fields.Function no requiere getter

FossilOrigin-Name: 47f9b9427ef55c688678001361260e5d00ea53d82977ea13e3414ed04878fb36

facho/model/fields/function.py

@@ -2,7 +2,7 @@ from .field import Field
 from .model import Model
 
 class Function(Field):
-    def __init__(self, getter, setter=None, field=None):
+    def __init__(self, getter=None, setter=None, field=None):
         self.field = field
         self.getter = getter
         self.setter = setter
@@ -12,6 +12,12 @@ class Function(Field):
             return self
         assert self.name is not None
 
+        if self.getter is None and self.field is None:
+            return None
+
+        if self.getter is None and self.field is not None:
+            return Model(self.field)
+
         if self.field is None:
             return self._call(inst, self.getter, self.name)
         else: