Added custom exception (inheriting from KeyError) when deserializatio…

…n fails.

dataclasses_json/core.py

@@ -17,6 +17,7 @@
 from typing_inspect import is_union_type  # type: ignore
 
 from dataclasses_json import cfg
+from dataclasses_json.errors import DataclassSerializationException
 from dataclasses_json.utils import (_get_type_cons,
                                     _handle_undefined_parameters_safe,
                                     _is_collection, _is_mapping, _is_new_type,
@@ -147,7 +148,14 @@ def _decode_dataclass(cls, kvs, infer_missing):
         if not field.init:
             continue
 
-        field_value = kvs[field.name]
+        try:
+            field_value = kvs[field.name]
+        except KeyError as ex:
+            qualified = f"{cls.__module__}.{cls.__qualname__}"
+            args = ", ".join(ex.args)
+            raise DataclassSerializationException(
+                f"Error decoding dataclass {qualified} encountered "
+                f"at field {args}") from ex
         field_type = types[field.name]
         if field_value is None and not _is_optional(field_type):
             warning = (f"value of non-optional type {field.name} detected "

dataclasses_json/errors.py

@@ -0,0 +1,3 @@
+class DataclassSerializationException(KeyError):
+    def __init__(self, *args, **kwargs) -> None:
+        super(DataclassSerializationException, self).__init__(*args, **kwargs)

tests/test_api.py

@@ -4,6 +4,7 @@
 
 import pytest
 
+from dataclasses_json.errors import DataclassSerializationException
 from tests.entities import (DataClassBoolImmutableDefault,
                             DataClassIntImmutableDefault,
                             DataClassJsonDecorator,
@@ -127,7 +128,7 @@ def test_warns_when_required_field_is_none(self):
 
 class TestErrors:
     def test_error_when_nonoptional_field_is_missing(self):
-        with pytest.raises(KeyError):
+        with pytest.raises(DataclassSerializationException):
             actual = DataClassWithDataClass.from_json('{"dc_with_list": {}}')
             expected = DataClassWithDataClass(DataClassWithList(None))
             assert (actual == expected)