bpo-41402: Fix email ContentManager calling .encode() on bytes (GH-21631)

Lib/email/contentmanager.py

@@ -238,9 +238,7 @@ def set_bytes_content(msg, data, maintype, subtype, cte='base64',
         data = binascii.b2a_qp(data, istext=False, header=False, quotetabs=True)
         data = data.decode('ascii')
     elif cte == '7bit':
-        # Make sure it really is only ASCII.  The early warning here seems
-        # worth the overhead...if you care write your own content manager :).
-        data.encode('ascii')
+        data = data.decode('ascii')
     elif cte in ('8bit', 'binary'):
         data = data.decode('ascii', 'surrogateescape')
     msg.set_payload(data)

Lib/test/test_email/test_contentmanager.py

@@ -776,6 +776,18 @@ class TestRawDataManager(TestEmailBase):
             foo
             """).encode('ascii'))
 
+    def test_set_content_bytes_cte_7bit(self):
+        m = self._make_message()
+        m.set_content(b'ASCII-only message.\n',
+            maintype='application', subtype='octet-stream', cte='7bit')
+        self.assertEqual(str(m), textwrap.dedent("""\
+            Content-Type: application/octet-stream
+            Content-Transfer-Encoding: 7bit
+            MIME-Version: 1.0
+
+            ASCII-only message.
+            """))
+
     content_object_params = {
         'text_plain': ('content', ()),
         'text_html': ('content', ('html',)),

Misc/NEWS.d/next/Library/2020-07-26-18-17-30.bpo-41402.YRkVkp.rst

@@ -0,0 +1 @@
+Fix :meth:`email.message.EmailMessage.set_content` when called with binary data and ``7bit`` content transfer encoding.