chore: resolve review comment

Author: fatelei

Doc/library/poplib.rst

@@ -249,12 +249,17 @@ A :class:`POP3` instance has the following methods:
 
    Authenticate using the POP3 ``AUTH`` command as specified in :rfc:`5034`.
 
-   If *initial_response* is provided (``bytes`` or ``str``), it is
-   base64-encoded and appended to the command after a single space.
+   If *initial_response_ok* is true, ``authobject()`` is called first with no
+   arguments to obtain an optional initial response. It may return :class:`bytes`
+   or :class:`str`; if it returns :const:`None`, no initial response is sent.
+   The returned value is base64-encoded before being sent.
 
-   If *authobject* is provided, it is called with the server’s ``bytes``
-   challenge (already base64-decoded) and must return the client response
-   (``bytes`` or ``str``).  Return ``b'*'`` to abort the exchange.
+   For each server challenge, ``authobject(challenge)`` is called with the
+   base64-decoded ``bytes`` challenge and must return the client response as
+   :class:`bytes` or :class:`str`. If it returns :const:`None`, an empty response
+   is sent. To abort the exchange, return ``b'*'`` (the client sends ``'*'``).
+
+   .. versionadded:: next
 
 
 Instances of :class:`POP3_SSL` have no additional methods. The interface of this

Lib/poplib.py

@@ -13,12 +13,12 @@
 
 # Imports
 
+import base64
 import binascii
 import errno
 import re
 import socket
 import sys
-import base64
 
 try:
     import ssl
@@ -221,6 +221,7 @@ def pass_(self, pswd):
         """
         return self._shortcmd('PASS %s' % pswd)
 
+
     def stat(self):
         """Get mailbox status.
 
@@ -428,18 +429,12 @@ def stls(self, context=None):
         return resp
 
     def auth(self, mechanism, authobject, *, initial_response_ok=True):
-        """Authenticate to the POP3 server using AUTH (RFC 5034).
-
-        Result is 'response'.
-        """
+        """Authenticate to the POP3 server using AUTH (RFC 5034)."""
         mech = mechanism.upper()
 
         initial = None
-        if initial_response_ok:
-            try:
-                initial = authobject()
-            except TypeError:
-                initial = None
+        if initial_response_ok and callable(authobject):
+            initial = authobject()
         if isinstance(initial, str):
             initial = initial.encode('ascii', 'strict')
         if initial is not None and not isinstance(initial, (bytes, bytearray)):
@@ -448,7 +443,7 @@ def auth(self, mechanism, authobject, *, initial_response_ok=True):
         if initial is not None:
             b64 = base64.b64encode(initial).decode('ascii')
             cmd = f'AUTH {mech} {b64}'
-            if len(cmd.encode('ascii')) + 2 <= 255:
+            if len(cmd.encode('ascii')) <= 253:
                 self._putcmd(cmd)
             else:
                 self._putcmd(f'AUTH {mech}')
@@ -465,12 +460,12 @@ def auth(self, mechanism, authobject, *, initial_response_ok=True):
             if line.startswith(b'-ERR'):
                 raise error_proto(line.decode('ascii', 'replace'))
             # Challenge line: "+ <b64>" or just "+" (empty challenge)
-            if not (line == b'+' or line.startswith(b'+ ')):
+            if line != b'+' and not line.startswith(b'+ '):
                 raise error_proto(f'malformed AUTH challenge line: {line!r}')
 
             auth_challenge_count += 1
             if auth_challenge_count > _MAXCHALLENGE:
-                raise error_proto('Server AUTH mechanism infinite loop')
+                raise error_proto('Server AUTH mechanism infinite loop. Last response: ', repr(line))
 
             chal = line[1:]
             if chal.startswith(b' '):
@@ -500,10 +495,7 @@ def auth(self, mechanism, authobject, *, initial_response_ok=True):
                 self._putcmd(base64.b64encode(resp).decode('ascii'))
 
     def auth_plain(self, user, password, authzid=''):
-        """Return an authobject suitable for SASL PLAIN.
-
-        Result is 'str'.
-        """
+        """Return an authobject suitable for SASL PLAIN."""
         def _auth_plain(challenge=None):
             # Per RFC 4616, the response is: authzid UTF8 NUL authcid UTF8 NUL passwd UTF8
             return f"{authzid}\0{user}\0{password}"

Lib/test/test_poplib.py

@@ -2,6 +2,7 @@
 
 # Modified by Giampaolo Rodola' to give poplib.POP3 and poplib.POP3_SSL
 # a real test suite
+
 import base64
 import poplib
 import socket

Misc/NEWS.d/next/Library/2025-12-12-15-09-34.gh-issue-64551.NfWlQX.rst

@@ -1 +1 @@
-Add RFC 5034 AUTH support to poplib
+:mod:`poplib`: add :rfc:`5034` ``AUTH`` support.