gh-45154: Fix imaplib handling of READ-ONLY mailboxes

Lib/imaplib.py

@@ -166,14 +166,15 @@ class IMAP4:
 
     Errors raise the exception class <instance>.error("<reason>").
     IMAP4 server errors raise <instance>.abort("<reason>"),
-    which is a sub-class of 'error'. Mailbox status changes
-    from READ-WRITE to READ-ONLY raise the exception class
-    <instance>.readonly("<reason>"), which is a sub-class of 'abort'.
+    which is a sub-class of 'error'.
+
+    When the server returns a READ-ONLY response code, the is_readonly
+    attribute is set to True. Applications should check this attribute
+    to determine mailbox access level.
 
     "error" exceptions imply a program error.
     "abort" exceptions imply the connection should be reset, and
             the command re-tried.
-    "readonly" exceptions imply the command should be re-tried.
 
     Note: to use this module, you must read the RFCs pertaining to the
     IMAP4 protocol, as the semantics of the arguments to each IMAP4
@@ -860,12 +861,8 @@ def select(self, mailbox='INBOX', readonly=False):
             self.state = 'AUTH'     # Might have been 'SELECTED'
             return typ, dat
         self.state = 'SELECTED'
-        if 'READ-ONLY' in self.untagged_responses \
-                and not readonly:
-            if __debug__:
-                if self.debug >= 1:
-                    self._dump_ur(self.untagged_responses)
-            raise self.readonly('%s is not writable' % mailbox)
+        if 'READ-ONLY' in self.untagged_responses:
+            self.is_readonly = True
         return typ, self.untagged_responses.get('EXISTS', [None])
 
 
@@ -1094,9 +1091,8 @@ def _command(self, name, *args):
             if typ in self.untagged_responses:
                 del self.untagged_responses[typ]
 
-        if 'READ-ONLY' in self.untagged_responses \
-        and not self.is_readonly:
-            raise self.readonly('mailbox status changed to READ-ONLY')
+        if 'READ-ONLY' in self.untagged_responses:
+            self.is_readonly = True
 
         tag = self._new_tag()
         name = bytes(name, self._encoding)

Lib/test/test_imaplib.py

@@ -657,6 +657,21 @@ def test_unselect(self):
         self.assertEqual(data[0], b'Returned to authenticated state. (Success)')
         self.assertEqual(client.state, 'AUTH')
 
+    def test_select_readonly_mailbox(self):
+        class ReadOnlyHandler(SimpleIMAPHandler):
+            def cmd_SELECT(self, tag, args):
+                self.server.is_selected = True
+                self._send_line(b'* 1 EXISTS')
+                self._send_line(b'* OK [READ-ONLY] Mailbox is read-only')
+                self._send_tagged(tag, 'OK', '[READ-ONLY] SELECT completed')
+        client, _ = self._setup(ReadOnlyHandler)
+        client.login('user', 'pass')
+        self.assertFalse(client.is_readonly)
+        typ, data = client.select('INBOX')
+        self.assertEqual(typ, 'OK')
+        self.assertTrue(client.is_readonly)
+        self.assertEqual(client.state, 'SELECTED')
+
     # property tests
 
     def test_file_property_should_not_be_accessed(self):

Misc/NEWS.d/next/Library/2025-11-18-10-44-46.gh-issue-45154.aBcDe1.rst

@@ -0,0 +1,3 @@
+Fix :mod:`imaplib` to handle mailboxes with ACL rights like ``lrs`` correctly
+by setting ``is_readonly`` flag instead of raising exception. Patched by Shamil
+Abdulaev.