Add default option to field (#12)

Author: mkrd

README.md

@@ -25,6 +25,7 @@ Features
 
 - **Type Validation**: Automatically validates types for attributes based on type hints.
 - **Constraint Validation**: Define constraints like minimum/maximum length, value ranges, and more.
+- **Default Field Values**: Set default values for fields that are used when not explicitly provided.
 - **Customizable Error Handling**: Use custom exception classes for type and constraint errors.
 - **Flexible Field Descriptors**: Add constraints, casting, and other behaviors to your fields.
 - **Optional Fields**: Support for optional fields with default values.
@@ -116,20 +117,108 @@ payload = OptionalPayload()
 print(payload.name)  # Output: None
 ```
 
+### Default Field Values
+
+You can specify default values for fields in two ways:
+
+#### Using Field() with default parameter
+
+```python
+class User(Statica):
+    name: str
+    age: int = Field(default=25)
+    active: bool = Field(default=True)
+
+# Using direct initialization
+user = User(name="John")
+print(user.name)    # Output: "John"
+print(user.age)     # Output: 25
+print(user.active)  # Output: True
+
+# Using from_map
+user = User.from_map({"name": "Jane"})
+print(user.age)     # Output: 25 (default used)
+
+# Explicit values override defaults
+user = User(name="Bob", age=30, active=False)
+print(user.age)     # Output: 30
+```
+
+#### Direct assignment (without Field)
+
+You can also assign default values directly to fields without using `Field()`:
+
+```python
+class Config(Statica):
+    name: str
+    timeout: float = 30.0      # Direct assignment
+    retries: int = 3           # Direct assignment
+    debug: bool = False        # Direct assignment
+
+config = Config(name="server")
+print(config.timeout)  # Output: 30.0
+print(config.retries)   # Output: 3
+print(config.debug)     # Output: False
+
+# Works with from_map too
+config = Config.from_map({"name": "api-server"})
+print(config.timeout)  # Output: 30.0 (default used)
+```
+
+Both approaches work identically and can be mixed within the same class. Use `Field(default=...)` when you need additional constraints or options, and direct assignment for simple defaults.
+
+#### Validation and Safety
+
+Default values are validated against any constraints you've defined:
+
+```python
+class Config(Statica):
+    timeout: float = Field(default=30.0, min_value=1.0, max_value=120.0)
+    retries: int = Field(default=3, min_value=1)
+
+config = Config()  # Uses defaults: timeout=30.0, retries=3
+```
+
+For mutable default values (like lists, dicts, sets), Statica automatically creates copies to prevent shared state issues:
+
+```python
+class UserProfile(Statica):
+    name: str
+    tags: list[str] = Field(default=[])  # or tags: list[str] = []
+
+user1 = UserProfile(name="Alice")
+user2 = UserProfile(name="Bob")
+
+user1.tags.append("admin")
+print(user1.tags)  # Output: ["admin"]
+print(user2.tags)  # Output: [] (not affected)
+```
+
 ### Field Constraints
 
-You can specify constraints on fields:
+You can specify constraints and options on fields:
 
+- **Default Values**: `default` (using `Field()`) or direct assignment
 - **String Constraints**: `min_length`, `max_length`, `strip_whitespace`
 - **Numeric Constraints**: `min_value`, `max_value`
 - **Casting**: `cast_to`
+- **Aliasing**: `alias`
 
 ```python
 class StringTest(Statica):
     name: str = Field(min_length=3, max_length=5, strip_whitespace=True)
 
 class IntTest(Statica):
     num: int = Field(min_value=1, max_value=10, cast_to=int)
+
+class DefaultTest(Statica):
+    # Using Field() for defaults with constraints
+    status: str = Field(default="active")
+    priority: int = Field(default=1, min_value=1, max_value=5)
+    
+    # Direct assignment for simple defaults
+    timeout: float = 30.0
+    retries: int = 3
 ```
 
 ### Custom Error Classes
@@ -181,21 +270,21 @@ Use the `alias` parameter to define an alternative name for both parsing and ser
 ```python
 class User(Statica):
     full_name: str = Field(alias="fullName")
-    age: int = Field(alias="userAge")
+    age: int = Field(alias="userAge", default=25)
 
 # Parse data with aliases
-data = {"fullName": "John Doe", "userAge": 30}
+data = {"fullName": "John Doe"}  # userAge not provided, uses default
 user = User.from_map(data)
 print(user.full_name)  # Output: "John Doe"
-print(user.age)        # Output: 30
+print(user.age)        # Output: 25
 
 # Serialize back with aliases (uses the alias for serialization by default)
 result = user.to_dict()
-print(result)  # Output: {"fullName": "John Doe", "userAge": 30}
+print(result)  # Output: {"fullName": "John Doe", "userAge": 25}
 
 # Serialize without aliases
 result_no_alias = user.to_dict(with_aliases=False)
-print(result_no_alias)  # Output: {"full_name": "John Doe", "age": 30}
+print(result_no_alias)  # Output: {"full_name": "John Doe", "age": 25}
 
 ```
 

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "statica"
-version = "1.3.0"
+version = "1.4.0"
 description = "A minimalistic data validation library"
 readme = "README.md"
 authors = [{ name = "Marcel Kröker", email = "kroeker.marcel@gmail.com" }]

statica/core.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import copy
 from dataclasses import dataclass
 from dataclasses import field as dataclass_field
 from types import UnionType
@@ -9,9 +10,9 @@
 	Generic,
 	Self,
 	TypeVar,
-	cast,
 	dataclass_transform,
 	get_type_hints,
+	overload,
 )
 
 from statica.config import StaticaConfig, default_config
@@ -76,6 +77,7 @@ class User(Statica):
 
 	# User-facing dataclass fields
 
+	default: T | Any | None = None
 	min_length: int | None = None
 	max_length: int | None = None
 	min_value: float | None = None
@@ -117,6 +119,14 @@ def get_statica_subclass(self, sub_types: tuple[type, ...]) -> type[Statica] | N
 			pass
 		return None
 
+	def get_default_safe(self) -> Any:
+		"""
+		Get the default value of the field, safely handling mutable defaults.
+		"""
+		if isinstance(self.default, (list, dict, set)):
+			return copy.copy(self.default)
+		return self.default
+
 	def __get__(self, instance: object | None, owner: Any) -> Any:
 		"""
 		Get the value of the field from the instance.
@@ -200,8 +210,36 @@ def get_field_descriptors(cls: type[Statica]) -> list[FieldDescriptor]:
 #### MARK: Type-safe field function
 
 
+@overload
+def Field(
+	*,
+	default: T,  # If default is used, the return type is T
+	min_length: int | None = None,
+	max_length: int | None = None,
+	min_value: float | None = None,
+	max_value: float | None = None,
+	strip_whitespace: bool | None = None,
+	cast_to: Callable[..., T] | None = None,
+	alias: str | None = None,
+) -> T: ...
+
+
+@overload
+def Field(
+	*,  # No default provided, return type is Any
+	min_length: int | None = None,
+	max_length: int | None = None,
+	min_value: float | None = None,
+	max_value: float | None = None,
+	strip_whitespace: bool | None = None,
+	cast_to: Callable[..., T] | None = None,
+	alias: str | None = None,
+) -> Any: ...
+
+
 def Field(  # noqa: N802
 	*,
+	default: T | Any | None = None,
 	min_length: int | None = None,
 	max_length: int | None = None,
 	min_value: float | None = None,
@@ -211,11 +249,14 @@ def Field(  # noqa: N802
 	alias: str | None = None,
 ) -> Any:
 	"""
-	Type-safe field function that returns the correct type for type checkers
-	but creates a Field descriptor at runtime.
-	"""
+	Type-safe field function that provides proper type checking for default values
+	while creating a FieldDescriptor at runtime.
 
-	fd = FieldDescriptor(
+	When a default value is provided, the return type matches the default's type.
+	This prevents type mismatches like: active: bool = Field(default="yes")
+	"""
+	return FieldDescriptor(
+		default=default,
 		min_length=min_length,
 		max_length=max_length,
 		min_value=min_value,
@@ -225,11 +266,6 @@ def Field(  # noqa: N802
 		alias=alias,
 	)
 
-	if TYPE_CHECKING:
-		return cast("Any", fd)
-
-	return fd  # type: ignore[unreachable]
-
 
 ########################################################################################
 #### MARK: Internal metaclass
@@ -265,7 +301,14 @@ def __new__(
 
 		def statica_init(self: Statica, **kwargs: Any) -> None:
 			for field_name in annotations:
-				setattr(self, field_name, kwargs.get(field_name))
+				field_descriptor = namespace.get(field_name)
+				assert isinstance(field_descriptor, FieldDescriptor)
+
+				# Use default value if key is missing and default is available
+				if field_name not in kwargs and field_descriptor.default is not None:
+					setattr(self, field_name, field_descriptor.get_default_safe())
+				else:
+					setattr(self, field_name, kwargs.get(field_name))
 
 		namespace["__init__"] = statica_init
 
@@ -280,8 +323,8 @@ def statica_init(self: Statica, **kwargs: Any) -> None:
 				continue
 
 			# Case 3: name: str (no assignment) or name: Field[str] (no assignment)
-			# Create a default Field descriptor
-			namespace[attr_annotated] = FieldDescriptor()
+			# Create a Field descriptor with the default if it exists
+			namespace[attr_annotated] = FieldDescriptor(default=namespace.get(attr_annotated))
 
 		return super().__new__(cls, name, bases, namespace)
 
@@ -293,20 +336,16 @@ def statica_init(self: Statica, **kwargs: Any) -> None:
 class Statica(metaclass=StaticaMeta):
 	@classmethod
 	def from_map(cls, mapping: Mapping[str, Any]) -> Self:
-		# Fields might have aliases, so we need to map them correctly.
-		# Here we map the chosen alias to the original field name.
-		# If no alias is provided, we use the field name itself.
-		# Priority: parsing alias > general alias > field name
-		mapping_key_to_field_keys = {}
+		# Fields might have aliases, so we need to map them correctly
 
+		kwargs = {}
 		for field_descriptor in get_field_descriptors(cls):
-			# Use alias for parsing if it exists
-			alias = field_descriptor.alias or field_descriptor.name
-			mapping_key_to_field_keys[alias] = field_descriptor.name
+			expected_field_name = field_descriptor.alias or field_descriptor.name
 
-		parsed_mapping = {mapping_key_to_field_keys[k]: v for k, v in mapping.items()}
+			if expected_field_name in mapping:
+				kwargs[field_descriptor.name] = mapping[expected_field_name]
 
-		return cls(**parsed_mapping)  # Init function will validate fields
+		return cls(**kwargs)  # Init function will validate fields and set defaults
 
 	def to_dict(self, *, with_aliases: bool = True) -> dict[str, Any]:
 		"""

tests/test_aliasing.py

@@ -21,7 +21,7 @@ class AliasTest(Statica):
 	assert instance.to_dict() == data
 
 	# Test that original field names don't work when alias is used
-	with pytest.raises(KeyError):
+	with pytest.raises(TypeValidationError):
 		AliasTest.from_map({"full_name": "John Doe", "age": INTEGER})
 
 
@@ -115,12 +115,12 @@ def test_empty_alias_mapping() -> None:
 	class EmptyMappingTest(Statica):
 		field_name: str = Field(alias="expectedAlias")
 
-	# Should raise KeyError when the aliased field is missing
-	with pytest.raises(KeyError):
+	# Should raise TypeValidationError when the aliased field is missing
+	with pytest.raises(TypeValidationError):
 		EmptyMappingTest.from_map({"wrongAlias": "value"})
 
-	# Should raise a key error when the original field name is used
-	with pytest.raises(KeyError):
+	# Should raise a TypeValidationError when the original field name is used
+	with pytest.raises(TypeValidationError):
 		EmptyMappingTest.from_map({"field_name": "value"})