fix field iteration crash in ssz typing

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

@@ -154,11 +154,11 @@ class Container(Series, metaclass=SSZType):
         super().__setattr__(name, value)
 
     def __repr__(self):
-        return repr({field: getattr(self, field) for field in self.get_fields()})
+        return repr({field: getattr(self, field) for field in self.get_fields().keys()})
 
     def __str__(self):
         output = [f'{self.__class__.__name__}']
-        for field in self.get_fields():
+        for field in self.get_fields().keys():
             output.append(f'  {field}: {getattr(self, field)}')
         return "\n".join(output)
 
@@ -176,7 +176,7 @@ class Container(Series, metaclass=SSZType):
 
     @classmethod
     def default(cls):
-        return cls(**{f: t.default() for f, t in cls.get_fields()})
+        return cls(**{f: t.default() for f, t in cls.get_fields().items()})
 
     @classmethod
     def is_fixed_size(cls):

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

@@ -76,6 +76,10 @@ def test_container():
         a: uint8
         b: uint32
 
+    empty = Foo()
+    assert empty.a == uint8(0)
+    assert empty.b == uint32(0)
+
     assert issubclass(Foo, Container)
     assert issubclass(Foo, SSZValue)
     assert issubclass(Foo, Series)