feat: Allow lambda functions in mappings

README.md

@@ -149,7 +149,7 @@ class Address:
     number: int
     zip_code: int
     city: str
-  
+
 class PersonInfo:
     def __init__(self, name: str, age: int, address: Address):
         self.name = name
@@ -181,6 +181,42 @@ print("Target public_info.address is same as source address: ", address is publi
 * [TortoiseORM](https://github.com/tortoise/tortoise-orm)
 * [SQLAlchemy](https://www.sqlalchemy.org/)
 
+## Complexer mapping registration
+
+Support for defining mappings using `lambda`s.
+
+```python
+class AgeGroup(Enum):
+  CHILD = "child"
+  TEENAGER = "teenager"
+  ADULT = "adult"
+  SENIOR = "senior"
+
+class UserInfo:
+    def __init__(self, name: str, profession: str, age: int):
+        self.name = name
+        self.profession = profession
+        self.age = age
+
+class PublicUserInfo:
+    def __init__(self, name: str, profession: str, age_group: AgeGroup):
+        self.name = name
+        self.profession = profession
+        self.age_group
+
+mapper.add(UserInfo, PublicUserInfo, fields_mapping={)
+    "age_group": lambda user: (
+        AgeGroup.CHILD if user.age < 13 else
+        AgeGroup.TEENAGER if user.age < 20 else
+        AgeGroup.ADULT if user.age < 65 else
+        AgeGroup.SENIOR
+    )
+})
+
+mapper.map(UserInfo("John Malkovich", "engineer", 35))
+# {'name': 'John Malkovich', 'profession': 'engineer', 'age_group': <AgeGroup.ADULT: 'adult'>}
+```
+
 ## Pydantic/FastAPI Support
 Out of the box Pydantic models support:
 ```python
@@ -273,7 +309,7 @@ class PublicUserInfo(Base):
     id = Column(Integer, primary_key=True)
     public_name = Column(String)
     hobbies = Column(String)
-    
+
 obj = UserInfo(
             id=2,
             full_name="Danny DeVito",
@@ -304,7 +340,7 @@ class TargetClass:
     def __init__(self, **kwargs):
         self.name = kwargs["name"]
         self.age = kwargs["age"]
-    
+
     @staticmethod
     def get_fields(cls):
         return ["name", "age"]
@@ -358,7 +394,7 @@ T = TypeVar("T")
 
 def class_has_fields_property(target_cls: Type[T]) -> bool:
     return callable(getattr(target_cls, "fields", None))
-    
+
 mapper.add_spec(class_has_fields_property, lambda t: getattr(t, "fields")())
 
 target_obj = mapper.to(TargetClass).map(source_obj)

automapper/mapper.py

@@ -28,13 +28,15 @@
 T = TypeVar("T")
 ClassifierFunction = Callable[[Type[T]], bool]
 SpecFunction = Callable[[Type[T]], Iterable[str]]
-FieldsMap = Optional[Dict[str, Any]]
+FieldsMap = Optional[Dict[str, Union[Callable[[S], Any], Any]]]
 
 
 def _try_get_field_value(
     field_name: str, original_obj: Any, custom_mapping: FieldsMap
 ) -> Tuple[bool, Any]:
-    if field_name in (custom_mapping or {}):
+    if field_name in (custom_mapping or {}): # type: ignore [index]
+        if callable(custom_mapping[field_name]): # type: ignore [index]
+            return True, custom_mapping[field_name](original_obj)  # type: ignore [index]
         return True, custom_mapping[field_name]  # type: ignore [index]
     if hasattr(original_obj, field_name):
         return True, getattr(original_obj, field_name)
@@ -184,7 +186,8 @@ def map(
             obj (object): Source object to map to `target class`.
             skip_none_values (bool, optional): Skip None values when creating `target class` obj. Defaults to False.
             fields_mapping (FieldsMap, optional): Custom mapping.
-                Specify dictionary in format {"field_name": value_object}. Defaults to None.
+                Specify dictionary in format {"field_name": value_object | lambda soure_obj}. Can take a lamdba
+                funtion as argument, that will get the source_cls as argument. Defaults to None.
             use_deepcopy (bool, optional): Apply deepcopy to all child objects when copy from source to target object.
                 Defaults to True.
 

tests/test_predefined_mapping.py

@@ -30,6 +30,9 @@ def __init__(self, text: Optional[str], num: int) -> None:
         self.text = text
         self.num = num
 
+class ComplexClass:
+    def __init__(self, text: Optional[str], num: int) -> None:
+        self.data = AnotherClass(text, num)
 
 class ClassWithoutInitAttrDef:
     def __init__(self, **kwargs: Any) -> None:
@@ -138,3 +141,15 @@ def test_map__pass_none_values_from_source_object(self):
         assert "num" in obj.data
         assert obj.data.get("text") is None
         assert obj.data.get("num") == 11
+
+    def test_add__lambda_resolver_works_with_lambda_function(self):
+        self.mapper.add(ComplexClass, AnotherClass, fields_mapping={
+            "text": lambda x: x.data.text.upper(),
+            "num": lambda x: x.data.num * 2
+        })
+        result: AnotherClass = self.mapper.map(ComplexClass("test_message", 10))
+
+        assert isinstance(result, AnotherClass)
+        assert result.text == "TEST_MESSAGE"
+        assert result.num == 20
+