fix: honor single-char symbol conjunctions regardless of name length (closes #151)

The `join_on_conjunction` heuristic skipped single-character conjunctions
when `total_length < 4` to avoid treating alphabetic initials like `e` or
`y` as conjunctions. This inadvertently also skipped non-alphabetic symbols
like `&`, which can never be initials.

Fix: only apply the "treat as initial" fallback when the character is
alphabetic. Non-alphabetic conjunctions (e.g. `&`) are now always joined
regardless of name length, so `"Mr. & Mrs. John Smith"` correctly parses
the title as `"Mr. & Mrs."`.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: derek73

nameparser/parser.py

@@ -870,7 +870,7 @@ def join_on_conjunctions(self, pieces: list[str], additional_parts_count: int =
         conj_index = [i for i, piece in enumerate(pieces) if self.is_conjunction(piece)]
 
         for i in conj_index:
-            if len(pieces[i]) == 1 and total_length < 4:
+            if len(pieces[i]) == 1 and total_length < 4 and pieces[i].isalpha():
                 # if there are only 3 total parts (minus known titles, suffixes
                 # and prefixes) and this conjunction is a single letter, prefer
                 # treating it as an initial rather than a conjunction.

tests/test_conjunctions.py

@@ -111,6 +111,14 @@ def test_couple_titles(self) -> None:
         self.m(hn.first, "John and Jane", hn)
         self.m(hn.last, "Smith", hn)
 
+    def test_couple_titles_ampersand_conjunction(self) -> None:
+        # issue 151: single-char conjunctions in the conjunctions list should
+        # be honored even when total_length < 4
+        hn = HumanName('Mr. & Mrs. John Smith')
+        self.m(hn.title, "Mr. & Mrs.", hn)
+        self.m(hn.first, "John", hn)
+        self.m(hn.last, "Smith", hn)
+
     def test_title_with_three_part_name_last_initial_is_suffix_uppercase_no_period(self) -> None:
         hn = HumanName("King John Alexander V")
         self.m(hn.title, "King", hn)