Use caching on properties

lib/nntp/tiny/message.py

@@ -125,7 +125,7 @@ class Message(DatabaseTable):
         return self._body
 
     def header(self, key: str):
-        if self.headers is None:
+        if self._headers is None:
             self.read(self.content)
 
         return self.headers.get(key.lower())
@@ -151,40 +151,66 @@ class Message(DatabaseTable):
 
     @created_on.setter
     def created_on(self, value):
-        self.headers['date']      = str(value)
+        if self._headers is not None:
+            self._headers['date'] = str(value)
+
         self._cache['created_on'] = str(value)
 
     @property
     def message_id(self) -> str:
+        if self._headers is None:
+            return self._cache.get('message_id')
+
         return self.header('Message-ID')
 
     @message_id.setter
     def message_id(self, value):
-        self.headers['message-id'] = value
+        if self._headers is None:
+            self._cache['message_id'] = value
+        else:
+            self.headers['message-id'] = value
 
     @property
     def parent_id(self) -> str:
+        if self._headers is None:
+            return self._cache.get('parent_id')
+
         return self.header('References')
 
     @parent_id.setter
     def parent_id(self, value):
-        self.headers['references'] = value
+        if self._headers is None:
+            self._cache['parent_id'] = value
+        else:
+            self.headers['references'] = value
 
     @property
     def sender(self) -> str:
+        if self._headers is None:
+            return self._cache.get('sender')
+
         return self.headers.get('from', 'Unknown')
 
     @sender.setter
     def sender(self, value):
-        self.headers['from'] = value
+        if self._headers is None:
+            self._cache['sender'] = value
+        else:
+            self.headers['from'] = value
 
     @property
     def subject(self) -> str:
+        if self._headers is None:
+            return self._cache.get('subject', '(no subject)')
+
         return self.headers.get('subject', '(no subject)')
 
     @subject.setter
     def subject(self, value):
-        self.headers['subject'] = value
+        if self._headers is None:
+            self._cache['subject'] = value
+        else:
+            self.headers['subject'] = value
 
     def is_first_line(self):
         return len(self.headers) == 1 and (self._body == '' or self._body is None)