[DeferredDict] improve handling of constants

masque/utils/deferreddict.py

@@ -25,9 +25,16 @@ class DeferredDict(dict, Generic[Key, Value]):
     """
     def __init__(self, *args, **kwargs) -> None:
         dict.__init__(self)
-        self.update(*args, **kwargs)
+        if args or kwargs:
+            self.update(*args, **kwargs)
 
     def __setitem__(self, key: Key, value: Callable[[], Value]) -> None:
+        """
+        Set a value, which must be a callable that returns the actual value.
+        The result of the callable is cached after the first access.
+        """
+        if not callable(value):
+            raise TypeError(f"DeferredDict value must be callable, got {type(value)}")
         cached_fn = lru_cache(maxsize=1)(value)
         dict.__setitem__(self, key, cached_fn)
 
@@ -35,8 +42,15 @@ class DeferredDict(dict, Generic[Key, Value]):
         return dict.__getitem__(self, key)()
 
     def update(self, *args, **kwargs) -> None:
+        """
+        Update the DeferredDict. If a value is callable, it is used as a generator.
+        Otherwise, it is wrapped as a constant.
+        """
         for k, v in dict(*args, **kwargs).items():
-            self[k] = v
+            if callable(v):
+                self[k] = v
+            else:
+                self.set_const(k, v)
 
     def __repr__(self) -> str:
         return '<DeferredDict with keys ' + repr(set(self.keys())) + '>'