Let data_key default to field name

Author: lafrech

src/marshmallow/fields.py

@@ -377,6 +377,7 @@ def _bind_to_schema(self, field_name, schema):
         """
         self.parent = self.parent or schema
         self.name = self.name or field_name
+        self.data_key = self.data_key if self.data_key is not None else field_name
         self.root = self.root or (
             self.parent.root if isinstance(self.parent, FieldABC) else self.parent
         )
@@ -688,7 +689,7 @@ def __init__(
     @property
     def _field_data_key(self):
         only_field = self.schema.fields[self.field_name]
-        return only_field.data_key or self.field_name
+        return only_field.data_key
 
     def _serialize(self, nested_obj, attr, obj, **kwargs):
         ret = super()._serialize(nested_obj, attr, obj, **kwargs)

src/marshmallow/schema.py

@@ -520,8 +520,7 @@ def _serialize(
             value = field_obj.serialize(attr_name, obj, accessor=self.get_attribute)
             if value is missing:
                 continue
-            key = field_obj.data_key if field_obj.data_key is not None else attr_name
-            ret[key] = value
+            ret[field_obj.data_key] = value
         return ret
 
     def dump(self, obj: typing.Any, *, many: typing.Optional[bool] = None):
@@ -634,9 +633,7 @@ def _deserialize(
         else:
             partial_is_collection = is_collection(partial)
             for attr_name, field_obj in self.load_fields.items():
-                field_name = (
-                    field_obj.data_key if field_obj.data_key is not None else attr_name
-                )
+                field_name = field_obj.data_key
                 raw_value = data.get(field_name, missing)
                 if raw_value is missing:
                     # Ignore missing field if we're allowed to.
@@ -669,10 +666,7 @@ def _deserialize(
                     key = field_obj.attribute or attr_name
                     set_value(ret_d, key, value)
             if unknown != EXCLUDE:
-                fields = {
-                    field_obj.data_key if field_obj.data_key is not None else field_name
-                    for field_name, field_obj in self.load_fields.items()
-                }
+                fields = {field_obj.data_key for field_obj in self.load_fields.values()}
                 for key in set(data) - fields:
                     value = data[key]
                     if unknown == INCLUDE:
@@ -986,10 +980,7 @@ def _init_fields(self) -> None:
             if not field_obj.load_only:
                 dump_fields[field_name] = field_obj
 
-        dump_data_keys = [
-            field_obj.data_key if field_obj.data_key is not None else name
-            for name, field_obj in dump_fields.items()
-        ]
+        dump_data_keys = [field_obj.data_key for field_obj in dump_fields.values()]
         if len(dump_data_keys) != len(set(dump_data_keys)):
             data_keys_duplicates = {
                 x for x in dump_data_keys if dump_data_keys.count(x) > 1
@@ -1110,9 +1101,7 @@ def _invoke_field_validators(self, *, error_store: ErrorStore, data, many: bool)
                     continue
                 raise ValueError(f'"{field_name}" field does not exist.') from error
 
-            data_key = (
-                field_obj.data_key if field_obj.data_key is not None else field_name
-            )
+            data_key = field_obj.data_key
             if many:
                 for idx, item in enumerate(data):
                     try:

tests/test_fields.py

@@ -92,6 +92,15 @@ class MySchema(Schema):
         result = MySchema().dump({"name": "Monty", "foo": 42})
         assert result == {"_NaMe": "Monty"}
 
+    def test_data_key_defaults_to_field_name(self):
+        class MySchema(Schema):
+            field_1 = fields.String(data_key="field_one")
+            field_2 = fields.String()
+
+        schema_fields = MySchema().fields
+        assert schema_fields["field_1"].data_key == "field_one"
+        assert schema_fields["field_2"].data_key == "field_2"
+
 
 class TestParentAndName:
     class MySchema(Schema):