suggestion to implement bitfield like

test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py

@@ -1,7 +1,7 @@
 from ..merkle_minimal import merkleize_chunks
 from ..hash_function import hash
 from .ssz_typing import (
-    SSZValue, SSZType, BasicValue, BasicType, Series, Elements, boolean, Container, List, Bytes,
+    SSZValue, SSZType, BasicValue, BasicType, Series, Elements, Bitfield, boolean, Container, List, Bytes,
     Bitlist, Bitvector, uint,
 )
 
@@ -128,6 +128,8 @@ def item_length(typ: SSZType) -> int:
 def chunk_count(typ: SSZType) -> int:
     if isinstance(typ, BasicType):
         return 1
+    elif issubclass(typ, Bitfield):
+        return (typ.length + 7) // 8 // 32
     elif issubclass(typ, Elements):
         return (typ.length * item_length(typ.elem_type) + 31) // 32
     elif issubclass(typ, Container):

test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py

@@ -281,10 +281,6 @@ class ElementsType(ParamsMeta):
     length: int
 
 
-class BitElementsType(ElementsType):
-    elem_type = boolean
-
-
 class Elements(ParamsBase, metaclass=ElementsType):
     pass
 
@@ -346,11 +342,16 @@ def last(self):
         return self[len(self) - 1]
 
 
-class BaseBitfield(BaseList, metaclass=BitElementsType):
-    elem_type = bool
+class BitElementsType(ElementsType):
+    elem_type: SSZType = boolean
+    length: int
+
+
+class Bitfield(BaseList, metaclass=BitElementsType):
+    pass
 
 
-class Bitlist(BaseBitfield):
+class Bitlist(Bitfield):
     @classmethod
     def is_fixed_size(cls):
         return False
@@ -360,15 +361,29 @@ def default(cls):
         return cls()
 
 
-class Bitvector(BaseBitfield):
+class Bitvector(Bitfield):
+
+    @classmethod
+    def extract_args(cls, *args):
+        if len(args) == 0:
+            return cls.default()
+        else:
+            return super().extract_args(*args)
+
+    @classmethod
+    def value_check(cls, value):
+        # check length limit strictly
+        return len(value) == cls.length and super().value_check(value)
+
     @classmethod
     def is_fixed_size(cls):
         return True
-    
+
     @classmethod
     def default(cls):
         return cls(0 for _ in range(cls.length))
 
+
 class List(BaseList):
 
     @classmethod