A handler can only be registered once

README.md

@@ -349,4 +349,5 @@ See LICENSE file for details.
 
 ## Version History
 * 0.1.0 - Initial release
-* 0.2.0 - Added custom reference handlers
+* 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

@@ -9,11 +9,11 @@ TAG_SPECIAL = "__special__"
 
 class BaseHandler:
   """Base class for all handlers. Subclasses must implement is_eligible_for() and tag()."""
-
+  
   def is_eligible_for(self, obj):
     """Check if this handler can process the given object."""
     raise NotImplementedError
-
+  
   def tag(self):
     """Return a unique tag identifying this handler type."""
     raise NotImplementedError
@@ -21,11 +21,11 @@ class BaseHandler:
 
 class BaseInlineHandler(BaseHandler):
   """Handler that serializes data directly into the JSON snapshot."""
-
+  
   def serialize(self, obj) -> dict:
     """Serialize object to dict for inline storage in JSON snapshot."""
     raise NotImplementedError
-
+  
   def deserialize(self, data: dict) -> object:
     """Deserialize object from dict stored in JSON snapshot."""
     raise NotImplementedError
@@ -33,15 +33,15 @@ class BaseInlineHandler(BaseHandler):
 
 class BaseRefHandler(BaseHandler):
   """Handler that serializes data to refs/ directory using content-addressable storage."""
-
+  
   def serialize_to_bytes(self, obj) -> bytes:
     """Serialize object to bytes for storage in refs/. Must be implemented by subclass."""
     raise NotImplementedError
-
+  
   def deserialize_from_bytes(self, data: bytes) -> object:
     """Deserialize object from bytes loaded from refs/. Must be implemented by subclass."""
     raise NotImplementedError
-
+  
   def serialize(self, obj, ref_helper) -> dict:
     """
     Default implementation: converts object to bytes, saves to refs/, returns dict with tag and digest.
@@ -52,10 +52,10 @@ class BaseRefHandler(BaseHandler):
     from dbengine.serializer import TAG_DIGEST
     TAG_SPECIAL = "__special__"
     return {
-      TAG_SPECIAL: self.tag(),
-      TAG_DIGEST: digest
+        TAG_SPECIAL: self.tag(),
+        TAG_DIGEST: digest
     }
-
+  
   def deserialize(self, data: dict, ref_helper) -> object:
     """
     Default implementation: loads bytes from refs/ using digest, deserializes from bytes.
@@ -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