feat: use a computed_field with cached_property for preferred usernames and subs

Author: ryuwd

diracx-core/src/diracx/core/config/schema.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from functools import cached_property
 import os
 from datetime import date, datetime
 from typing import Annotated, Any, MutableMapping, TypeVar
@@ -10,6 +11,7 @@
     EmailStr,
     Field,
     PrivateAttr,
+    computed_field,
     field_validator,
     model_validator,
 )
@@ -158,6 +160,15 @@ class RegistryConfig(BaseModel):
     Groups: MutableMapping[str, GroupConfig]
     """DIRAC groups section, subsections represent the name of the group."""
 
+    @computed_field  # type: ignore[misc]
+    @cached_property
+    def _preferred_username_to_sub(self) -> dict[str, str]:
+        """Compute reverse lookup map from preferred username to user sub.
+        
+        This computed field is automatically cached by Pydantic v2.
+        """
+        return {user.PreferedUsername: sub for sub, user in self.Users.items()}
+
     def sub_from_preferred_username(self, preferred_username: str) -> str:
         """Get the user sub from the preferred username.
 
@@ -171,10 +182,10 @@ def sub_from_preferred_username(self, preferred_username: str) -> str:
             KeyError: If no user with the given preferred username is found.
 
         """
-        for sub, user in self.Users.items():
-            if user.PreferedUsername == preferred_username:
-                return sub
-        raise KeyError(f"User {preferred_username} not found in registry")
+        try:
+            return self._preferred_username_to_sub[preferred_username]
+        except KeyError:
+            raise KeyError(f"User {preferred_username} not found in registry") from None
 
 
 class DIRACConfig(BaseModel):