Expose error details with a class (#257)

Signed-off-by: Anuraag Agrawal <anuraaga@gmail.com>

Author: anuraaga

conformance/test/client.py

@@ -667,7 +667,7 @@ async def send_unary_request(
         except ConnectError as e:
             test_response.response.error.code = _convert_code(e.code)
             test_response.response.error.message = e.message
-            test_response.response.error.details.extend(e.details)
+            test_response.response.error.details.extend(d._any for d in e.details)
         except (asyncio.CancelledError, Exception) as e:
             traceback.print_tb(e.__traceback__, file=sys.stderr)
             test_response.error.message = str(e)

src/connectrpc/_protocol.py

@@ -6,13 +6,12 @@
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Protocol, TypeVar, cast
 
-from google.protobuf import symbol_database
 from google.protobuf.any_pb2 import Any
 from google.protobuf.json_format import MessageToDict
 
 from ._compression import Compression
 from .code import Code
-from .errors import ConnectError
+from .errors import ConnectError, ErrorDetail
 
 if TYPE_CHECKING:
     from collections.abc import Mapping, Sequence
@@ -91,7 +90,7 @@ def from_http_status(status: HTTPStatus) -> ExtendedHTTPStatus:
 class ConnectWireError:
     code: Code
     message: str
-    details: Sequence[Any]
+    details: Sequence[ErrorDetail]
 
     @staticmethod
     def from_exception(exc: Exception) -> ConnectWireError:
@@ -122,7 +121,7 @@ def from_dict(
         else:
             code = _http_status_code_to_error.get(http_status, Code.UNKNOWN)
         message = data.get("message", "")
-        details: Sequence[Any] = ()
+        details: Sequence[ErrorDetail] = ()
         details_json = cast("list[dict[str, str]] | None", data.get("details"))
         if details_json:
             details = []
@@ -133,9 +132,11 @@ def from_dict(
                     # Ignore malformed details
                     continue
                 details.append(
-                    Any(
-                        type_url="type.googleapis.com/" + detail_type,
-                        value=b64decode(detail_value + "==="),
+                    ErrorDetail(
+                        Any(
+                            type_url="type.googleapis.com/" + detail_type,
+                            value=b64decode(detail_value + "==="),
+                        )
                     )
                 )
         return ConnectWireError(code, message, details)
@@ -161,26 +162,17 @@ def to_dict(self) -> dict:
         if self.details:
             details: list[dict] = []
             for detail in self.details:
-                if detail.type_url.startswith("type.googleapis.com/"):
-                    detail_type = detail.type_url[len("type.googleapis.com/") :]
-                else:
-                    detail_type = detail.type_url
                 detail_dict: dict = {
-                    "type": detail_type,
+                    "type": detail.type_name,
                     # Connect requires unpadded base64
-                    "value": b64encode(detail.value).decode("utf-8").rstrip("="),
+                    "value": b64encode(detail.message_bytes)
+                    .decode("utf-8")
+                    .rstrip("="),
                 }
                 # Try to produce debug info, but expect failure when we don't
                 # have descriptors for the message type.
-                debug = None
-                try:
-                    msg_instance = symbol_database.Default().GetSymbol(detail_type)()
-                    if detail.Unpack(msg_instance):
-                        debug = MessageToDict(msg_instance)
-                except Exception:
-                    debug = None
-                if debug is not None:
-                    detail_dict["debug"] = debug
+                if (debug := detail.value()) is not None:
+                    detail_dict["debug"] = MessageToDict(debug)
                 details.append(detail_dict)
             data["details"] = details
         return data

src/connectrpc/_protocol_grpc.py

@@ -169,7 +169,9 @@ def end(self, user_trailers: Headers, error: ConnectWireError | None) -> Headers
                 trailers["grpc-message"] = message
             if error.details:
                 grpc_status = Status(
-                    code=int(status), message=error.message, details=error.details
+                    code=int(status),
+                    message=error.message,
+                    details=[d._any for d in error.details],  # noqa: SLF001
                 )
                 grpc_status_bin = (
                     b64encode(grpc_status.SerializeToString()).decode().rstrip("=")

src/connectrpc/errors.py

@@ -1,19 +1,73 @@
 from __future__ import annotations
 
-__all__ = ["ConnectError"]
+__all__ = ["ConnectError", "ErrorDetail"]
 
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, TypeVar, overload
 
+from google.protobuf import symbol_database
 from google.protobuf.any_pb2 import Any
+from google.protobuf.message import Message
 
 if TYPE_CHECKING:
     from collections.abc import Iterable, Sequence
 
-    from google.protobuf.message import Message
-
     from .code import Code
 
+T = TypeVar("T", bound=Message)
+
+
+class ErrorDetail:
+    """A self-describing Protobuf message attached to a [ConnectError][].
+
+    Error details are sent over the network to clients, which can then work with
+    strongly-typed data rather than trying to parse a complex error message. For
+    example, you might use details to send a localized error message or retry
+    parameters to a client.
+    """
+
+    def __init__(self, message: Message) -> None:
+        if isinstance(message, Any):
+            self._message = None
+            self._any = message
+            return
+        self._message = message
+        self._any = pack_any(message)
+
+    @property
+    def type_name(self) -> str:
+        """The fully-qualified name of the details Protobuf message (for example, acme.foo.v1.FooDetail)."""
+        return self._any.type_url.removeprefix("type.googleapis.com/")
+
+    @property
+    def message_bytes(self) -> bytes:
+        """The Protobuf message serialized as bytes."""
+        return self._any.value
+
+    @overload
+    def value(self) -> Message | None: ...
+
+    @overload
+    def value(self, typ: type[T], /) -> T | None: ...
+
+    def value(self, desc: type[Message] | None = None) -> Message | None:
+        """The details message as a Protobuf message, or None if it cannot be deserialized."""
+        if self._message:
+            return self._message
+        if isinstance(desc, type):
+            msg = desc()
+            if self._any.Unpack(msg):
+                return msg
+            return None
+        try:
+            detail_type = self._any.type_url.removeprefix("type.googleapis.com/")
+            msg_instance = symbol_database.Default().GetSymbol(detail_type)()
+            if self._any.Unpack(msg_instance):
+                return msg_instance
+            return None
+        except Exception:
+            return None
+
 
 class ConnectError(Exception):
     """An exception in a Connect RPC.
@@ -25,7 +79,7 @@ class ConnectError(Exception):
     """
 
     def __init__(
-        self, code: Code, message: str, details: Iterable[Message] = ()
+        self, code: Code, message: str, details: Iterable[Message | ErrorDetail] = ()
     ) -> None:
         """
         Creates a new Connect error.
@@ -40,7 +94,7 @@ def __init__(
         self._message = message
 
         self._details = (
-            [m if isinstance(m, Any) else pack_any(m) for m in details]
+            [m if isinstance(m, ErrorDetail) else ErrorDetail(m) for m in details]
             if details
             else ()
         )
@@ -54,7 +108,7 @@ def message(self) -> str:
         return self._message
 
     @property
-    def details(self) -> Sequence[Any]:
+    def details(self) -> Sequence[ErrorDetail]:
         return self._details
 
 

test/test_details.py

@@ -11,7 +11,7 @@
 
 from connectrpc._protocol import ConnectWireError
 from connectrpc.code import Code
-from connectrpc.errors import ConnectError, pack_any
+from connectrpc.errors import ConnectError, ErrorDetail
 
 from .haberdasher_connect import (
     Haberdasher,
@@ -32,7 +32,7 @@ def make_hat(self, request, ctx) -> NoReturn:
                 "Resource exhausted",
                 details=[
                     Struct(fields={"animal": Value(string_value="bear")}),
-                    pack_any(Struct(fields={"color": Value(string_value="red")})),
+                    ErrorDetail(Struct(fields={"color": Value(string_value="red")})),
                 ],
             )
 
@@ -47,11 +47,11 @@ def make_hat(self, request, ctx) -> NoReturn:
     assert exc_info.value.code == Code.RESOURCE_EXHAUSTED
     assert exc_info.value.message == "Resource exhausted"
     assert len(exc_info.value.details) == 2
-    s0 = Struct()
-    assert exc_info.value.details[0].Unpack(s0)
+    s0 = exc_info.value.details[0].value(Struct)
+    assert s0 is not None
     assert s0.fields["animal"].string_value == "bear"
-    s1 = Struct()
-    assert exc_info.value.details[1].Unpack(s1)
+    s1 = exc_info.value.details[1].value(Struct)
+    assert s1 is not None
     assert s1.fields["color"].string_value == "red"
 
 
@@ -64,7 +64,7 @@ async def make_hat(self, request, ctx) -> NoReturn:
                 "Resource exhausted",
                 details=[
                     Struct(fields={"animal": Value(string_value="bear")}),
-                    pack_any(Struct(fields={"color": Value(string_value="red")})),
+                    ErrorDetail(Struct(fields={"color": Value(string_value="red")})),
                 ],
             )
 
@@ -78,11 +78,11 @@ async def make_hat(self, request, ctx) -> NoReturn:
     assert exc_info.value.code == Code.RESOURCE_EXHAUSTED
     assert exc_info.value.message == "Resource exhausted"
     assert len(exc_info.value.details) == 2
-    s0 = Struct()
-    assert exc_info.value.details[0].Unpack(s0)
+    s0 = exc_info.value.details[0].value(Struct)
+    assert s0 is not None
     assert s0.fields["animal"].string_value == "bear"
-    s1 = Struct()
-    assert exc_info.value.details[1].Unpack(s1)
+    s1 = exc_info.value.details[1].value(Struct)
+    assert s1 is not None
     assert s1.fields["color"].string_value == "red"
 
 
@@ -124,7 +124,7 @@ def test_error_detail_debug_field_absent_for_unknown_type() -> None:
         type_url="type.googleapis.com/completely.Unknown.Message", value=b"\x08\x01"
     )
     wire_error = ConnectWireError(
-        code=Code.INTERNAL, message="test", details=[unknown_detail]
+        code=Code.INTERNAL, message="test", details=[ErrorDetail(unknown_detail)]
     )
     data = wire_error.to_dict()
     assert len(data["details"]) == 1