Implement consistent handling of TransactionCanceled errors.

Previously, the code handling errors would raise the cause of the
Cancellation, for example ConditionalCheckFailed or ThrottlingError, if
it was caused by the first item in the list of items in the transaction.

If the failure was caused by any other item in the list, the caller
would see the TransactionCanceled.

This makes is awkward for calling code to handle errors when interested
in the exact item which caused the failure - it needs two `except`
blocks for handling essentially the same thing. One for when it is the
first item and another for any other item.

This change removes any special handling and bubbles the
TransactionCanceled error up to the caller. I've added a type value for
the cancellation reasons. These are documented here:

https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_TransactWriteItems.html#API_TransactWriteItems_Errors

This should allow clients to introspect the errors, if needed, with a
typed object.

Closes #170

src/aiodynamo/errors.py

@@ -1,5 +1,10 @@
 import json
-from typing import Any, Dict
+from typing import Any, Dict, List, Optional, TypedDict
+
+
+class CancellationReason(TypedDict):
+    Code: Optional[str]
+    Message: Optional[str]
 
 
 class AIODynamoError(Exception):
@@ -110,8 +115,10 @@ class PointInTimeRecoveryUnavailable(AIODynamoError):
 
 
 class TransactionCanceled(AIODynamoError):
-    pass
-
+    def __init__(self, body: Dict[str, Any]):
+        self.body = body
+        self.cancellation_reasons: List[CancellationReason] = self.body["CancellationReasons"]
+        super().__init__(body)
 
 class TransactionEmpty(AIODynamoError):
     pass
@@ -207,17 +214,7 @@ def exception_from_response(status: int, body: bytes) -> Exception:
         return ServiceUnavailable()
     try:
         data = json.loads(body)
-        error = ERRORS[data["__type"].split("#", 1)[-1]](data)
-        if isinstance(error, TransactionCanceled):
-            error = extract_error_from_transaction_canceled(data)
+        error: Exception = ERRORS[data["__type"].split("#", 1)[-1]](data)
         return error
     except Exception:
         return UnknownError(status, body)
-
-
-def extract_error_from_transaction_canceled(data: Dict[str, Any]) -> AIODynamoError:
-    try:
-        error = data["CancellationReasons"][0]
-        return ERRORS[f"{error['Code']}Exception"](error["Message"])
-    except Exception:
-        return ERRORS[data["__type"].split("#", 1)[-1]](data)

tests/integration/test_client.py

@@ -624,14 +624,34 @@ async def test_transact_write_items_put(client: Client, table: TableName) -> Non
     await client.transact_write_items(items=puts)
     assert len([item async for item in client.query(table, HashKey("h", "h"))]) == 2
 
-    with pytest.raises(errors.ConditionalCheckFailed):
+    with pytest.raises(errors.TransactionCanceled) as excinfo:
         put = Put(
             table=table,
             item={"h": "h", "r": "0", "s": "initial"},
             condition=F("h").does_not_exist(),
         )
         await client.transact_write_items(items=[put])
 
+    assert len(excinfo.value.cancellation_reasons) == 1
+    assert excinfo.value.cancellation_reasons[0]["Code"] == "ConditionalCheckFailed"
+
+    with pytest.raises(errors.TransactionCanceled) as excinfo:
+        put = Put(
+            table=table,
+            item={"h": "h", "r": "3", "s": "initial"},
+            condition=F("h").does_not_exist(),
+        )
+        put_fail = Put(
+            table=table,
+            item={"h": "h", "r": "0", "s": "initial"},
+            condition=F("h").does_not_exist(),
+        )
+        await client.transact_write_items(items=[put, put_fail])
+
+    assert len(excinfo.value.cancellation_reasons) == 2
+    assert excinfo.value.cancellation_reasons[0]["Code"] == 'None'
+    assert excinfo.value.cancellation_reasons[1]["Code"] == "ConditionalCheckFailed"
+
 
 @pytest.mark.usefixtures("supports_transactions")
 async def test_transact_write_items_update(client: Client, table: TableName) -> None:
@@ -647,7 +667,7 @@ async def test_transact_write_items_update(client: Client, table: TableName) ->
     query = await client.query_single_page(table, HashKey("h", "h"))
     assert query.items[0]["s"] == "changed"
 
-    with pytest.raises(errors.ConditionalCheckFailed):
+    with pytest.raises(errors.TransactionCanceled) as excinfo:
         update = Update(
             table=table,
             key={"h": "h", "r": "1"},
@@ -656,6 +676,9 @@ async def test_transact_write_items_update(client: Client, table: TableName) ->
         )
         await client.transact_write_items(items=[update])
 
+    assert len(excinfo.value.cancellation_reasons) == 1
+    assert excinfo.value.cancellation_reasons[0]["Code"] == "ConditionalCheckFailed"
+
 
 @pytest.mark.usefixtures("supports_transactions")
 async def test_transact_write_items_delete(client: Client, table: TableName) -> None:
@@ -670,14 +693,18 @@ async def test_transact_write_items_delete(client: Client, table: TableName) ->
     assert len([item async for item in client.query(table, HashKey("h", "h"))]) == 0
 
     await client.put_item(table=table, item={"h": "h", "r": "1", "s": "initial"})
-    with pytest.raises(errors.ConditionalCheckFailed):
+
+    with pytest.raises(errors.TransactionCanceled) as excinfo:
         delete = Delete(
             table=table,
             key={"h": "h", "r": "1"},
             condition=F("s").not_equals("initial"),
         )
         await client.transact_write_items(items=[delete])
 
+    assert len(excinfo.value.cancellation_reasons) == 1
+    assert excinfo.value.cancellation_reasons[0]["Code"] == "ConditionalCheckFailed"
+
 
 @pytest.mark.usefixtures("supports_transactions")
 async def test_transact_write_items_condition_check(
@@ -687,9 +714,12 @@ async def test_transact_write_items_condition_check(
     condition = ConditionCheck(
         table=table, key={"h": "h", "r": "1"}, condition=F("s").not_equals("initial")
     )
-    with pytest.raises(errors.ConditionalCheckFailed):
+    with pytest.raises(errors.TransactionCanceled) as excinfo:
         await client.transact_write_items(items=[condition])
 
+    assert len(excinfo.value.cancellation_reasons) == 1
+    assert excinfo.value.cancellation_reasons[0]["Code"] == "ConditionalCheckFailed"
+
     condition = ConditionCheck(
         table=table, key={"h": "h", "r": "1"}, condition=F("s").equals("initial")
     )