A handler can only be registered once

README.md

@@ -350,3 +350,4 @@ See LICENSE file for details.
 ## Version History
 * 0.1.0 - Initial release
 * 0.2.0 - Added custom reference handlers
+* 0.2.1 - A handler can only be registered once

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "mydbengine"
-version = "0.2.0"
+version = "0.2.1"
 description = "A lightweight, git-inspired database engine that maintains complete history of all modifications"
 readme = "README.md"
 requires-python = ">=3.8"

src/dbengine/handlers.py

@@ -89,7 +89,7 @@ class DateHandler(BaseInlineHandler):
 class Handlers:
   
   def __init__(self, handlers_):
-    self.handlers = handlers_
+    self.handlers = handlers_ or []
   
   def get_handler(self, obj):
     if has_tag(obj, TAG_SPECIAL):
@@ -102,7 +102,14 @@ class Handlers:
     return None
   
   def register_handler(self, handler):
+    for h in self.handlers:
+      if type(h) == type(handler):
+        return  # won't register twice the same handler
+    
     self.handlers.append(handler)
   
+  def unregister_handler(self, handler):
+    self.handlers = [h for h in self.handlers if type(h) != type(handler)]
+
 
 handlers = Handlers([DateHandler()])

tests/test_handlers.py

@@ -0,0 +1,16 @@
+from dbengine.handlers import BaseHandler, handlers
+
+
+class DummyHandler(BaseHandler):
+  pass
+
+
+def test_i_can_register_only_once():
+  handlers.register_handler(DummyHandler())
+  count = len(handlers.handlers)
+  
+  handlers.register_handler(DummyHandler())  # second time is skipped as the class is already registered
+  assert count == len(handlers.handlers)
+  
+  handlers.unregister_handler(DummyHandler())  # clean the handlers list
+  assert len(handlers.handlers) == count - 1