gh-153854: Raise ValueError from imaplib.Time2Internaldate on empty string

Lib/imaplib.py

@@ -2272,7 +2272,7 @@ def Time2Internaldate(date_time):
         if date_time.tzinfo is None:
             raise ValueError("date_time must be aware")
         dt = date_time
-    elif isinstance(date_time, str) and (date_time[0],date_time[-1]) == ('"','"'):
+    elif isinstance(date_time, str) and date_time.startswith('"') and date_time.endswith('"'):
         return date_time        # Assume in correct format
     else:
         raise ValueError("date_time not of a known type")

Lib/test/test_imaplib.py

@@ -159,6 +159,20 @@ def test_that_Time2Internaldate_returns_a_result(self):
         for t in self.timevalues():
             imaplib.Time2Internaldate(t)
 
+    def test_Time2Internaldate_quoted_string_passthrough(self):
+        # An already double-quoted string is assumed to be in the
+        # correct format and returned unchanged.
+        quoted = '"18-May-2033 05:33:20 +0200"'
+        self.assertEqual(imaplib.Time2Internaldate(quoted), quoted)
+
+    def test_Time2Internaldate_bad_string(self):
+        # A string that is not double-quoted is not of a known type and
+        # must raise ValueError, including the empty string (gh-153854).
+        for value in ('', 'x', '"x', 'x"'):
+            with self.subTest(value=value):
+                with self.assertRaises(ValueError):
+                    imaplib.Time2Internaldate(value)
+
     @socket_helper.skip_if_tcp_blackhole
     def test_imap4_host_default_value(self):
         # Check whether the IMAP4_PORT is truly unavailable.

Misc/ACKS

@@ -2008,6 +2008,7 @@ Wm. Keith van der Meulen
 Eric N. Vander Weele
 Andrew Vant
 Atul Varma
+Vyron Vasileiadis
 Dmitry Vasiliev
 Sebastian Ortiz Vasquez
 Alexandre Vassalotti

Misc/NEWS.d/next/Library/2026-07-18-01-50-34.gh-issue-153854.z8ec1p.rst

@@ -0,0 +1,3 @@
+:func:`imaplib.Time2Internaldate` now raises :exc:`ValueError` instead of
+:exc:`IndexError` when passed an empty string, consistent with its documented
+behaviour for strings that are not of a known type.