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gh-155775: Do not consume the CAPABILITY response in imaplib #155789

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gh-155775: Do not consume the CAPABILITY response in imaplib #155789

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14 changes: 14 additions & 0 deletions Doc/library/imaplib.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -907,6 +907,20 @@ An :class:`IMAP4` instance has the following methods:

The following attributes are defined on instances of :class:`IMAP4`:

.. attribute:: IMAP4.capabilities

A tuple of the capabilities advertised by the server, in upper case.

It is set when the connection is established,
and refreshed after a successful :meth:`~IMAP4.login`,
:meth:`~IMAP4.authenticate` or :meth:`~IMAP4.starttls`,
because the server can advertise different capabilities
in different connection states.

.. versionchanged:: 3.14.7
Refreshed after :meth:`~IMAP4.login` and :meth:`~IMAP4.authenticate`.


.. attribute:: IMAP4.PROTOCOL_VERSION

The most recent supported protocol in the ``CAPABILITY`` response from the
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11 changes: 8 additions & 3 deletions Lib/imaplib.py
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Expand Up @@ -366,7 +366,8 @@ def _connect(self):
self._encoding, 'replace')
raise self.error('invalid greeting: ' + greeting)

self._refresh_capabilities()
# The greeting is not a response to a command.
self._refresh_capabilities(consume=True)
if __debug__:
if self.debug >= 3:
self._mesg('CAPABILITIES: %r' % (self.capabilities,))
Expand Down Expand Up @@ -1509,10 +1510,14 @@ def _get_capabilities(self):
self.capabilities = tuple(dat.split())


def _refresh_capabilities(self):
def _refresh_capabilities(self, consume=False):
# Use a CAPABILITY response sent by the server, or ask for it.
# Unless it is consumed, the response can still be read with
# response('CAPABILITY').
if 'CAPABILITY' in self.untagged_responses:
dat = self.untagged_responses.pop('CAPABILITY')[-1]
dat = self.untagged_responses['CAPABILITY'][-1]
if consume:
del self.untagged_responses['CAPABILITY']
self.capabilities = tuple(str(dat, self._encoding).upper().split())
else:
self._get_capabilities()
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6 changes: 6 additions & 0 deletions Lib/test/test_imaplib.py
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Expand Up @@ -1069,6 +1069,8 @@ def cmd_ENABLE(self, tag, args):
client.login('user', 'pass')
self.assertIn('ENABLE', client.capabilities)
self.assertIn('UTF8=ACCEPT', client.capabilities)
self.assertEqual(client.response('CAPABILITY'),
('CAPABILITY', [b'IMAP4rev1 ENABLE UTF8=ACCEPT']))
typ, _ = client.enable('UTF8=ACCEPT')
self.assertEqual(typ, 'OK')

Expand All @@ -1087,6 +1089,8 @@ def cmd_AUTHENTICATE(self, tag, args):
self.assertNotIn('ENABLE', client.capabilities)
client.authenticate('MYAUTH', lambda x: b'fake')
self.assertIn('ENABLE', client.capabilities)
self.assertEqual(client.response('CAPABILITY'),
('CAPABILITY', [b'IMAP4rev1 ENABLE']))

def test_greeting_capabilities(self):
# Capabilities advertised in the greeting are used directly,
Expand All @@ -1100,6 +1104,8 @@ def cmd_CAPABILITY(self, tag, args):
client, server = self._setup(GreetingHandler)
self.assertEqual(client.capabilities, ('IMAP4REV1', 'ENABLE'))
self.assertFalse(getattr(server, 'capability_queried', False))
# The greeting is not a response to a command, so it is consumed.
self.assertEqual(client.response('CAPABILITY'), ('CAPABILITY', [None]))

def test_login_requery_capabilities(self):
# If the server does not advertise capabilities after login,
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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-14-14-10-00.gh-issue-155775.Kq3vTx.rst
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@@ -0,0 +1,4 @@
Fix a regression in :mod:`imaplib` introduced in the fix for :gh:`63121`:
refreshing the capabilities after a successful :meth:`~imaplib.IMAP4.login`
or :meth:`~imaplib.IMAP4.authenticate` consumed the ``CAPABILITY`` response,
so it could no longer be read with :meth:`~imaplib.IMAP4.response`.
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