Improve the dataclasses kw_only documentation. (GH-25799)

Doc/library/dataclasses.rst

@@ -169,9 +169,13 @@ Module-level decorators, classes, and functions
      ``__match_args__`` will not be generated.
 
    - ``kw_only``: If true (the default value is ``False``), then all
-     fields will be marked as keyword-only.  See the :term:`parameter`
-     glossary entry for details.  Also see the ``dataclasses.KW_ONLY``
-     section.
+     fields will be marked as keyword-only.  If a field is marked as
+     keyword-only, then the only affect is that the :meth:`__init__`
+     parameter generated from a keyword-only field must be specified
+     with a keyword when :meth:`__init__` is called.  There is no
+     effect on any other aspect of dataclasses.  See the
+     :term:`parameter` glossary entry for details.  Also see the
+     ``dataclasses.KW_ONLY`` section.
 
    - ``slots``: If true (the default is ``False``), :attr:`__slots__` attribute
      will be generated and new class will be returned instead of the original one.
@@ -195,7 +199,7 @@ Module-level decorators, classes, and functions
    follows a field with a default value.  This is true either when this
    occurs in a single class, or as a result of class inheritance.
 
-.. function:: field(*, default=MISSING, default_factory=MISSING, repr=True, hash=None, init=True, compare=True, metadata=None)
+.. function:: field(*, default=MISSING, default_factory=MISSING, init=True, repr=True, hash=None, compare=True, metadata=None, kw_only=MISSING):
 
    For common and simple use cases, no other functionality is
    required.  There are, however, some dataclass features that
@@ -234,10 +238,6 @@ Module-level decorators, classes, and functions
    - ``repr``: If true (the default), this field is included in the
      string returned by the generated :meth:`__repr__` method.
 
-   - ``compare``: If true (the default), this field is included in the
-     generated equality and comparison methods (:meth:`__eq__`,
-     :meth:`__gt__`, et al.).
-
    - ``hash``: This can be a bool or ``None``.  If true, this field is
      included in the generated :meth:`__hash__` method.  If ``None`` (the
      default), use the value of ``compare``: this would normally be
@@ -251,6 +251,10 @@ Module-level decorators, classes, and functions
      fields that contribute to the type's hash value.  Even if a field
      is excluded from the hash, it will still be used for comparisons.
 
+   - ``compare``: If true (the default), this field is included in the
+     generated equality and comparison methods (:meth:`__eq__`,
+     :meth:`__gt__`, et al.).
+
    - ``metadata``: This can be a mapping or None. None is treated as
      an empty dict.  This value is wrapped in
      :func:`~types.MappingProxyType` to make it read-only, and exposed
@@ -259,6 +263,10 @@ Module-level decorators, classes, and functions
      Multiple third-parties can each have their own key, to use as a
      namespace in the metadata.
 
+   - ``kw_only``: If true, this field will be marked as keyword-only.
+     This is used when the generated :meth:`__init__` method's
+     parameters are computed.
+
    If the default value of a field is specified by a call to
    :func:`field()`, then the class attribute for this field will be
    replaced by the specified ``default`` value.  If no ``default`` is
@@ -532,15 +540,18 @@ The generated :meth:`__init__` method for ``C`` will look like::
 Re-ordering of keyword-only parameters in __init__
 --------------------------------------------------
 
-After the fields needed for :meth:`__init__` are computed, any
-keyword-only fields are put after regular fields.  In this example,
-``Base.y`` and ``D.t`` are keyword-only fields::
+After the parameters needed for :meth:`__init__` are computed, any
+keyword-only parameters are moved to come after regular
+(non-keyword-only) fields.  In this example, ``Base.y``, ``Base.w``,
+and ``D.t`` are keyword-only fields, and ``Base.x`` and ``D.z`` are
+regular fields::
 
   @dataclass
   class Base:
       x: Any = 15.0
       _: KW_ONLY
       y: int = 0
+      w: int = 1
 
   @dataclass
   class D(Base):
@@ -549,9 +560,13 @@ keyword-only fields are put after regular fields.  In this example,
 
 The generated :meth:`__init__` method for ``D`` will look like::
 
-  def __init__(self, x: Any = 15.0, z: int = 10, *, y: int = 0, t: int = 0):
+  def __init__(self, x: Any = 15.0, z: int = 10, *, y: int = 0, w: int = 1, t: int = 0):
 
-The relative ordering of keyword-only arguments is not changed from
+Note that the parameters have been re-ordered from how they appear in
+the list of fields: parameters derived from regular fields are
+followed by parameters derived from keyword-only fields.
+
+The relative ordering of keyword-only parameters is not changed from
 the order they are in computed field :meth:`__init__` list.