A handler can only be registered once

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