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gh-132111: Document dataclasses.InitVar (#132446)

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Doc/library/dataclasses.rst
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@ -344,6 +344,15 @@ Module contents
Other attributes may exist, but they are private and must not be Other attributes may exist, but they are private and must not be
inspected or relied on. inspected or relied on.
.. class:: InitVar
``InitVar[T]`` type annotations describe variables that are :ref:`init-only
<dataclasses-init-only-variables>`. Fields annotated with :class:`!InitVar`
are considered pseudo-fields, and thus are neither returned by the
:func:`fields` function nor used in any way except adding them as
parameters to :meth:`~object.__init__` and an optional
:meth:`__post_init__`.
.. function:: fields(class_or_instance) .. function:: fields(class_or_instance)
Returns a tuple of :class:`Field` objects that define the fields for this Returns a tuple of :class:`Field` objects that define the fields for this
@ -600,8 +609,8 @@ Init-only variables
Another place where :func:`@dataclass <dataclass>` inspects a type annotation is to Another place where :func:`@dataclass <dataclass>` inspects a type annotation is to
determine if a field is an init-only variable. It does this by seeing determine if a field is an init-only variable. It does this by seeing
if the type of a field is of type ``dataclasses.InitVar``. If a field if the type of a field is of type :class:`InitVar`. If a field
is an ``InitVar``, it is considered a pseudo-field called an init-only is an :class:`InitVar`, it is considered a pseudo-field called an init-only
field. As it is not a true field, it is not returned by the field. As it is not a true field, it is not returned by the
module-level :func:`fields` function. Init-only fields are added as module-level :func:`fields` function. Init-only fields are added as
parameters to the generated :meth:`~object.__init__` method, and are passed to parameters to the generated :meth:`~object.__init__` method, and are passed to