Add pre/post_load parameters to Field

src/marshmallow/fields.py

@@ -80,6 +80,11 @@
     "Url",
 ]
 
+_ProcessorT = typing.TypeVar(
+    "_ProcessorT",
+    bound=typing.Union[types.PostLoadCallable, types.PreLoadCallable, types.Validator],
+)
+
 
 class Field(FieldABC):
     """Base field from which other fields inherit.
@@ -153,6 +158,12 @@ def __init__(
         data_key: str | None = None,
         attribute: str | None = None,
         validate: types.Validator | typing.Iterable[types.Validator] | None = None,
+        pre_load: (
+            types.PreLoadCallable | typing.Iterable[types.PreLoadCallable] | None
+        ) = None,
+        post_load: (
+            types.PostLoadCallable | typing.Iterable[types.PostLoadCallable] | None
+        ) = None,
         required: bool = False,
         allow_none: bool | None = None,
         load_only: bool = False,
@@ -193,17 +204,9 @@ def __init__(
         self.attribute = attribute
         self.data_key = data_key
         self.validate = validate
-        if validate is None:
-            self.validators = []
-        elif callable(validate):
-            self.validators = [validate]
-        elif utils.is_iterable_but_not_string(validate):
-            self.validators = list(validate)
-        else:
-            raise ValueError(
-                "The 'validate' parameter must be a callable "
-                "or a collection of callables."
-            )
+        self.validators = self._normalize_processors(validate, param="validate")
+        self.pre_load = self._normalize_processors(pre_load, param="pre_load")
+        self.post_load = self._normalize_processors(post_load, param="post_load")
 
         # If allow_none is None and load_default is None
         # None should be considered valid by default
@@ -369,10 +372,23 @@ def deserialize(
         if value is missing_:
             _miss = self.load_default
             return _miss() if callable(_miss) else _miss
+
+        # Apply pre_load functions
+        for func in self.pre_load:
+            if func is not None:
+                value = func(value)
+
         if self.allow_none and value is None:
             return None
+
         output = self._deserialize(value, attr, data, **kwargs)
+        # Apply validators
         self._validate(output)
+
+        # Apply post_load functions
+        for func in self.post_load:
+            if func is not None:
+                output = func(output)
         return output
 
     # Methods for concrete classes to override.
@@ -484,6 +500,23 @@ def missing(self, value):
         )
         self.load_default = value
 
+    @staticmethod
+    def _normalize_processors(
+        processors: _ProcessorT | typing.Iterable[_ProcessorT] | None,
+        *,
+        param: str,
+    ) -> list[_ProcessorT]:
+        if processors is None:
+            return []
+        if callable(processors):
+            return [typing.cast(_ProcessorT, processors)]
+        if not utils.is_iterable_but_not_string(processors):
+            raise ValueError(
+                f"The '{param}' parameter must be a callable "
+                "or an iterable of callables."
+            )
+        return list(processors)
+
 
 class Raw(Field):
     """Field that applies no formatting."""

src/marshmallow/types.py

@@ -9,11 +9,17 @@
 
 import typing
 
+T = typing.TypeVar("T")
+
 #: A type that can be either a sequence of strings or a set of strings
 StrSequenceOrSet = typing.Union[typing.Sequence[str], typing.AbstractSet[str]]
 
 #: Type for validator functions
 Validator = typing.Callable[[typing.Any], typing.Any]
+#: Type for field-level pre-load functions
+PreLoadCallable = typing.Callable[[typing.Any], typing.Any]
+#: Type for field-level post-load functions
+PostLoadCallable = typing.Callable[[T], T]
 
 
 class SchemaValidator(typing.Protocol):

tests/test_fields.py

@@ -663,3 +663,125 @@ class Family(Schema):
                     "daughter": {"value": {"age": ["Missing data for required field."]}}
                 }
             }
+
+
+class TestFieldPreAndPostLoad:
+    def test_field_pre_load(self):
+        class UserSchema(Schema):
+            name = fields.Str(pre_load=str)
+
+        schema = UserSchema()
+        result = schema.load({"name": 808})
+        assert result["name"] == "808"
+
+    def test_field_pre_load_multiple(self):
+        def decrement(value):
+            return value - 1
+
+        def add_prefix(value):
+            return "test_" + value
+
+        class UserSchema(Schema):
+            name = fields.Str(pre_load=[decrement, str, add_prefix])
+
+        schema = UserSchema()
+        result = schema.load({"name": 809})
+        assert result["name"] == "test_808"
+
+    def test_field_post_load(self):
+        class UserSchema(Schema):
+            age = fields.Int(post_load=str)
+
+        schema = UserSchema()
+        result = schema.load({"age": 42})
+        assert result["age"] == "42"
+
+    def test_field_post_load_multiple(self):
+        def multiply_by_2(value):
+            return value * 2
+
+        def decrement(value):
+            return value - 1
+
+        class UserSchema(Schema):
+            age = fields.Float(post_load=[multiply_by_2, decrement])
+
+        schema = UserSchema()
+        result = schema.load({"age": 21.5})
+        assert result["age"] == 42.0
+
+    def test_field_pre_and_post_load(self):
+        def multiply_by_2(value):
+            return value * 2
+
+        class UserSchema(Schema):
+            age = fields.Int(pre_load=[str.strip, int], post_load=[multiply_by_2])
+
+        schema = UserSchema()
+        result = schema.load({"age": " 21 "})
+        assert result["age"] == 42
+
+    def test_field_pre_load_validation_error(self):
+        def always_fail(value):
+            raise ValidationError("oops")
+
+        class UserSchema(Schema):
+            age = fields.Int(pre_load=always_fail)
+
+        schema = UserSchema()
+        with pytest.raises(ValidationError) as exc:
+            schema.load({"age": 42})
+        assert exc.value.messages == {"age": ["oops"]}
+
+    def test_field_post_load_validation_error(self):
+        def always_fail(value):
+            raise ValidationError("oops")
+
+        class UserSchema(Schema):
+            age = fields.Int(post_load=always_fail)
+
+        schema = UserSchema()
+        with pytest.raises(ValidationError) as exc:
+            schema.load({"age": 42})
+        assert exc.value.messages == {"age": ["oops"]}
+
+    def test_field_pre_load_none(self):
+        def handle_none(value):
+            if value is None:
+                return 0
+            return value
+
+        class UserSchema(Schema):
+            age = fields.Int(pre_load=handle_none, allow_none=True)
+
+        schema = UserSchema()
+        result = schema.load({"age": None})
+        assert result["age"] == 0
+
+    def test_field_post_load_not_called_with_none_input_when_not_allowed(self):
+        def handle_none(value):
+            if value is None:
+                return 0
+            return value
+
+        class UserSchema(Schema):
+            age = fields.Int(post_load=handle_none, allow_none=False)
+
+        schema = UserSchema()
+        with pytest.raises(ValidationError) as exc:
+            schema.load({"age": None})
+        assert exc.value.messages == {"age": ["Field may not be null."]}
+
+    def test_invalid_type_passed_to_pre_load(self):
+        with pytest.raises(
+            ValueError,
+            match="The 'pre_load' parameter must be a callable or an iterable of callables.",
+        ):
+            fields.Int(pre_load="not_callable")
+
+    def test_invalid_type_passed_to_post_load(self):
+        with pytest.raises(
+            ValueError,
+            match="The 'post_load' parameter must be a callable or an iterable of callables.",
+        ):
+            fields.Int(post_load="not_callable")