Add support for dataclass fields init (#8552)

Co-authored-by: sydney-runkle <sydneymarierunkle@gmail.com>

docs/concepts/fields.md

@@ -459,6 +459,7 @@ print(foo)
 
 There are fields that can be used to constrain dataclasses:
 
+* `init`: Whether the field should be included in the `__init__` of the dataclass.
 * `init_var`: Whether the field should be seen as an [init-only field] in the dataclass.
 * `kw_only`: Whether the field should be a keyword-only argument in the constructor of the dataclass.
 

docs/errors/usage_errors.md

@@ -1046,4 +1046,47 @@ class Model(BaseModel):
     date: datetime.date = Field(description='A date')
 ```
 
+## Incompatible `dataclass` `init` and `extra` settings {#dataclass-init-false-extra-allow}
+
+Pydantic does not allow the specification of the `extra='allow'` setting on a dataclass
+while any of the fields have `init=False` set.
+
+Thus, you may not do something like the following:
+
+```py test="skip"
+from pydantic import ConfigDict, Field
+from pydantic.dataclasses import dataclass
+
+
+@dataclass(config=ConfigDict(extra='allow'))
+class A:
+    a: int = Field(init=False, default=1)
+```
+
+The above snippet results in the following error during schema building for the `A` dataclass:
+
+```
+pydantic.errors.PydanticUserError: Field a has `init=False` and dataclass has config setting `extra="allow"`.
+This combination is not allowed.
+```
+
+## Incompatible `init` and `init_var` settings on `dataclass` field {#clashing-init-and-init-var}
+
+The `init=False` and `init_var=True` settings are mutually exclusive. Doing so results in the `PydanticUserError` shown in the example below.
+
+```py test="skip"
+from pydantic import Field
+from pydantic.dataclasses import dataclass
+
+
+@dataclass
+class Foo:
+    bar: str = Field(..., init=False, init_var=True)
+
+
+"""
+pydantic.errors.PydanticUserError: Dataclass field bar has init=False and init_var=True, but these are mutually exclusive.
+"""
+```
+
 {% endraw %}

pydantic/_internal/_fields.py

@@ -10,6 +10,8 @@
 
 from pydantic_core import PydanticUndefined
 
+from pydantic.errors import PydanticUserError
+
 from . import _typing_extra
 from ._config import ConfigWrapper
 from ._repr import Representation
@@ -284,11 +286,18 @@ def collect_dataclass_fields(
 
         if isinstance(dataclass_field.default, FieldInfo):
             if dataclass_field.default.init_var:
-                # TODO: same note as above
+                if dataclass_field.default.init is False:
+                    raise PydanticUserError(
+                        f'Dataclass field {ann_name} has init=False and init_var=True, but these are mutually exclusive.',
+                        code='clashing-init-and-init-var',
+                    )
+
+                # TODO: same note as above re validate_assignment
                 continue
             field_info = FieldInfo.from_annotated_attribute(ann_type, dataclass_field.default)
         else:
             field_info = FieldInfo.from_annotated_attribute(ann_type, dataclass_field)
+
         fields[ann_name] = field_info
 
         if field_info.default is not PydanticUndefined and isinstance(getattr(cls, ann_name, field_info), FieldInfo):

pydantic/_internal/_generate_schema.py

@@ -941,6 +941,7 @@ def _generate_dc_field_schema(
         return core_schema.dataclass_field(
             name,
             common_field['schema'],
+            init=field_info.init,
             init_only=field_info.init_var or None,
             kw_only=None if field_info.kw_only else False,
             serialization_exclude=common_field['serialization_exclude'],
@@ -1468,6 +1469,18 @@ def _dataclass_schema(
                         self._types_namespace,
                         typevars_map=typevars_map,
                     )
+
+                # disallow combination of init=False on a dataclass field and extra='allow' on a dataclass
+                if config and config.get('extra') == 'allow':
+                    # disallow combination of init=False on a dataclass field and extra='allow' on a dataclass
+                    for field_name, field in fields.items():
+                        if field.init is False:
+                            raise PydanticUserError(
+                                f'Field {field_name} has `init=False` and dataclass has config setting `extra="allow"`. '
+                                f'This combination is not allowed.',
+                                code='dataclass-init-false-extra-allow',
+                            )
+
                 decorators = dataclass.__dict__.get('__pydantic_decorators__') or DecoratorInfos.build(dataclass)
                 # Move kw_only=False args to the start of the list, as this is how vanilla dataclasses work.
                 # Note that when kw_only is missing or None, it is treated as equivalent to kw_only=True

pydantic/_internal/_signature.py

@@ -68,7 +68,7 @@ def _process_param_defaults(param: Parameter) -> Parameter:
     return param
 
 
-def _generate_signature_parameters(
+def _generate_signature_parameters(  # noqa: C901 (ignore complexity, could use a refactor)
     init: Callable[..., None],
     fields: dict[str, FieldInfo],
     config_wrapper: ConfigWrapper,
@@ -85,6 +85,9 @@ def _generate_signature_parameters(
         # inspect does "clever" things to show annotations as strings because we have
         # `from __future__ import annotations` in main, we don't want that
         if fields.get(param.name):
+            # exclude params with init=False
+            if getattr(fields[param.name], 'init', True) is False:
+                continue
             param = param.replace(name=_field_name_for_signature(param.name, fields[param.name]))
         if param.annotation == 'Any':
             param = param.replace(annotation=Any)

pydantic/errors.py

@@ -59,6 +59,8 @@
     'type-adapter-config-unused',
     'root-model-extra',
     'unevaluable-type-annotation',
+    'dataclass-init-false-extra-allow',
+    'clashing-init-and-init-var',
 ]
 
 

pydantic/fields.py

@@ -64,6 +64,7 @@ class _FromFieldInfoInputs(typing_extensions.TypedDict, total=False):
     frozen: bool | None
     validate_default: bool | None
     repr: bool
+    init: bool | None
     init_var: bool | None
     kw_only: bool | None
 
@@ -101,7 +102,8 @@ class FieldInfo(_repr.Representation):
         frozen: Whether the field is frozen.
         validate_default: Whether to validate the default value of the field.
         repr: Whether to include the field in representation of the model.
-        init_var: Whether the field should be included in the constructor of the dataclass.
+        init: Whether the field should be included in the constructor of the dataclass.
+        init_var: Whether the field should _only_ be included in the constructor of the dataclass, and not stored.
         kw_only: Whether the field should be a keyword-only argument in the constructor of the dataclass.
         metadata: List of metadata constraints.
     """
@@ -122,6 +124,7 @@ class FieldInfo(_repr.Representation):
     frozen: bool | None
     validate_default: bool | None
     repr: bool
+    init: bool | None
     init_var: bool | None
     kw_only: bool | None
     metadata: list[Any]
@@ -143,6 +146,7 @@ class FieldInfo(_repr.Representation):
         'frozen',
         'validate_default',
         'repr',
+        'init',
         'init_var',
         'kw_only',
         'metadata',
@@ -203,6 +207,7 @@ def __init__(self, **kwargs: Unpack[_FieldInfoInputs]) -> None:
         self.validate_default = kwargs.pop('validate_default', None)
         self.frozen = kwargs.pop('frozen', None)
         # currently only used on dataclasses
+        self.init = kwargs.pop('init', None)
         self.init_var = kwargs.pop('init_var', None)
         self.kw_only = kwargs.pop('kw_only', None)
 
@@ -360,6 +365,7 @@ class MyModel(pydantic.BaseModel):
             )
             pydantic_field.frozen = final or pydantic_field.frozen
             pydantic_field.init_var = init_var
+            pydantic_field.init = getattr(default, 'init', None)
             pydantic_field.kw_only = getattr(default, 'kw_only', None)
             return pydantic_field
         else:
@@ -596,6 +602,7 @@ class _EmptyKwargs(typing_extensions.TypedDict):
     frozen=None,
     validate_default=None,
     repr=True,
+    init=None,
     init_var=None,
     kw_only=None,
     pattern=None,
@@ -630,6 +637,7 @@ def Field(  # noqa: C901
     frozen: bool | None = _Unset,
     validate_default: bool | None = _Unset,
     repr: bool = _Unset,
+    init: bool | None = _Unset,
     init_var: bool | None = _Unset,
     kw_only: bool | None = _Unset,
     pattern: str | None = _Unset,
@@ -675,7 +683,9 @@ def Field(  # noqa: C901
         validate_default: If `True`, apply validation to the default value every time you create an instance.
             Otherwise, for performance reasons, the default value of the field is trusted and not validated.
         repr: A boolean indicating whether to include the field in the `__repr__` output.
-        init_var: Whether the field should be included in the constructor of the dataclass.
+        init: Whether the field should be included in the constructor of the dataclass.
+            (Only applies to dataclasses.)
+        init_var: Whether the field should _only_ be included in the constructor of the dataclass.
             (Only applies to dataclasses.)
         kw_only: Whether the field should be a keyword-only argument in the constructor of the dataclass.
             (Only applies to dataclasses.)
@@ -784,6 +794,7 @@ def Field(  # noqa: C901
         pattern=pattern,
         validate_default=validate_default,
         repr=repr,
+        init=init,
         init_var=init_var,
         kw_only=kw_only,
         strict=strict,

tests/test_dataclasses.py

@@ -2648,3 +2648,95 @@ class Model(BaseModel):
         n: Nested
 
     assert Model.model_validate_strings({'n': {'d': '2017-01-01'}}).n.d == date(2017, 1, 1)
+
+
+@pytest.mark.parametrize('field_constructor', [dataclasses.field, pydantic.dataclasses.Field])
+@pytest.mark.parametrize('extra', ['ignore', 'forbid'])
+def test_init_false_not_in_signature(extra, field_constructor):
+    @pydantic.dataclasses.dataclass(config=ConfigDict(extra=extra))
+    class MyDataclass:
+        a: int = field_constructor(init=False, default=-1)
+        b: int = pydantic.dataclasses.Field(default=2)
+
+    signature = inspect.signature(MyDataclass)
+    # `a` should not be in the __init__
+    assert 'a' not in signature.parameters.keys()
+    assert 'b' in signature.parameters.keys()
+
+
+init_test_cases = [
+    ({'a': 2, 'b': -1}, 'ignore', {'a': 2, 'b': 1}),
+    ({'a': 2}, 'ignore', {'a': 2, 'b': 1}),
+    (
+        {'a': 2, 'b': -1},
+        'forbid',
+        [
+            {
+                'type': 'unexpected_keyword_argument',
+                'loc': ('b',),
+                'msg': 'Unexpected keyword argument',
+                'input': -1,
+            }
+        ],
+    ),
+    ({'a': 2}, 'forbid', {'a': 2, 'b': 1}),
+]
+
+
+@pytest.mark.parametrize('field_constructor', [dataclasses.field, pydantic.dataclasses.Field])
+@pytest.mark.parametrize(
+    'input_data,extra,expected',
+    init_test_cases,
+)
+def test_init_false_with_post_init(input_data, extra, expected, field_constructor):
+    @pydantic.dataclasses.dataclass(config=ConfigDict(extra=extra))
+    class MyDataclass:
+        a: int
+        b: int = field_constructor(init=False)
+
+        def __post_init__(self):
+            self.b = 1
+
+    if isinstance(expected, list):
+        with pytest.raises(ValidationError) as exc_info:
+            MyDataclass(**input_data)
+
+        assert exc_info.value.errors(include_url=False) == expected
+    else:
+        assert dataclasses.asdict(MyDataclass(**input_data)) == expected
+
+
+@pytest.mark.parametrize('field_constructor', [dataclasses.field, pydantic.dataclasses.Field])
+@pytest.mark.parametrize(
+    'input_data,extra,expected',
+    init_test_cases,
+)
+def test_init_false_with_default(input_data, extra, expected, field_constructor):
+    @pydantic.dataclasses.dataclass(config=ConfigDict(extra=extra))
+    class MyDataclass:
+        a: int
+        b: int = field_constructor(init=False, default=1)
+
+    if isinstance(expected, list):
+        with pytest.raises(ValidationError) as exc_info:
+            MyDataclass(**input_data)
+
+        assert exc_info.value.errors(include_url=False) == expected
+    else:
+        assert dataclasses.asdict(MyDataclass(**input_data)) == expected
+
+
+def test_disallow_extra_allow_and_init_false() -> None:
+    with pytest.raises(PydanticUserError, match='This combination is not allowed.'):
+
+        @pydantic.dataclasses.dataclass(config=ConfigDict(extra='allow'))
+        class A:
+            a: int = Field(init=False, default=1)
+
+
+def test_disallow_init_false_and_init_var_true() -> None:
+    with pytest.raises(PydanticUserError, match='mutually exclusive.'):
+
+        @pydantic.dataclasses.dataclass
+        class Foo:
+            bar: str = Field(..., init=False, init_var=True)