Add new compute function to field

Author: LeanderCS

AGENTS.md

@@ -87,6 +87,33 @@ def create_user():
 - **Logic**: `OneOfCondition`, `NOfCondition`, `ExactlyNOfCondition`
 - **Comparison**: `EqualCondition`, `NotEqualCondition`
 
+### Computed Fields
+Define read-only fields that are automatically calculated from other fields:
+
+```python
+class OrderInputFilter(InputFilter):
+    quantity: int = field(required=True)
+    price: float = field(required=True)
+
+    # Computed field - automatically calculated
+    total: float = field(
+        computed=lambda data: data['quantity'] * data['price']
+    )
+
+# Usage:
+result = OrderInputFilter().validate_data({"quantity": 5, "price": 10.0})
+# → {"quantity": 5, "price": 10.0, "total": 50.0}
+```
+
+**Computed Field Characteristics:**
+- Read-only (input values are ignored)
+- Evaluated during validation with access to previously processed fields
+- Non-blocking (errors set field to `None` and log warning)
+- Can use lambda or named functions
+- Can depend on other computed fields (order matters)
+- Works with filters, validators, copy, defaults, and fallbacks on dependencies
+- Useful for: totals, full names, derived booleans, calculations
+
 ## Project Structure
 
 ```

docs/source/changelog.rst

@@ -4,6 +4,18 @@ Changelog
 All notable changes to this project will be documented in this file.
 
 
+[0.7.4] - 2025-10-08
+--------------------
+
+Added
+^^^^^
+- **Computed Fields**: New feature to define read-only fields that are
+  automatically calculated from other fields. Use the ``computed``
+  parameter in ``field()`` to provide a callable that receives the current
+  data dictionary and returns the computed value.
+  See :doc:`Computed Fields documentation <options/field_decorator>` for more details.
+
+
 [0.7.3] - 2025-10-08
 --------------------
 

docs/source/options/field_decorator.rst

@@ -333,6 +333,51 @@ independently.
 
 For detailed information, see :doc:`Copy <copy>`.
 
+computed
+~~~~~~~~
+
+**Type**: ``Callable[[dict[str, Any]], Any]``
+**Default**: ``None``
+
+Define a read-only field that is automatically calculated from other fields.
+The provided callable receives the current data dictionary and should return the
+computed value.
+
+Computed fields are:
+
+- **Read-only**: Input values are ignored
+- **Non-blocking**: Errors let the field stay on ``None`` and log a warning
+- Evaluated **during validation**: Have access to all previously processed fields
+
+.. code-block:: python
+
+    class OrderInputFilter(InputFilter):
+        quantity: int = field(required=True)
+        price: float = field(required=True)
+
+        # Computed field using lambda
+        total: float = field(
+            computed=lambda data: data['quantity'] * data['price']
+        )
+
+.. code-block:: python
+
+    # Using named function for complex calculations
+    def calculate_tax(data):
+        subtotal = data.get('subtotal', 0)
+        tax_rate = data.get('tax_rate', 0.19)
+        return subtotal * tax_rate
+
+    class InvoiceInputFilter(InputFilter):
+        subtotal: float = field(required=True)
+        tax_rate: float = field(default=0.19)
+
+        tax: float = field(computed=calculate_tax)
+        total: float = field(
+            required=True,
+            computed=lambda data: data['subtotal'] + data.get('tax', 0)
+        )
+
 Advanced Field Patterns
 -----------------------
 

flask_inputfilter/_input_filter.pyx

@@ -260,20 +260,23 @@ cdef class InputFilter:
         inheritance chain."""
         cdef object cls, attr_value, conditions, validators, filters, base_cls
         cdef str attr_name
-        cdef list dir_attrs
-        cdef FieldDescriptor field_desc
         cdef set added_conditions, added_global_validators, added_global_filters
         cdef object condition_id, validator_id, filter_id
         cdef object condition, validator, filter_instance
 
         cls = self.__class__
-        dir_attrs = dir(cls)
+        added_conditions = set()
+        added_global_validators = set()
+        added_global_filters = set()
 
-        for attr_name in dir_attrs:
-            if attr_name.startswith("_"):
+        for base_cls in reversed(cls.__mro__):
+            if base_cls is object:
                 continue
-            if hasattr(cls, attr_name):
-                attr_value = getattr(cls, attr_name)
+
+            for attr_name, attr_value in base_cls.__dict__.items():
+                if attr_name.startswith("_"):
+                    continue
+
                 if isinstance(attr_value, FieldDescriptor):
                     self.fields[attr_name] = FieldModel(
                         attr_value.required,
@@ -284,13 +287,9 @@ cdef class InputFilter:
                         attr_value.steps,
                         attr_value.external_api,
                         attr_value.copy,
+                        attr_value.computed,
                     )
 
-        added_conditions = set()
-        added_global_validators = set()
-        added_global_filters = set()
-
-        for base_cls in reversed(cls.__mro__):
             conditions = getattr(base_cls, "_conditions", None)
             if conditions is not None:
                 for condition in conditions:
@@ -564,6 +563,7 @@ cdef class InputFilter:
             steps or [],
             external_api,
             copy,
+            None,  # computed (not supported in deprecated add method)
         )
 
     cpdef bint has(self, str field_name):
@@ -689,6 +689,7 @@ cdef class InputFilter:
             steps or [],
             external_api,
             copy,
+            None,  # computed (not supported in deprecated add method)
         )
 
     cpdef void add_global_filter(self, BaseFilter filter):

flask_inputfilter/declarative/_field_descriptor.pxd

@@ -12,6 +12,7 @@ cdef class FieldDescriptor:
         list steps
         ExternalApiConfig external_api
         str copy
+        object computed
     cdef public:
         str name
 

flask_inputfilter/declarative/_field_descriptor.pyx

@@ -39,6 +39,7 @@ cdef class FieldDescriptor:
         list steps = None,
         object external_api = None,
         str copy = None,
+        object computed = None,
     ) -> None:
         """
         Initialize a field descriptor.
@@ -59,6 +60,8 @@ cdef class FieldDescriptor:
           external API call.
         - **copy** (*Optional[str]*): The name of the field to copy the value
           from.
+        - **computed** (*Optional[Callable[[dict[str, Any]], Any]]*): A callable
+          that computes the field value from validated data.
         """
         self.required = required
         self._default = default
@@ -68,6 +71,7 @@ cdef class FieldDescriptor:
         self.steps = steps if steps is not None else []
         self.external_api = external_api
         self.copy = copy
+        self.computed = computed
         self.name = None
 
     @property

flask_inputfilter/declarative/field.py

@@ -22,6 +22,7 @@ def field(
     steps: Optional[list[Union[BaseFilter, BaseValidator]]] = None,
     external_api: Optional[ExternalApiConfig] = None,
     copy: Optional[str] = None,
+    computed: Optional[Any] = None,
 ) -> FieldDescriptor:
     """
     Create a field descriptor for declarative field definition.
@@ -47,6 +48,10 @@ def field(
       external API call. Default: None.
     - **copy** (*Optional[str]*): The name of the field to copy the value
       from. Default: None.
+    - **computed** (*Optional[Callable[[dict[str, Any]], Any]]*): A callable
+      that computes the field value from validated data. Computed fields are
+      read-only and calculated after all regular fields are validated.
+      Default: None.
 
     **Returns:**
 
@@ -73,4 +78,5 @@ class UserInputFilter(InputFilter):
         steps=steps,
         external_api=external_api,
         copy=copy,
+        computed=computed,
     )

flask_inputfilter/declarative/field_descriptor.py

@@ -32,6 +32,8 @@ class FieldDescriptor:
       configuration.
     - **copy** (*Optional[str]*): Field to copy value from if this field
       is missing.
+    - **computed** (*Optional[Callable[[dict[str, Any]], Any]]*): A callable
+      that computes the field value from validated data.
 
     **Expected Behavior:**
 
@@ -50,6 +52,7 @@ def __init__(
         steps: Optional[list[Union[BaseFilter, BaseValidator]]] = None,
         external_api: Optional[ExternalApiConfig] = None,
         copy: Optional[str] = None,
+        computed: Optional[Any] = None,
     ) -> None:
         """
         Initialize a field descriptor.
@@ -72,6 +75,8 @@ def __init__(
           external API call.
         - **copy** (*Optional[str]*): The name of the field to copy the value
           from.
+        - **computed** (*Optional[Callable[[dict[str, Any]], Any]]*): A callable
+          that computes the field value from validated data.
         """
         self.required = required
         self.default = default
@@ -81,6 +86,7 @@ def __init__(
         self.steps = steps or []
         self.external_api = external_api
         self.copy = copy
+        self.computed = computed
         self.name: Optional[str] = None
 
     def __set_name__(self, owner: type, name: str) -> None:
@@ -135,6 +141,10 @@ def __repr__(self) -> str:
             f"required={self.required}, "
             f"default={self.default!r}, "
             f"filters={len(self.filters)}, "
-            f"validators={len(self.validators)}"
+            f"validators={len(self.validators)}, "
+            f"steps={len(self.steps)}, "
+            f"external_api={self.external_api!r}, "
+            f"copy={self.copy!r}, "
+            f"computed={self.computed!r}, "
             f")"
         )

flask_inputfilter/declarative/field_descriptor.pyi

@@ -48,6 +48,7 @@ class FieldDescriptor:
     external_api: Optional[ExternalApiConfig]
     copy: Optional[str]
     name: Optional[str]
+    computed: Optional[Any]
 
     def __init__(
         self,
@@ -59,6 +60,7 @@ class FieldDescriptor:
         steps: Optional[list[Union[BaseFilter, BaseValidator]]] = None,
         external_api: Optional[ExternalApiConfig] = None,
         copy: Optional[str] = None,
+        computed: Optional[Any] = None,
     ) -> None: ...
     def __set_name__(self, owner: type, name: str) -> None: ...
     def __get__(self, obj: Any, objtype: Optional[type] = None) -> Any: ...

flask_inputfilter/input_filter.py

@@ -225,13 +225,19 @@ def _register_decorator_components(self) -> None:
         """Register decorator-based components from the current class and
         inheritance chain."""
         cls = self.__class__
-        dir_attrs = dir(cls)
 
-        for attr_name in dir_attrs:
-            if attr_name.startswith("_"):
+        added_conditions = set()
+        added_global_validators = set()
+        added_global_filters = set()
+
+        for base_cls in reversed(cls.__mro__):
+            if base_cls is object:
                 continue
-            if hasattr(cls, attr_name):
-                attr_value = getattr(cls, attr_name)
+
+            for attr_name, attr_value in base_cls.__dict__.items():
+                if attr_name.startswith("_"):
+                    continue
+
                 if isinstance(attr_value, FieldDescriptor):
                     self.fields[attr_name] = FieldModel(
                         attr_value.required,
@@ -242,13 +248,9 @@ def _register_decorator_components(self) -> None:
                         attr_value.steps,
                         attr_value.external_api,
                         attr_value.copy,
+                        attr_value.computed,
                     )
 
-        added_conditions = set()
-        added_global_validators = set()
-        added_global_filters = set()
-
-        for base_cls in reversed(cls.__mro__):
             conditions = getattr(base_cls, "_conditions", None)
             if conditions is not None:
                 for condition in conditions: