support list casting

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

@@ -296,7 +296,7 @@ class BaseList(list, Elements):
     @classmethod
     def extract_args(cls, *args):
         x = list(args)
-        if len(x) == 1 and isinstance(x[0], GeneratorType):
+        if len(x) == 1 and isinstance(x[0], (GeneratorType, list, tuple)):
             x = list(x[0])
         x = [coerce_type_maybe(v, cls.elem_type) for v in x]
         return x

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

@@ -161,6 +161,8 @@ def test_list():
     assert issubclass(v.type(), Elements)
     assert isinstance(v.type(), ElementsType)
 
+    assert len(typ([i for i in range(10)])) == 10  # cast py list to SSZ list
+
     foo = List[uint32, 128](0 for i in range(128))
     foo[0] = 123
     foo[1] = 654