gh-63121: Refresh imaplib capabilities on state changes

imaplib fetched the server capabilities only once, at connection time.
They are now also refreshed after a successful LOGIN or AUTHENTICATE,
from the CAPABILITY response the server sent or, if it sent none, by
querying it.  This lets methods such as enable() see capabilities added
after login, for example ENABLE on Gmail (gh-103451).

Capabilities advertised in the server greeting are now used too, saving
a redundant CAPABILITY command.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/imaplib.py

@@ -255,7 +255,7 @@ def _connect(self):
         else:
             raise self.error(self.welcome)
 
-        self._get_capabilities()
+        self._refresh_capabilities()
         if __debug__:
             if self.debug >= 3:
                 self._mesg('CAPABILITIES: %r' % (self.capabilities,))
@@ -533,6 +533,7 @@ def authenticate(self, mechanism, authobject):
         if typ != 'OK':
             raise self.error(dat[-1].decode('utf-8', 'replace'))
         self.state = 'AUTH'
+        self._refresh_capabilities()
         return typ, dat
 
 
@@ -714,6 +715,7 @@ def login(self, user, password):
         if typ != 'OK':
             raise self.error(dat[-1].decode('UTF-8', 'replace'))
         self.state = 'AUTH'
+        self._refresh_capabilities()
         return typ, dat
 
 
@@ -1201,6 +1203,15 @@ def _get_capabilities(self):
         self.capabilities = tuple(dat.split())
 
 
+    def _refresh_capabilities(self):
+        # Use a CAPABILITY response sent by the server, or ask for it.
+        if 'CAPABILITY' in self.untagged_responses:
+            dat = self.untagged_responses.pop('CAPABILITY')[-1]
+            self.capabilities = tuple(str(dat, self._encoding).upper().split())
+        else:
+            self._get_capabilities()
+
+
     def _get_response(self, start_timeout=False):
 
         # Read response and store.

Lib/test/test_imaplib.py

@@ -136,9 +136,11 @@ def _send_textline(self, message):
     def _send_tagged(self, tag, code, message):
         self._send_textline(' '.join((tag, code, message)))
 
+    welcome = '* OK IMAP4rev1'
+
     def handle(self):
         # Send a welcome message.
-        self._send_textline('* OK IMAP4rev1')
+        self._send_textline(self.welcome)
         while 1:
             # Gather up input until we receive a line terminator or we timeout.
             # Accumulate read(1) because it's simpler to handle the differences
@@ -634,6 +636,71 @@ def test_login(self):
         self.assertEqual(data[0], b'LOGIN completed')
         self.assertEqual(client.state, 'AUTH')
 
+    def test_login_capabilities(self):
+        # A server may advertise new capabilities after login (as an
+        # untagged CAPABILITY response); imaplib must refresh its cached
+        # capability list (gh-63121, gh-103451).
+        class CapabilityLoginHandler(SimpleIMAPHandler):
+            def cmd_LOGIN(self, tag, args):
+                self.server.logged = args[0]
+                self._send_textline('* CAPABILITY IMAP4rev1 ENABLE UTF8=ACCEPT')
+                self._send_tagged(tag, 'OK', 'LOGIN completed')
+            def cmd_ENABLE(self, tag, args):
+                self._send_tagged(tag, 'OK', 'ENABLE completed')
+
+        client, _ = self._setup(CapabilityLoginHandler)
+        self.assertNotIn('ENABLE', client.capabilities)
+        client.login('user', 'pass')
+        self.assertIn('ENABLE', client.capabilities)
+        self.assertIn('UTF8=ACCEPT', client.capabilities)
+        typ, _ = client.enable('UTF8=ACCEPT')
+        self.assertEqual(typ, 'OK')
+
+    def test_authenticate_capabilities(self):
+        # Capabilities are also refreshed after AUTHENTICATE, here from a
+        # CAPABILITY response code in the tagged OK response.
+        class CapabilityAuthHandler(SimpleIMAPHandler):
+            def cmd_AUTHENTICATE(self, tag, args):
+                self._send_textline('+')
+                self.server.response = yield
+                self._send_tagged(
+                    tag, 'OK',
+                    '[CAPABILITY IMAP4rev1 ENABLE] AUTHENTICATE completed')
+
+        client, _ = self._setup(CapabilityAuthHandler)
+        self.assertNotIn('ENABLE', client.capabilities)
+        client.authenticate('MYAUTH', lambda x: b'fake')
+        self.assertIn('ENABLE', client.capabilities)
+
+    def test_greeting_capabilities(self):
+        # Capabilities advertised in the greeting are used directly,
+        # without sending a separate CAPABILITY command.
+        class GreetingHandler(SimpleIMAPHandler):
+            welcome = '* OK [CAPABILITY IMAP4rev1 ENABLE] Server ready'
+            def cmd_CAPABILITY(self, tag, args):
+                self.server.capability_queried = True
+                super().cmd_CAPABILITY(tag, args)
+
+        client, server = self._setup(GreetingHandler)
+        self.assertEqual(client.capabilities, ('IMAP4REV1', 'ENABLE'))
+        self.assertFalse(getattr(server, 'capability_queried', False))
+
+    def test_login_requery_capabilities(self):
+        # If the server does not advertise capabilities after login,
+        # imaplib re-queries them (as it does after STARTTLS), so a
+        # capability that becomes available only after authentication is
+        # still recognized (gh-63121).
+        class RequeryHandler(SimpleIMAPHandler):
+            def cmd_CAPABILITY(self, tag, args):
+                caps = 'IMAP4rev1 ENABLE' if self.server.logged else 'IMAP4rev1'
+                self._send_textline('* CAPABILITY ' + caps)
+                self._send_tagged(tag, 'OK', 'CAPABILITY completed')
+
+        client, _ = self._setup(RequeryHandler)
+        self.assertNotIn('ENABLE', client.capabilities)
+        client.login('user', 'pass')
+        self.assertIn('ENABLE', client.capabilities)
+
     def test_logout(self):
         client, _ = self._setup(SimpleIMAPHandler)
         typ, data = client.login('user', 'pass')

Misc/NEWS.d/next/Library/2026-07-01-10-30-00.gh-issue-63121.Rm4tZp.rst

@@ -0,0 +1,7 @@
+:mod:`imaplib` now refreshes the cached capability list after a successful
+:meth:`~imaplib.IMAP4.login` or :meth:`~imaplib.IMAP4.authenticate`, using
+the ``CAPABILITY`` response sent by the server or, if none was sent, by
+querying it, so that capabilities that become available only after
+authentication (such as ``ENABLE`` on Gmail) are recognized.  Capabilities
+advertised in the server greeting are now also used, avoiding a redundant
+``CAPABILITY`` command.