fix: treat write_text linesep as a literal separator

path/__init__.py

@@ -899,7 +899,8 @@ def write_text(
         conversion.
         """
         if linesep is not None:
-            text = U_NEWLINE.sub(linesep, text)
+            # lambda keeps linesep literal; str repl is a regex template
+            text = U_NEWLINE.sub(lambda _: linesep, text)
         bytes = text.encode(encoding or sys.getdefaultencoding(), errors)
         self.write_bytes(bytes, append=append)
 

tests/test_path.py

@@ -318,6 +318,15 @@ def test_read_write(self, tmpdir):
         assert file.read_text(encoding='utf-8') == 'hello world'
         assert file.read_bytes() == b'hello world'
 
+    def test_write_text_linesep_is_literal(self, tmpdir):
+        file = path.Path(tmpdir) / 'filename'
+        file.write_text('a\nb\rc\r\nd', encoding='utf-8', linesep='\\1')
+        assert file.read_bytes() == b'a\\1b\\1c\\1d'
+        file.write_text('a\nb', encoding='utf-8', linesep='\\n')
+        assert file.read_bytes() == b'a\\nb'
+        file.write_text('a\nb', encoding='utf-8', linesep='\n')
+        assert file.read_bytes() == b'a\nb'
+
 
 class TestPerformance:
     @staticmethod