container field coercion

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

@@ -1,4 +1,4 @@
-from typing import Tuple, Dict, Iterator
+from typing import Tuple, Iterator
 from types import GeneratorType
 
 
@@ -142,6 +142,16 @@ class Container(Series, metaclass=SSZType):
         from .ssz_impl import signing_root
         return signing_root(self)
 
+    def __setattr__(self, name, value):
+        if name not in self.__class__.__annotations__:
+            raise AttributeError("Cannot change non-existing SSZ-container attribute")
+        field_typ = self.__class__.__annotations__[name]
+        value = coerce_type_maybe(value, field_typ)
+        if not isinstance(value, field_typ):
+            raise ValueCheckError(f"Cannot set field of {self.__class__}:"
+                                  f" field: {name} type: {field_typ} value: {value} value type: {type(value)}")
+        super().__setattr__(name, value)
+
     def get_field_values(self) -> Tuple[SSZValue, ...]:
         cls = self.__class__
         return tuple(getattr(self, field) for field in cls.get_field_names())

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

@@ -105,6 +105,20 @@ def test_container():
     assert v.type().elem_type == uint8
     assert v.type().length == 1024
 
+    y.a = 42
+    y.a = uint16(255)
+    try:
+        y.a = uint16(256)
+        assert False
+    except ValueError:
+        pass
+
+    try:
+        y.not_here = 5
+        assert False
+    except AttributeError:
+        pass
+
 
 def test_list():
     typ = List[uint64, 128]