gh-134152: Fix UnboundLocalError in email._header_value_parser _get_ptext_to_endchars (GH-134233)

Fix an UnboundLocalError that can occur when parsing certain delimited constructs in headers (domain literals, quoted strings, comments). After the fix the _get_ptext_to_endchars returns an empty string if there is no content after the opening delimiter. The calling code is responsible for handling the lack of the trailing delimiter, which it already does; this edge case was the header ending immediately after the opening delimiter.
(cherry picked from commit a32ea45)

Co-authored-by: R. David Murray <rdmurray@bitdance.com>

Author: bitdancer

Lib/email/_header_value_parser.py

@@ -1020,6 +1020,8 @@ def _get_ptext_to_endchars(value, endchars):
     a flag that is True iff there were any quoted printables decoded.
 
     """
+    if not value:
+        return '', '', False
     fragment, *remainder = _wsp_splitter(value, 1)
     vchars = []
     escape = False

Lib/test/test_email/test__header_value_parser.py

@@ -463,6 +463,19 @@ def test_get_qp_ctext_non_printables(self):
                                 [errors.NonPrintableDefect], ')')
         self.assertEqual(ptext.defects[0].non_printables[0], '\x00')
 
+    def test_get_qp_ctext_close_paren_only(self):
+        self._test_get_x(parser.get_qp_ctext,
+                        ')', '', ' ', [], ')')
+
+    def test_get_qp_ctext_open_paren_only(self):
+        self._test_get_x(parser.get_qp_ctext,
+                        '(', '', ' ', [], '(')
+
+    def test_get_qp_ctext_no_end_char(self):
+        self._test_get_x(parser.get_qp_ctext,
+                        '', '', ' ', [], '')
+
+
     # get_qcontent
 
     def test_get_qcontent_only(self):
@@ -503,6 +516,14 @@ def test_get_qcontent_non_printables(self):
                                 [errors.NonPrintableDefect], '"')
         self.assertEqual(ptext.defects[0].non_printables[0], '\x00')
 
+    def test_get_qcontent_empty(self):
+        self._test_get_x(parser.get_qcontent,
+                         '"', '', '', [], '"')
+
+    def test_get_qcontent_no_end_char(self):
+        self._test_get_x(parser.get_qcontent,
+                         '', '', '', [], '')
+
     # get_atext
 
     def test_get_atext_only(self):
@@ -1283,6 +1304,18 @@ def test_get_dtext_open_bracket_mid_word(self):
         self._test_get_x(parser.get_dtext,
                         'foo[bar', 'foo', 'foo', [], '[bar')
 
+    def test_get_dtext_open_bracket_only(self):
+        self._test_get_x(parser.get_dtext,
+                        '[', '', '', [], '[')
+
+    def test_get_dtext_close_bracket_only(self):
+        self._test_get_x(parser.get_dtext,
+                        ']', '', '', [], ']')
+
+    def test_get_dtext_empty(self):
+        self._test_get_x(parser.get_dtext,
+                        '', '', '', [], '')
+
     # get_domain_literal
 
     def test_get_domain_literal_only(self):

Misc/NEWS.d/next/Library/2025-05-19-10-32-11.gh-issue-134152.INJC2j.rst

@@ -0,0 +1,2 @@
+Fixed :exc:`UnboundLocalError` that could occur during :mod:`email` header
+parsing if an expected trailing delimiter is missing in some contexts.