gh-70039: smtplib: store the server name in ._host in .connect()

Lib/smtplib.py

@@ -244,7 +244,6 @@ def __init__(self, host='', port=0, local_hostname=None,
         will be used.
 
         """
-        self._host = host
         self.timeout = timeout
         self.esmtp_features = {}
         self.command_encoding = 'ascii'
@@ -335,6 +334,7 @@ def connect(self, host='localhost', port=0, source_address=None):
                     port = int(port)
                 except ValueError:
                     raise OSError("nonnumeric port")
+        self._host = host
         if not port:
             port = self.default_port
         sys.audit("smtplib.connect", self, host, port)

Lib/test/test_smtplib.py

@@ -282,6 +282,34 @@ def testBasic(self):
         # connect
         smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost',
                             timeout=support.LOOPBACK_TIMEOUT)
+        # implicit .connect must set ._host since it is used by .starttls
+        self.assertEqual(smtp._host, HOST)
+        smtp.quit()
+
+    def test_host_port_host(self):
+        # create instance with host:port notation
+        smtp = smtplib.SMTP(f"{HOST}:{self.port}", local_hostname='localhost',
+                            timeout=support.LOOPBACK_TIMEOUT)
+        # implicit .connect must set ._host since it is used by .starttls
+        self.assertEqual(smtp._host, HOST)
+        smtp.quit()
+
+    def test_explicit_connect(self):
+        # create instance without arguments
+        smtp = smtplib.SMTP(local_hostname='localhost',
+                            timeout=support.LOOPBACK_TIMEOUT)
+        # explicit .connect must set ._host since it is used by .starttls
+        smtp.connect(HOST, self.port)
+        self.assertEqual(smtp._host, HOST)
+        smtp.quit()
+
+    def test_explicit_connect_host_port(self):
+        # create instance without arguments
+        smtp = smtplib.SMTP(local_hostname='localhost',
+                            timeout=support.LOOPBACK_TIMEOUT)
+        # explicit .connect with host:port notation must set ._host, too
+        smtp.connect(f"{HOST}:{self.port}")
+        self.assertEqual(smtp._host, HOST)
         smtp.quit()
 
     def testSourceAddress(self):

Lib/test/test_smtpnet.py

@@ -42,6 +42,59 @@ def test_connect_starttls(self):
             server.ehlo()
             server.quit()
 
+    def test_connect_host_port_starttls(self):
+        support.get_attribute(smtplib, 'SMTP_SSL')
+        context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
+        context.check_hostname = False
+        context.verify_mode = ssl.CERT_NONE
+        with socket_helper.transient_internet(self.testServer):
+            server = smtplib.SMTP(f'{self.testServer}:{self.remotePort}')
+            try:
+                server.starttls(context=context)
+            except smtplib.SMTPException as e:
+                if e.args[0] == 'STARTTLS extension not supported by server.':
+                    unittest.skip(e.args[0])
+                else:
+                    raise
+            server.ehlo()
+            server.quit()
+
+    def test_explicit_connect_starttls(self):
+        support.get_attribute(smtplib, 'SMTP_SSL')
+        context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
+        context.check_hostname = False
+        context.verify_mode = ssl.CERT_NONE
+        with socket_helper.transient_internet(self.testServer):
+            server = smtplib.SMTP()
+            server.connect(self.testServer, self.remotePort)
+            try:
+                server.starttls(context=context)
+            except smtplib.SMTPException as e:
+                if e.args[0] == 'STARTTLS extension not supported by server.':
+                    unittest.skip(e.args[0])
+                else:
+                    raise
+            server.ehlo()
+            server.quit()
+
+    def test_explicit_connect_host_port_starttls(self):
+        support.get_attribute(smtplib, 'SMTP_SSL')
+        context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
+        context.check_hostname = False
+        context.verify_mode = ssl.CERT_NONE
+        with socket_helper.transient_internet(self.testServer):
+            server = smtplib.SMTP()
+            server.connect(f'{self.testServer}:{self.remotePort}')
+            try:
+                server.starttls(context=context)
+            except smtplib.SMTPException as e:
+                if e.args[0] == 'STARTTLS extension not supported by server.':
+                    unittest.skip(e.args[0])
+                else:
+                    raise
+            server.ehlo()
+            server.quit()
+
 
 class SmtpSSLTest(unittest.TestCase):
     testServer = 'smtp.gmail.com'

Misc/NEWS.d/next/Library/2024-02-10-21-25-22.gh-issue-70039.6wvcAP.rst

@@ -0,0 +1 @@
+In smtplib.SMTP, set ._host in .connect() and not immediately during instantiation. This allows .starttls() to work if the instance was created without arguments and host and port were only provided via an explicit .connect() call. If will also set ._host to the correct host value if the host and port were specified using the 'host:port' notation, which according to the documentation is supported by .connect().