Raise more descriptive exceptions (ethereum/execution-specs#1021)

src/ethereum_rlp/__init__.py

@@ -4,4 +4,4 @@
 
 from .rlp import RLP, Extended, Simple, decode, decode_to, encode  # noqa: F401
 
-__version__ = "0.1.0"
+__version__ = "0.1.1"

src/ethereum_rlp/exceptions.py

@@ -2,6 +2,8 @@
 Exceptions that can be thrown while serializing/deserializing RLP.
 """
 
+from typing import override
+
 
 class RLPException(Exception):
     """
@@ -14,6 +16,19 @@ class DecodingError(RLPException):
     Indicates that RLP decoding failed.
     """
 
+    @override
+    def __str__(self) -> str:
+        message = [super().__str__()]
+        current: BaseException = self
+        while isinstance(current, DecodingError) and current.__cause__:
+            current = current.__cause__
+            if isinstance(current, DecodingError):
+                as_str = super(DecodingError, current).__str__()
+            else:
+                as_str = str(current)
+            message.append(f"\tbecause {as_str}")
+        return "\n".join(message)
+
 
 class EncodingError(RLPException):
     """

src/ethereum_rlp/rlp.py

@@ -152,7 +152,10 @@ def decode_to(cls: Type[U], encoded_data: Bytes) -> U:
     a `Bytes` subclass, a dataclass, `Uint`, `U256` or `Tuple[cls]`.
     """
     decoded = decode(encoded_data)
-    return _deserialize_to(cls, decoded)
+    try:
+        return _deserialize_to(cls, decoded)
+    except Exception as e:
+        raise DecodingError(f"cannot decode into `{cls.__name__}`") from e
 
 
 @overload
@@ -200,7 +203,11 @@ def _deserialize_to_dataclass(cls: Type[U], decoded: Simple) -> U:
 
     for value, target_field in zip(decoded, target_fields):
         resolved_type = hints[target_field.name]
-        values[target_field.name] = _deserialize_to(resolved_type, value)
+        try:
+            values[target_field.name] = _deserialize_to(resolved_type, value)
+        except Exception as e:
+            msg = f"cannot decode field `{cls.__name__}.{target_field.name}`"
+            raise DecodingError(msg) from e
 
     result = cls(**values)
     assert isinstance(result, cls)
@@ -286,8 +293,13 @@ def _deserialize_to_tuple(
         arguments = list(arguments) + [arguments[-1]] * fill_count
 
     decoded = []
-    for argument, value in zip(arguments, values):
-        decoded.append(_deserialize_to(argument, value))
+    for index, (argument, value) in enumerate(zip(arguments, values)):
+        try:
+            deserialized = _deserialize_to(argument, value)
+        except Exception as e:
+            msg = f"cannot decode tuple element {index} of type `{argument}`"
+            raise DecodingError(msg) from e
+        decoded.append(deserialized)
 
     return tuple(decoded)
 
@@ -298,7 +310,15 @@ def _deserialize_to_list(
     if isinstance(values, bytes):
         raise DecodingError("invalid list")
     argument = get_args(annotation)[0]
-    return [_deserialize_to(argument, v) for v in values]
+    results = []
+    for index, value in enumerate(values):
+        try:
+            deserialized = _deserialize_to(argument, value)
+        except Exception as e:
+            msg = f"cannot decode list item {index} of type `{annotation}`"
+            raise DecodingError(msg) from e
+        results.append(deserialized)
+    return results
 
 
 def decode_to_bytes(encoded_bytes: Bytes) -> Bytes:

tests/test_rlp.py

@@ -300,7 +300,7 @@ class WithInt:
 
 
 def test_decode_to__int() -> None:
-    with pytest.raises(NotImplementedError):
+    with pytest.raises(DecodingError):
         rlp.decode_to(WithInt, b"\xc1\x00")
 
 
@@ -414,20 +414,25 @@ class WithNonRlp:
 
 
 def test_decode_to__annotation_non_rlp() -> None:
-    with pytest.raises(NotImplementedError, match="RLP non-type"):
+    with pytest.raises(DecodingError, match="RLP non-type"):
         rlp.decode_to(WithNonRlp, b"\xc2\xc1\x01")
 
 
 @dataclass
 class WithList:
-    items: List[Uint]
+    items: List[Union[Bytes1, Bytes4]]
 
 
 def test_decode_to__list_bytes() -> None:
     with pytest.raises(DecodingError, match="invalid list"):
         rlp.decode_to(WithList, b"\xc1\x80")
 
 
+def test_decode_to__list_invalid_union() -> None:
+    with pytest.raises(DecodingError, match="list item 0"):
+        rlp.decode_to(WithList, b"\xc2\xc1\xc0")
+
+
 #
 # Testing uint decoding
 #