bpo-28879: Add Date header if missing in smtplib send_message

Doc/library/smtplib.rst

@@ -522,12 +522,17 @@ An :class:`SMTP` instance has the following methods:
    specified in :rfc:`5322`\: *from_addr* is set to the :mailheader:`Sender`
    field if it is present, and otherwise to the :mailheader:`From` field.
    *to_addrs* combines the values (if any) of the :mailheader:`To`,
-   :mailheader:`Cc`, and :mailheader:`Bcc` fields from *msg*.  If exactly one
-   set of :mailheader:`Resent-*` headers appear in the message, the regular
-   headers are ignored and the :mailheader:`Resent-*` headers are used instead.
-   If the message contains more than one set of :mailheader:`Resent-*` headers,
-   a :exc:`ValueError` is raised, since there is no way to unambiguously detect
-   the most recent set of :mailheader:`Resent-` headers.
+   :mailheader:`Cc`, and :mailheader:`Bcc` fields from *msg*.  If there's no
+   :mailheader:`Date` header inside the message, ``send_message`` will add one to the data.
+   If exactly one set of :mailheader:`Resent-*` headers appear in the message,
+   the regular headers are ignored and the :mailheader:`Resent-*` headers are
+   used instead.  If the message contains more than one set of
+   :mailheader:`Resent-*` headers, a :exc:`ValueError` is raised, since there
+   is no way to unambiguously detect the most recent set of
+   :mailheader:`Resent-` headers.
+
+   .. versionchanged:: 3.2
+      Support to add :mailheader:`Date` header to the message if one does not exist.
 
    ``send_message`` serializes *msg* using
    :class:`~email.generator.BytesGenerator` with ``\r\n`` as the *linesep*, and

Lib/smtplib.py

@@ -932,6 +932,10 @@ def send_message(self, msg, from_addr=None, to_addrs=None,
             header_prefix = 'Resent-'
         else:
             raise ValueError("message has more than one 'Resent-' header block")
+
+        # RFC 5322 section 3.6, 4th Paragraph
+        if msg.get('Date', None) is None:
+            msg['Date'] = email.utils.formatdate()
         if from_addr is None:
             # Prefer the sender field per RFC 2822:3.6.2.
             from_addr = (msg[header_prefix + 'Sender']

Lib/test/test_smtplib.py

@@ -23,7 +23,7 @@
 from test.support import hashlib_helper
 from test.support import socket_helper
 from test.support import threading_helper
-from unittest.mock import Mock
+from unittest.mock import Mock, patch
 
 HOST = socket_helper.HOST
 
@@ -1342,36 +1342,40 @@ def test_send_unicode_with_SMTPUTF8_via_low_level_API(self):
         self.assertEqual(self.serv.last_rcpt_options, [])
 
     def test_send_message_uses_smtputf8_if_addrs_non_ascii(self):
-        msg = EmailMessage()
-        msg['From'] = "Páolo <fő[email protected]>"
-        msg['To'] = 'Dinsdale'
-        msg['Subject'] = 'Nudge nudge, wink, wink \u1F609'
-        # XXX I don't know why I need two \n's here, but this is an existing
-        # bug (if it is one) and not a problem with the new functionality.
-        msg.set_content("oh là là, know what I mean, know what I mean?\n\n")
-        # XXX smtpd converts received /r/n to /n, so we can't easily test that
-        # we are successfully sending /r/n :(.
-        expected = textwrap.dedent("""\
-            From: Páolo <fő[email protected]>
-            To: Dinsdale
-            Subject: Nudge nudge, wink, wink \u1F609
-            Content-Type: text/plain; charset="utf-8"
-            Content-Transfer-Encoding: 8bit
-            MIME-Version: 1.0
-
-            oh là là, know what I mean, know what I mean?
-            """)
-        smtp = smtplib.SMTP(
-            HOST, self.port, local_hostname='localhost',
-            timeout=support.LOOPBACK_TIMEOUT)
-        self.addCleanup(smtp.close)
-        self.assertEqual(smtp.send_message(msg), {})
-        self.assertEqual(self.serv.last_mailfrom, 'fő[email protected]')
-        self.assertEqual(self.serv.last_rcpttos, ['Dinsdale'])
-        self.assertEqual(self.serv.last_message.decode(), expected)
-        self.assertIn('BODY=8BITMIME', self.serv.last_mail_options)
-        self.assertIn('SMTPUTF8', self.serv.last_mail_options)
-        self.assertEqual(self.serv.last_rcpt_options, [])
+        expected_date = "Thu, 19 Mar 2020 00:59:43 -0000"
+        with patch("email.utils.formatdate") as date_mock:
+            date_mock.return_value = expected_date
+            msg = EmailMessage()
+            msg['From'] = "Páolo <fő[email protected]>"
+            msg['To'] = 'Dinsdale'
+            msg['Subject'] = 'Nudge nudge, wink, wink \u1F609'
+            # XXX I don't know why I need two \n's here, but this is an existing
+            # bug (if it is one) and not a problem with the new functionality.
+            msg.set_content("oh là là, know what I mean, know what I mean?\n\n")
+            # XXX smtpd converts received /r/n to /n, so we can't easily test that
+            # we are successfully sending /r/n :(.
+            expected = textwrap.dedent("""\
+                From: Páolo <fő[email protected]>
+                To: Dinsdale
+                Subject: Nudge nudge, wink, wink \u1F609
+                Content-Type: text/plain; charset="utf-8"
+                Content-Transfer-Encoding: 8bit
+                MIME-Version: 1.0
+                Date: {}
+
+                oh là là, know what I mean, know what I mean?
+                """.format(expected_date))
+            smtp = smtplib.SMTP(
+                HOST, self.port, local_hostname='localhost',
+                timeout=support.LOOPBACK_TIMEOUT)
+            self.addCleanup(smtp.close)
+            self.assertEqual(smtp.send_message(msg), {})
+            self.assertEqual(self.serv.last_mailfrom, 'fő[email protected]')
+            self.assertEqual(self.serv.last_rcpttos, ['Dinsdale'])
+            self.assertEqual(self.serv.last_message.decode(), expected)
+            self.assertIn('BODY=8BITMIME', self.serv.last_mail_options)
+            self.assertIn('SMTPUTF8', self.serv.last_mail_options)
+            self.assertEqual(self.serv.last_rcpt_options, [])
 
 
 EXPECTED_RESPONSE = encode_base64(b'\0psu\0doesnotexist', eol='')

Misc/NEWS.d/next/Library/2018-01-13-15-52-00.bpo-28879.aW2gj0.rst

@@ -0,0 +1,2 @@
+Fix ``smtplib.send_message`` to add a ``Date`` header if it is missing as per
+RFC5322.