by_author("George")
+        assert len(results) == 2
+
+    def test_partial_name_no_match_returns_empty(self, populated_collection):
+        results = populated_collection.find_by_author("Asimov")
+        assert results == []
+
+    # --- Case variations ---
+
+    @pytest.mark.parametrize("query", [
+        "orwell",
+        "ORWELL",
+        "Orwell",
+        "oRwElL",
+    ])
+    def test_partial_match_is_case_insensitive(self, populated_collection, query):
+        results = populated_collection.find_by_author(query)
+        assert len(results) == 1
+        assert results[0].title == "1984"
+
+    def test_uppercase_full_name_matches(self, populated_collection):
+        results = populated_collection.find_by_author("GEORGE ORWELL")
+        assert len(results) == 1
+
+    # --- Edge cases ---
+
+    def test_empty_string_matches_all(self, populated_collection):
+        """Empty string is a substring of every author name."""
+        results = populated_collection.find_by_author("")
+        assert len(results) == len(populated_collection.books)
+
+    def test_single_character_matches_correct_books(self, populated_collection):
+        results = populated_collection.find_by_author("J")
+        titles = [b.title for b in results]
+        assert "The Hobbit" in titles  # J.R.R. Tolkien
+
+    def test_returns_list_type(self, collection):
+        assert isinstance(collection.find_by_author("Anyone"), list)
+
 
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 # TestMarkAsRead