#10839: raise an error on add of duplicate unique headers in new email policies

This feature was supposed to be part of the initial email6 checkin, but it got
lost in my big refactoring.

In this patch I'm not providing an easy way to turn off the errors, but they
only happen when a header is added programmatically, and it is almost never
the right thing to do to allow the duplicate to be added.  An application that
needs to add duplicates of unique headers can create a policy subclass to
allow it.

1 files changed, 8 insertions, 0 deletions

diff --git a/Lib/email/policy.py b/Lib/email/policy.py
index 47ed66b..bfffb45 100644
--- a/Lib/email/policy.py
+++ b/Lib/email/policy.py
@@ -69,6 +69,14 @@ class EmailPolicy(Policy):
             object.__setattr__(self, 'header_factory', HeaderRegistry())
         super().__init__(**kw)
 
+    def header_max_count(self, name):
+        """+
+        The implementation for this class returns the max_count attribute from
+        the specialized header class that would be used to construct a header
+        of type 'name'.
+        """
+        return self.header_factory[name].max_count
+
     # The logic of the next three methods is chosen such that it is possible to
     # switch a Message object between a Compat32 policy and a policy derived
     # from this class and have the results stay consistent.  This allows a