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bpo-34246: Use no mutable default args in smtplib (GH-8554)

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 d5fbe9b)

Co-authored-by: Pablo Aguiar <>
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miss-islington and scorphus committed Sep 7, 2018
1 parent 7399407 commit 9835696ec4c57a9a30f1c11cfb4c5d3e121bf97c
@@ -419,7 +419,7 @@ An :class:`SMTP` instance has the following methods:

.. method:: SMTP.sendmail(from_addr, to_addrs, msg, mail_options=[], rcpt_options=[])
.. method:: SMTP.sendmail(from_addr, to_addrs, msg, mail_options=(), rcpt_options=())

Send mail. The required arguments are an :rfc:`822` from-address string, a list
of :rfc:`822` to-address strings (a bare string will be treated as a list with 1
@@ -491,7 +491,7 @@ An :class:`SMTP` instance has the following methods:

.. method:: SMTP.send_message(msg, from_addr=None, to_addrs=None, \
mail_options=[], rcpt_options=[])
mail_options=(), rcpt_options=())

This is a convenience method for calling :meth:`sendmail` with the message
represented by an :class:`email.message.Message` object. The arguments have
@@ -513,7 +513,7 @@ def noop(self):
"""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 @@ def mail(self, sender, options=[]):
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 @@ def starttls(self, keyfile=None, certfile=None, context=None):
raise SMTPResponseException(resp, reply)
return (resp, reply)

def sendmail(self, from_addr, to_addrs, msg, mail_options=[],
def sendmail(self, from_addr, to_addrs, msg, mail_options=(),
"""This command performs an entire mail transaction.
The arguments are:
@@ -890,7 +890,7 @@ def sendmail(self, from_addr, to_addrs, msg, mail_options=[],
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 @@ def send_message(self, msg, from_addr=None, to_addrs=None,
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')
g = email.generator.BytesGenerator(bytesmsg)
g.flatten(msg_copy, linesep='\r\n')
@@ -20,6 +20,7 @@
import unittest
from test import support, mock_socket
from import HOST, HOSTv4, HOSTv6
from unittest.mock import Mock

if sys.platform == 'darwin':
@@ -578,6 +579,33 @@ def testNonnumericPort(self):

class DefaultArgumentsTests(unittest.TestCase):

def setUp(self):
self.msg = EmailMessage()
self.msg['From'] = 'Páolo <fő>'
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')

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'])

# test response of client to a non-successful HELO message
class BadHELOServerTests(unittest.TestCase):

@@ -22,6 +22,7 @@ Eitan Adler
Anton Afanasyev
Ali Afshar
Nitika Agarwal
Pablo S. Blum de Aguiar
Jim Ahlstrom
Farhan Ahmad
Matthew Ahrens
@@ -0,0 +1,2 @@
:meth:`smtplib.SMTP.send_message` no longer modifies the content of the
*mail_options* argument. Patch by Pablo S. Blum de Aguiar.

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