Set defaults only for fields not passed in __init__

ctapipe/core/container.py

@@ -122,12 +122,12 @@ class Container(metaclass=ContainerMeta):
     def __init__(self, **fields):
         self.meta = {}
         # __slots__ cannot be provided with defaults
-        # via class variables, so we use a `__prefix` class variable
-        # and a `_prefix` in `__slots__` together with a property.
+        # via class variables, so we use a `container_prefix` class variable
+        # and an instance variable `prefix` in `__slots__`
         self.prefix = self.container_prefix
 
-        for k, v in self.fields.items():
-            setattr(self, k, deepcopy(v.default))
+        for k in set(self.fields).difference(fields):
+            setattr(self, k, deepcopy(self.fields[k].default))
 
         for k, v in fields.items():
             setattr(self, k, v)