#20098: add mangle_from_ policy option.

This defaults to True in the compat32 policy for backward compatibility,
but to False for all new policies.

Patch by Milan Oberkirch, with a few tweaks.

4 files changed, 40 insertions, 5 deletions

diff --git a/Lib/email/_policybase.py b/Lib/email/_policybase.py
index 8106114..c0d98a4 100644
--- a/Lib/email/_policybase.py
+++ b/Lib/email/_policybase.py
@@ -149,12 +149,18 @@ class Policy(_PolicyBase, metaclass=abc.ABCMeta):
                            during serialization.  None or 0 means no line
                            wrapping is done.  Default is 78.
 
+    mangle_from_        -- a flag that, when True escapes From_ lines in the
+                           body of the message by putting a `>' in front of
+                           them. This is used when the message is being
+                           serialized by a generator. Default: True.
+
     """
 
     raise_on_defect = False
     linesep = '\n'
     cte_type = '8bit'
     max_line_length = 78
+    mangle_from_ = False
 
     def handle_defect(self, obj, defect):
         """Based on policy, either raise defect or call register_defect.
@@ -266,6 +272,8 @@ class Compat32(Policy):
     replicates the behavior of the email package version 5.1.
     """
 
+    mangle_from_ = True
+
     def _sanitize_header(self, name, value):
         # If the header value contains surrogates, return a Header using
         # the unknown-8bit charset to encode the bytes as encoded words.
diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index 4735721..11ff16d 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -32,16 +32,16 @@ class Generator:
     # Public interface
     #
 
-    def __init__(self, outfp, mangle_from_=True, maxheaderlen=None, *,
+    def __init__(self, outfp, mangle_from_=None, maxheaderlen=None, *,
                  policy=None):
         """Create the generator for message flattening.
 
         outfp is the output file-like object for writing the message to.  It
         must have a write() method.
 
-        Optional mangle_from_ is a flag that, when True (the default), escapes
-        From_ lines in the body of the message by putting a `>' in front of
-        them.
+        Optional mangle_from_ is a flag that, when True (the default if policy
+        is not set), escapes From_ lines in the body of the message by putting
+        a `>' in front of them.
 
         Optional maxheaderlen specifies the longest length for a non-continued
         header.  When a header line is longer (in characters, with tabs
@@ -56,6 +56,9 @@ class Generator:
         flatten method is used.
 
         """
+
+        if mangle_from_ is None:
+            mangle_from_ = True if policy is None else policy.mangle_from_
         self._fp = outfp
         self._mangle_from_ = mangle_from_
         self.maxheaderlen = maxheaderlen
@@ -449,7 +452,7 @@ class DecodedGenerator(Generator):
     Like the Generator base class, except that non-text parts are substituted
     with a format string representing the part.
     """
-    def __init__(self, outfp, mangle_from_=True, maxheaderlen=78, fmt=None):
+    def __init__(self, outfp, mangle_from_=None, maxheaderlen=78, fmt=None):
         """Like Generator.__init__() except that an additional optional
         argument is allowed.
 
diff --git a/Lib/test/test_email/test_generator.py b/Lib/test/test_email/test_generator.py
index 920f870..b1cbce2 100644
--- a/Lib/test/test_email/test_generator.py
+++ b/Lib/test/test_email/test_generator.py
@@ -140,6 +140,28 @@ class TestGeneratorBase:
         g.flatten(msg, linesep='\n')
         self.assertEqual(s.getvalue(), self.typ(expected))
 
+    def test_set_mangle_from_via_policy(self):
+        source = textwrap.dedent("""\
+            Subject: test that
+             from is mangeld in the body!
+
+            From time to time I write a rhyme.
+            """)
+        variants = (
+            (None, True),
+            (policy.compat32, True),
+            (policy.default, False),
+            (policy.default.clone(mangle_from_=True), True),
+            )
+        for p, mangle in variants:
+            expected = source.replace('From ', '>From ') if mangle else source
+            with self.subTest(policy=p, mangle_from_=mangle):
+                msg = self.msgmaker(self.typ(source))
+                s = self.ioclass()
+                g = self.genclass(s, policy=p)
+                g.flatten(msg)
+                self.assertEqual(s.getvalue(), self.typ(expected))
+
 
 class TestGenerator(TestGeneratorBase, TestEmailBase):
 
diff --git a/Lib/test/test_email/test_policy.py b/Lib/test/test_email/test_policy.py
index 4b0a04e..9bb32f0 100644
--- a/Lib/test/test_email/test_policy.py
+++ b/Lib/test/test_email/test_policy.py
@@ -22,6 +22,7 @@ class PolicyAPITests(unittest.TestCase):
         'linesep':                  '\n',
         'cte_type':                 '8bit',
         'raise_on_defect':          False,
+        'mangle_from_':             True,
         }
     # These default values are the ones set on email.policy.default.
     # If any of these defaults change, the docs must be updated.
@@ -32,6 +33,7 @@ class PolicyAPITests(unittest.TestCase):
         'header_factory':           email.policy.EmailPolicy.header_factory,
         'refold_source':            'long',
         'content_manager':          email.policy.EmailPolicy.content_manager,
+        'mangle_from_':             False,
         })
 
     # For each policy under test, we give here what we expect the defaults to