Make kw_only=True behavior consistent with dataclasses (#1457)

* initial change

* move comment

* fix docs

* Apply suggestions from code review

Co-authored-by: Hynek Schlawack <hs@ox.cx>

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* Add changelog

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---------

Co-authored-by: Hynek Schlawack <hs@ox.cx>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

changelog.d/1457.breaking.md

@@ -0,0 +1,28 @@
+Class-level `kw_only=True` behavior is now consistent with `dataclasses`.
+
+Previously, a class that sets `kw_only=True` makes all attributes keyword-only,
+including those from base classes.  If an attribute sets `kw_only=False`, that
+setting is ignored, and it is still made keyword-only.
+
+Now, only the attributes defined in that class that doesn't explicitly set
+`kw_only=False` are made keyword-only.
+
+This shouldn't be a problem for most users, unless you have a pattern like this:
+
+```python
+@attrs.define(kw_only=True)
+class Base:
+    a: int
+    b: int = attrs.field(default=1, kw_only=False)
+
+@attrs.define
+class Subclass(Base):
+    c: int
+```
+
+Here, we have a `kw_only=True` attrs class (`Base`) with an attribute that sets
+`kw_only=False` and has a default (`Base.b`), and then create a subclass (`Subclass`)
+with required arguments (`Subclass.c`).  Previously this would work, since it
+would make `Base.b` keyword-only, but now this fails since `Base.b` is positional, and
+we have a required positional argument (`Subclass.c`) following another argument with
+defaults.

src/attr/__init__.pyi

@@ -176,7 +176,7 @@ def attrib(
     type: None = ...,
     converter: None = ...,
     factory: None = ...,
-    kw_only: bool = ...,
+    kw_only: bool | None = ...,
     eq: _EqOrderType | None = ...,
     order: _EqOrderType | None = ...,
     on_setattr: _OnSetAttrArgType | None = ...,
@@ -200,7 +200,7 @@ def attrib(
     | tuple[_ConverterType]
     | None = ...,
     factory: Callable[[], _T] | None = ...,
-    kw_only: bool = ...,
+    kw_only: bool | None = ...,
     eq: _EqOrderType | None = ...,
     order: _EqOrderType | None = ...,
     on_setattr: _OnSetAttrArgType | None = ...,
@@ -223,7 +223,7 @@ def attrib(
     | tuple[_ConverterType]
     | None = ...,
     factory: Callable[[], _T] | None = ...,
-    kw_only: bool = ...,
+    kw_only: bool | None = ...,
     eq: _EqOrderType | None = ...,
     order: _EqOrderType | None = ...,
     on_setattr: _OnSetAttrArgType | None = ...,
@@ -246,7 +246,7 @@ def attrib(
     | tuple[_ConverterType]
     | None = ...,
     factory: Callable[[], _T] | None = ...,
-    kw_only: bool = ...,
+    kw_only: bool | None = ...,
     eq: _EqOrderType | None = ...,
     order: _EqOrderType | None = ...,
     on_setattr: _OnSetAttrArgType | None = ...,

src/attr/_make.py

@@ -114,7 +114,7 @@ def attrib(
     type=None,
     converter=None,
     factory=None,
-    kw_only=False,
+    kw_only=None,
     eq=None,
     order=None,
     on_setattr=None,
@@ -157,6 +157,9 @@ def attrib(
        *eq*, *order*, and *cmp* also accept a custom callable
     .. versionchanged:: 21.1.0 *cmp* undeprecated
     .. versionadded:: 22.2.0 *alias*
+    .. versionchanged:: 25.4.0
+       *kw_only* can now be None, and its default is also changed from False to
+       None.
     """
     eq, eq_key, order, order_key = _determine_attrib_eq_order(
         cmp, eq, order, True
@@ -374,7 +377,13 @@ def _collect_base_attrs_broken(cls, taken_attr_names):
 
 
 def _transform_attrs(
-    cls, these, auto_attribs, kw_only, collect_by_mro, field_transformer
+    cls,
+    these,
+    auto_attribs,
+    kw_only,
+    force_kw_only,
+    collect_by_mro,
+    field_transformer,
 ) -> _Attributes:
     """
     Transform all `_CountingAttr`s on a class into `Attribute`s.
@@ -430,7 +439,8 @@ def _transform_attrs(
 
     fca = Attribute.from_counting_attr
     own_attrs = [
-        fca(attr_name, ca, anns.get(attr_name)) for attr_name, ca in ca_list
+        fca(attr_name, ca, kw_only, anns.get(attr_name))
+        for attr_name, ca in ca_list
     ]
 
     if collect_by_mro:
@@ -442,7 +452,7 @@ def _transform_attrs(
             cls, {a.name for a in own_attrs}
         )
 
-    if kw_only:
+    if kw_only and force_kw_only:
         own_attrs = [a.evolve(kw_only=True) for a in own_attrs]
         base_attrs = [a.evolve(kw_only=True) for a in base_attrs]
 
@@ -669,6 +679,7 @@ def __init__(
         getstate_setstate,
         auto_attribs,
         kw_only,
+        force_kw_only,
         cache_hash,
         is_exc,
         collect_by_mro,
@@ -681,6 +692,7 @@ def __init__(
             these,
             auto_attribs,
             kw_only,
+            force_kw_only,
             collect_by_mro,
             field_transformer,
         )
@@ -1352,6 +1364,7 @@ def attrs(
     field_transformer=None,
     match_args=True,
     unsafe_hash=None,
+    force_kw_only=True,
 ):
     r"""
     A class decorator that adds :term:`dunder methods` according to the
@@ -1418,6 +1431,10 @@ def attrs(
        If a class has an *inherited* classmethod called
        ``__attrs_init_subclass__``, it is executed after the class is created.
     .. deprecated:: 24.1.0 *hash* is deprecated in favor of *unsafe_hash*.
+    .. versionchanged:: 25.4.0
+       *kw_only* now only applies to attributes defined in the current class,
+       and respects attribute-level ``kw_only=False`` settings.
+    .. versionadded:: 25.4.0 *force_kw_only*
     """
     if repr_ns is not None:
         import warnings
@@ -1464,6 +1481,7 @@ def wrap(cls):
             ),
             auto_attribs,
             kw_only,
+            force_kw_only,
             cache_hash,
             is_exc,
             collect_by_mro,
@@ -2516,7 +2534,11 @@ def __setattr__(self, name, value):
         raise FrozenInstanceError
 
     @classmethod
-    def from_counting_attr(cls, name: str, ca: _CountingAttr, type=None):
+    def from_counting_attr(
+        cls, name: str, ca: _CountingAttr, kw_only: bool, type=None
+    ):
+        # The 'kw_only' argument is the class-level setting, and is used if the
+        # attribute itself does not explicitly set 'kw_only'.
         # type holds the annotated value. deal with conflicts:
         if type is None:
             type = ca.type
@@ -2535,7 +2557,7 @@ def from_counting_attr(cls, name: str, ca: _CountingAttr, type=None):
             ca.metadata,
             type,
             ca.converter,
-            ca.kw_only,
+            kw_only if ca.kw_only is None else ca.kw_only,
             ca.eq,
             ca.eq_key,
             ca.order,

src/attr/_next_gen.py

@@ -43,6 +43,7 @@ def define(
     on_setattr=None,
     field_transformer=None,
     match_args=True,
+    force_kw_only=False,
 ):
     r"""
     A class decorator that adds :term:`dunder methods` according to
@@ -214,8 +215,12 @@ def define(
                 5. Subclasses of a frozen class are frozen too.
 
         kw_only (bool):
-            Make all attributes keyword-only in the generated ``__init__`` (if
-            *init* is False, this parameter is ignored).
+            Make attributes keyword-only in the generated ``__init__`` (if
+            *init* is False, this parameter is ignored).  Attributes that
+            explicitly set ``kw_only=False`` are not affected; base class
+            attributes are also not affected.
+
+            Also see *force_kw_only*.
 
         weakref_slot (bool):
             Make instances weak-referenceable.  This has no effect unless
@@ -244,6 +249,15 @@ def define(
             See also `issue #428
             <https://github.com/python-attrs/attrs/issues/428>`_.
 
+        force_kw_only (bool):
+            A back-compat flag for restoring pre-25.4.0 behavior.  If True and
+            ``kw_only=True``, all attributes are made keyword-only, including
+            base class attributes, and those set to ``kw_only=False`` at the
+            attribute level.  Defaults to False.
+
+            See also `issue #980
+            <https://github.com/python-attrs/attrs/issues/980>`_.
+
         getstate_setstate (bool | None):
             .. note::
 
@@ -319,6 +333,11 @@ def define(
     .. versionadded:: 24.3.0
        Unless already present, a ``__replace__`` method is automatically
        created for `copy.replace` (Python 3.13+ only).
+    .. versionchanged:: 25.4.0
+       *kw_only* now only applies to attributes defined in the current class,
+       and respects attribute-level ``kw_only=False`` settings.
+    .. versionadded:: 25.4.0
+       Added *force_kw_only* to go back to the previous *kw_only* behavior.
 
     .. note::
 
@@ -337,6 +356,7 @@ def define(
         - *auto_exc=True*
         - *auto_detect=True*
         - *order=False*
+        - *force_kw_only=False*
         - Some options that were only relevant on Python 2 or were kept around
           for backwards-compatibility have been removed.
 
@@ -366,6 +386,7 @@ def do_it(cls, auto_attribs):
             on_setattr=on_setattr,
             field_transformer=field_transformer,
             match_args=match_args,
+            force_kw_only=force_kw_only,
         )
 
     def wrap(cls):
@@ -424,7 +445,7 @@ def field(
     type=None,
     converter=None,
     factory=None,
-    kw_only=False,
+    kw_only=None,
     eq=None,
     order=None,
     on_setattr=None,
@@ -550,9 +571,10 @@ def field(
             itself. You can use it as part of your own code or for `static type
             checking <types>`.
 
-        kw_only (bool):
+        kw_only (bool | None):
             Make this attribute keyword-only in the generated ``__init__`` (if
-            ``init`` is False, this parameter is ignored).
+            *init* is False, this parameter is ignored).  If None (default),
+            mirror the setting from `attrs.define`.
 
         on_setattr (~typing.Callable | list[~typing.Callable] | None | ~typing.Literal[attrs.setters.NO_OP]):
             Allows to overwrite the *on_setattr* setting from `attr.s`. If left
@@ -572,6 +594,9 @@ def field(
     .. versionadded:: 23.1.0
        The *type* parameter has been re-added; mostly for `attrs.make_class`.
        Please note that type checkers ignore this metadata.
+    .. versionchanged:: 25.4.0
+       *kw_only* can now be None, and its default is also changed from False to
+       None.
 
     .. seealso::
 

src/attrs/__init__.pyi

@@ -77,7 +77,7 @@ def field(
     metadata: Mapping[Any, Any] | None = ...,
     converter: None = ...,
     factory: None = ...,
-    kw_only: bool = ...,
+    kw_only: bool | None = ...,
     eq: bool | None = ...,
     order: bool | None = ...,
     on_setattr: _OnSetAttrArgType | None = ...,
@@ -101,7 +101,7 @@ def field(
     | tuple[_ConverterType]
     | None = ...,
     factory: Callable[[], _T] | None = ...,
-    kw_only: bool = ...,
+    kw_only: bool | None = ...,
     eq: _EqOrderType | None = ...,
     order: _EqOrderType | None = ...,
     on_setattr: _OnSetAttrArgType | None = ...,
@@ -124,7 +124,7 @@ def field(
     | tuple[_ConverterType]
     | None = ...,
     factory: Callable[[], _T] | None = ...,
-    kw_only: bool = ...,
+    kw_only: bool | None = ...,
     eq: _EqOrderType | None = ...,
     order: _EqOrderType | None = ...,
     on_setattr: _OnSetAttrArgType | None = ...,
@@ -147,7 +147,7 @@ def field(
     | tuple[_ConverterType]
     | None = ...,
     factory: Callable[[], _T] | None = ...,
-    kw_only: bool = ...,
+    kw_only: bool | None = ...,
     eq: _EqOrderType | None = ...,
     order: _EqOrderType | None = ...,
     on_setattr: _OnSetAttrArgType | None = ...,