Add checking foreign rrn API

rrn.py

@@ -112,8 +112,20 @@ def _is_sex_corresponding(rrn: str, female: bool) -> Optional[bool]:
 
 
 def _is_foreignness_corresponding(rrn: str, foreign: bool) -> Optional[bool]:
+    f = is_foreign(rrn)
+    return f == foreign if f is not None else None
+
+
+def is_foreign(rrn: str) -> Optional[bool]:
+    """
+    Check if given RRN literal is foreigner or not.
+    It returns None when given RRN literal is too short to determine.
+
+    :param rrn: RRN literal
+    :return: expectation to be foreigner or not
+    """
     try:
-        return (5 <= int(rrn[SEX]) <= 8) == foreign
+        return 5 <= int(rrn[SEX]) <= 8
     except IndexError:
         return None
 

tests/test_rrn.py

@@ -6,6 +6,40 @@
 
 class TestRRN(unittest.TestCase):
 
+    def test_is_foreign(self):
+        undetermined = None
+        foreign, domestic = True, False
+        for s, expected in [
+            ('', undetermined),
+            ('9', undetermined),
+            ('94', undetermined),
+            ('940', undetermined),
+            ('9408', undetermined),
+            ('94081', undetermined),
+            ('940812', undetermined),
+            ('9408120', domestic),
+            ('94081201', domestic),
+            ('9408121', domestic),
+            ('94081212', domestic),
+            ('9408122', domestic),
+            ('94081223', domestic),
+            ('9408123', domestic),
+            ('94081234', domestic),
+            ('9408124', domestic),
+            ('94081245', domestic),
+            ('9408125', foreign),
+            ('94081256', foreign),
+            ('9408126', foreign),
+            ('94081267', foreign),
+            ('9408127', foreign),
+            ('94081278', foreign),
+            ('9408128', foreign),
+            ('94081289', foreign),
+            ('9408129', domestic),
+            ('94081290', domestic)
+        ]:
+            self.assertEqual(expected, rrn.is_foreign(s))
+
     def test_is_valid_rrn(self):
         valid, invalid = True, False
         for s, expected in [