fix: recognize trailing suffix_not_acronyms in lastname-comma format (closes #144)

In 'Last, First Middle I' format, a trailing single-letter suffix like 'I'
was silently absorbed into the middle name because is_suffix() rejects single
uppercase letters via is_an_initial().

Add is_suffix_at_lastname_comma_end() which applies when the piece is the
final token of the post-comma segment and no explicit comma-separated suffix
follows (len(parts)==2). When parts[2] exists the caller already declared a
suffix, making the trailing token more likely a middle initial
(e.g. 'Doe, Rev. John V, Jr.'), so that case is excluded.

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

Author: derek73

nameparser/parser.py

@@ -504,6 +504,20 @@ def are_suffixes_after_comma(self, pieces: Iterable[str]) -> bool:
                 return False
         return True
 
+    def is_suffix_at_lastname_comma_end(self, piece: str, nxt: str | None, parts: list[str]) -> bool:
+        """True when a suffix_not_acronyms member is the final token in the
+        post-comma segment of a lastname-comma name with no explicit
+        comma-separated suffix (e.g. 'Maier, Amy Lauren I').
+
+        When parts[2] exists the caller already has an explicit suffix, making
+        the trailing single-letter more likely a middle initial
+        (e.g. 'Doe, Rev. John V, Jr.'), so the guard len(parts)==2 excludes
+        that case.
+        """
+        return (nxt is None
+                and len(parts) == 2
+                and lc(piece) in self.C.suffix_not_acronyms)
+
     def is_rootname(self, piece: str) -> bool:
         """
         Is not a known title, suffix or prefix. Just first, middle, last names.
@@ -767,7 +781,7 @@ def parse_full_name(self) -> None:
                     if not self.first:
                         self.first_list.append(piece)
                         continue
-                    if self.is_suffix(piece):
+                    if self.is_suffix(piece) or self.is_suffix_at_lastname_comma_end(piece, nxt, parts):
                         self.suffix_list.append(piece)
                         continue
                     self.middle_list.append(piece)

tests/test_suffixes.py

@@ -166,6 +166,14 @@ def test_suffix_with_periods_with_lastname_comma(self) -> None:
         self.m(hn.last, "Doe", hn)
         self.m(hn.suffix, "Msc.Ed.", hn)
 
+    def test_roman_numeral_i_lastname_comma_format(self) -> None:
+        # trailing 'I' in lastname-comma format must be a suffix, not a middle initial (issue #144)
+        hn = HumanName("Maier, Amy Lauren I")
+        self.m(hn.first, "Amy", hn)
+        self.m(hn.middle, "Lauren", hn)
+        self.m(hn.last, "Maier", hn)
+        self.m(hn.suffix, "I", hn)
+
     def test_suffix_delimiter_default_on_constants(self) -> None:
         from nameparser.config import CONSTANTS
         self.assertIs(CONSTANTS.suffix_delimiter, None)