From e68978978f49cfc489fa0f888281ce84460818c0 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Wed, 8 Jul 2020 14:21:00 -0700
Subject: bpo-40597: Allow email.contextmanager set_content() to set a null
 string. (GH-20542)

(cherry picked from commit 4fa61a7732923f92de0f7830c12da48c4cec937f)

Co-authored-by: Mark Sapiro <mark@msapiro.net>
---
 Lib/email/contentmanager.py                                 |  2 +-
 Lib/test/test_email/test_contentmanager.py                  | 13 +++++++++++++
 .../next/Library/2020-05-30-12-44-29.bpo-39384.Iqxy3q.rst   |  1 +
 3 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-05-30-12-44-29.bpo-39384.Iqxy3q.rst

diff --git a/Lib/email/contentmanager.py b/Lib/email/contentmanager.py
index 2b4b875..b91fb0e 100644
--- a/Lib/email/contentmanager.py
+++ b/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:
diff --git a/Lib/test/test_email/test_contentmanager.py b/Lib/test/test_email/test_contentmanager.py
index 64dca2d..f4f6bb7 100644
--- a/Lib/test/test_email/test_contentmanager.py
+++ b/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"
diff --git a/Misc/NEWS.d/next/Library/2020-05-30-12-44-29.bpo-39384.Iqxy3q.rst b/Misc/NEWS.d/next/Library/2020-05-30-12-44-29.bpo-39384.Iqxy3q.rst
new file mode 100644
index 0000000..482ae62
--- /dev/null
+++ b/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.
-- 
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