1 files changed, 19 insertions, 0 deletions
diff --git a/Lib/email/_policybase.py b/Lib/email/_policybase.py
index 6bc298b..d5e8df9 100644
--- a/Lib/email/_policybase.py
+++ b/Lib/email/_policybase.py
@@ -194,6 +194,25 @@ class Policy(_PolicyBase, metaclass=_DocstringExtenderMetaclass):
         """
         obj.defects.append(defect)
 
+    def header_max_count(self, name):
+        """Return the maximum allowed number of headers named 'name'.
+
+        Called when a header is added to a Message object.  If the returned
+        value is not 0 or None, and there are already a number of headers with
+        the name 'name' equal to the value returned, a ValueError is raised.
+
+        Because the default behavior of Message's __setitem__ is to append the
+        value to the list of headers, it is easy to create duplicate headers
+        without realizing it.  This method allows certain headers to be limited
+        in the number of instances of that header that may be added to a
+        Message programmatically.  (The limit is not observed by the parser,
+        which will faithfully produce as many headers as exist in the message
+        being parsed.)
+
+        The default implementation returns None for all header names.
+        """
+        return None
+
     @abc.abstractmethod
     def header_source_parse(self, sourcelines):
         """Given a list of linesep terminated strings constituting the lines of