Fix: if field has custom decoder, schema takes it into account (#462)

dataclasses_json/mm.py

@@ -253,10 +253,10 @@ def inner(type_, options):
         origin = getattr(type_, '__origin__', type_)
         args = [inner(a, {}) for a in getattr(type_, '__args__', []) if
                 a is not type(None)]
-        
+
         if type_ == Ellipsis:
             return type_
-        
+
         if _is_optional(type_):
             options["allow_none"] = True
         if origin is tuple:
@@ -318,6 +318,16 @@ def schema(cls, mixin, infer_missing):
                 options['data_key'] = metadata.letter_case(field.name)
 
             t = build_type(type_, options, mixin, field, cls)
+            if field.metadata.get('dataclasses_json', {}).get('decoder'):
+                # If the field defines a custom decoder, it should completely replace the Marshmallow field's conversion
+                # logic.
+                # From Marshmallow's documentation for the _deserialize method:
+                # "Deserialize value. Concrete :class:`Field` classes should implement this method. "
+                # This is the method that Field implementations override to perform the actual deserialization logic.
+                # In this case we specifically override this method instead of `deserialize` to minimize potential
+                # side effects, and only cancel the actual value deserialization.
+                t._deserialize = lambda v, *_a, **_kw: v
+
             # if type(t) is not fields.Field:  # If we use `isinstance` we would return nothing.
             if field.type != typing.Optional[CatchAllVar]:
                 schema[field.name] = t

tests/entities.py

@@ -271,6 +271,16 @@ class DataClassWithErroneousDecode:
     id: float = field(metadata=config(decoder=lambda: None))
 
 
+def split_str(data: str, *_args, **_kwargs):
+    return data.split(',')
+
+
+@dataclass_json
+@dataclass
+class DataClassDifferentTypeDecode:
+    lst: List[str] = field(default=None, metadata=config(decoder=split_str))
+
+
 @dataclass_json
 @dataclass
 class DataClassMappingBadDecode:

tests/test_schema.py

@@ -2,7 +2,8 @@
 import pytest
 
 from .entities import (DataClassDefaultListStr, DataClassDefaultOptionalList, DataClassList, DataClassOptional,
-                       DataClassWithNestedOptional, DataClassWithNestedOptionalAny, DataClassWithNestedAny)
+                       DataClassWithNestedOptional, DataClassWithNestedOptionalAny, DataClassWithNestedAny,
+                       DataClassDifferentTypeDecode)
 from .test_letter_case import CamelCasePerson, KebabCasePerson, SnakeCasePerson, FieldNamePerson
 
 test_do_list = """[{}, {"children": [{"name": "a"}, {"name": "b"}]}]"""
@@ -47,3 +48,7 @@ def test_nested_optional_any(self):
     def test_nested_any_accepts_optional(self):
         DataClassWithNestedAny.schema().loads(nested_optional_data)
         assert True
+
+    def test_accounts_for_decode(self):
+        assert DataClassDifferentTypeDecode.schema().load({'lst': '1,2,3'}) == \
+               DataClassDifferentTypeDecode(lst=['1', '2', '3'])