gh-135307: Fix email error when policy max_line_length is set to 0 or None

Lib/email/contentmanager.py

@@ -3,6 +3,7 @@
 import email.message
 import email.errors
 from email import quoprimime
+import sys
 
 class ContentManager:
 
@@ -142,22 +143,23 @@ def _encode_base64(data, max_line_length):
 
 
 def _encode_text(string, charset, cte, policy):
+    # max_line_length 0/None means no limit, ie: infinitely long.
+    maxlen = policy.max_line_length or sys.maxsize
     lines = string.encode(charset).splitlines()
     linesep = policy.linesep.encode('ascii')
     def embedded_body(lines): return linesep.join(lines) + linesep
     def normal_body(lines): return b'\n'.join(lines) + b'\n'
     if cte is None:
         # Use heuristics to decide on the "best" encoding.
-        if max((len(x) for x in lines), default=0) <= policy.max_line_length:
+        if max((len(x) for x in lines), default=0) <= maxlen:
             try:
                 return '7bit', normal_body(lines).decode('ascii')
             except UnicodeDecodeError:
                 pass
             if policy.cte_type == '8bit':
                 return '8bit', normal_body(lines).decode('ascii', 'surrogateescape')
         sniff = embedded_body(lines[:10])
-        sniff_qp = quoprimime.body_encode(sniff.decode('latin-1'),
-                                          policy.max_line_length)
+        sniff_qp = quoprimime.body_encode(sniff.decode('latin-1'), maxlen)
         sniff_base64 = binascii.b2a_base64(sniff)
         # This is a little unfair to qp; it includes lineseps, base64 doesn't.
         if len(sniff_qp) > len(sniff_base64):
@@ -172,9 +174,9 @@ def normal_body(lines): return b'\n'.join(lines) + b'\n'
         data = normal_body(lines).decode('ascii', 'surrogateescape')
     elif cte == 'quoted-printable':
         data = quoprimime.body_encode(normal_body(lines).decode('latin-1'),
-                                      policy.max_line_length)
+                                      maxlen)
     elif cte == 'base64':
-        data = _encode_base64(embedded_body(lines), policy.max_line_length)
+        data = _encode_base64(embedded_body(lines), maxlen)
     else:
         raise ValueError("Unknown content transfer encoding {}".format(cte))
     return cte, data

Lib/test/test_email/test_message.py

@@ -1004,6 +1004,26 @@ def test_folding_with_long_nospace_http_policy_1(self):
         parsed_msg = message_from_bytes(m.as_bytes(), policy=policy.default)
         self.assertEqual(parsed_msg['Message-ID'], m['Message-ID'])
 
+    def test_no_wrapping_max_line_length(self):
+        def do_test_no_wrapping_max_line_length(n):
+            pol = policy.default.clone(max_line_length=n)
+            subj = "S" * 100
+            body = "B" * 100
+            msg = EmailMessage(policy=pol)
+            msg["From"] = "a@ex.com"
+            msg["To"] = "b@ex.com"
+            msg["Subject"] = subj
+            msg.set_content(body)
+
+            raw = msg.as_bytes()
+            self.assertNotIn(b"\r\n ", raw, "Found fold indicator; wrapping not disabled")
+
+            parsed = message_from_bytes(msg.as_bytes(), policy=policy.default)
+            self.assertEqual(parsed["Subject"], subj)
+            self.assertEqual(parsed.get_body().get_content().rstrip('\n'), body)
+        do_test_no_wrapping_max_line_length(None)
+        do_test_no_wrapping_max_line_length(0)
+
     def test_invalid_header_names(self):
         invalid_headers = [
             ('Invalid Header', 'contains space'),

Misc/NEWS.d/next/Library/2025-06-10-18-02-29.gh-issue-135307.fXGrcK.rst

@@ -0,0 +1,2 @@
+:mod:`email`: Ensure policy accepts unlimited line lengths by
+treating falsey values as :data:`sys.maxsize`.