[3.6] bpo-34246: Use no mutable default args in smtplib (GH-8554) (#9112)

Some methods of the SMTP class use mutable default arguments. Specially
`send_message` is affected as it mutates one of the args by appending items
to it, which has side effects on further calls..
(cherry picked from commit d5fbe9b1a3d65ceeb9159c5ba999ee966a945f76)

Co-authored-by: Pablo Aguiar <scorphus@gmail.com>

2 files changed, 38 insertions, 6 deletions

diff --git a/Lib/smtplib.py b/Lib/smtplib.py
index b679875fd2c..048c6bfb067 100755
--- a/Lib/smtplib.py
+++ b/Lib/smtplib.py
@@ -513,7 +513,7 @@ class SMTP:
         """SMTP 'noop' command -- doesn't do anything :>"""
         return self.docmd("noop")
 
-    def mail(self, sender, options=[]):
+    def mail(self, sender, options=()):
         """SMTP 'mail' command -- begins mail xfer session.
 
         This method may raise the following exceptions:
@@ -534,7 +534,7 @@ class SMTP:
         self.putcmd("mail", "FROM:%s%s" % (quoteaddr(sender), optionlist))
         return self.getreply()
 
-    def rcpt(self, recip, options=[]):
+    def rcpt(self, recip, options=()):
         """SMTP 'rcpt' command -- indicates 1 recipient for this mail."""
         optionlist = ''
         if options and self.does_esmtp:
@@ -785,8 +785,8 @@ class SMTP:
             raise SMTPResponseException(resp, reply)
         return (resp, reply)
 
-    def sendmail(self, from_addr, to_addrs, msg, mail_options=[],
-                 rcpt_options=[]):
+    def sendmail(self, from_addr, to_addrs, msg, mail_options=(),
+                 rcpt_options=()):
         """This command performs an entire mail transaction.
 
         The arguments are:
@@ -890,7 +890,7 @@ class SMTP:
         return senderrs
 
     def send_message(self, msg, from_addr=None, to_addrs=None,
-                mail_options=[], rcpt_options={}):
+                     mail_options=(), rcpt_options=()):
         """Converts message to a bytestring and passes it to sendmail.
 
         The arguments are as for sendmail, except that msg is an
@@ -958,7 +958,7 @@ class SMTP:
             if international:
                 g = email.generator.BytesGenerator(
                     bytesmsg, policy=msg.policy.clone(utf8=True))
-                mail_options += ['SMTPUTF8', 'BODY=8BITMIME']
+                mail_options = (*mail_options, 'SMTPUTF8', 'BODY=8BITMIME')
             else:
                 g = email.generator.BytesGenerator(bytesmsg)
             g.flatten(msg_copy, linesep='\r\n')
diff --git a/Lib/test/test_smtplib.py b/Lib/test/test_smtplib.py
index 79b3bd43687..18110191dc3 100644
--- a/Lib/test/test_smtplib.py
+++ b/Lib/test/test_smtplib.py
@@ -18,6 +18,11 @@ import textwrap
 
 import unittest
 from test import support, mock_socket
+from unittest.mock import Mock
+
+HOST = "localhost"
+HOSTv4 = "127.0.0.1"
+HOSTv6 = "::1"
 
 try:
     import threading
@@ -569,6 +574,33 @@ class NonConnectingTests(unittest.TestCase):
                           "localhost:bogus")
 
 
+class DefaultArgumentsTests(unittest.TestCase):
+
+    def setUp(self):
+        self.msg = EmailMessage()
+        self.msg['From'] = 'Páolo <főo@bar.com>'
+        self.smtp = smtplib.SMTP()
+        self.smtp.ehlo = Mock(return_value=(200, 'OK'))
+        self.smtp.has_extn, self.smtp.sendmail = Mock(), Mock()
+
+    def testSendMessage(self):
+        expected_mail_options = ('SMTPUTF8', 'BODY=8BITMIME')
+        self.smtp.send_message(self.msg)
+        self.smtp.send_message(self.msg)
+        self.assertEqual(self.smtp.sendmail.call_args_list[0][0][3],
+                         expected_mail_options)
+        self.assertEqual(self.smtp.sendmail.call_args_list[1][0][3],
+                         expected_mail_options)
+
+    def testSendMessageWithMailOptions(self):
+        mail_options = ['STARTTLS']
+        expected_mail_options = ('STARTTLS', 'SMTPUTF8', 'BODY=8BITMIME')
+        self.smtp.send_message(self.msg, None, None, mail_options)
+        self.assertEqual(mail_options, ['STARTTLS'])
+        self.assertEqual(self.smtp.sendmail.call_args_list[0][0][3],
+                         expected_mail_options)
+
+
 # test response of client to a non-successful HELO message
 @unittest.skipUnless(threading, 'Threading required for this test.')
 class BadHELOServerTests(unittest.TestCase):