bpo-40597: Allow email.contextmanager set_content() to set a null string. (GH-20542)

(cherry picked from commit 4fa61a7732)

Co-authored-by: Mark Sapiro <mark@msapiro.net>

Lib/email/contentmanager.py

@@ -146,7 +146,7 @@ def _encode_text(string, charset, cte, policy):
     def normal_body(lines): return b'\n'.join(lines) + b'\n'
     if cte==None:
         # Use heuristics to decide on the "best" encoding.
-        if max(len(x) for x in lines) <= policy.max_line_length:
+        if max((len(x) for x in lines), default=0) <= policy.max_line_length:
             try:
                 return '7bit', normal_body(lines).decode('ascii')
             except UnicodeDecodeError:

Lib/test/test_email/test_contentmanager.py

@@ -303,6 +303,19 @@ class TestRawDataManager(TestEmailBase):
         self.assertEqual(m.get_payload(decode=True).decode('utf-8'), content)
         self.assertEqual(m.get_content(), content)
 
+    def test_set_text_plain_null(self):
+        m = self._make_message()
+        content = ''
+        raw_data_manager.set_content(m, content)
+        self.assertEqual(str(m), textwrap.dedent("""\
+            Content-Type: text/plain; charset="utf-8"
+            Content-Transfer-Encoding: 7bit
+
+
+            """))
+        self.assertEqual(m.get_payload(decode=True).decode('utf-8'), '\n')
+        self.assertEqual(m.get_content(), '\n')
+
     def test_set_text_html(self):
         m = self._make_message()
         content = "<p>Simple message.</p>\n"

Misc/NEWS.d/next/Library/2020-05-30-12-44-29.bpo-39384.Iqxy3q.rst

@@ -0,0 +1 @@
+Fixed email.contentmanager to allow set_content() to set a null string.