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More clarification of kw_only args. (GH-25838)

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Also, clarify that the dataclass decorator is what raises an error for some mutable defaults.
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Eric V. Smith 2021-05-03 01:55:13 -04:00 • committed by GitHub
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33
Doc/library/dataclasses.rst
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@ -307,8 +307,8 @@ Module contents
- ``type``: The type of the field.
- ``default``, ``default_factory``, ``init``, ``repr``, ``hash``,
``compare``, and ``metadata`` have the identical meaning and
values as they do in the :func:`field` declaration.
``compare``, ``metadata``, and ``kw_only`` have the identical
meaning and values as they do in the :func:`field` declaration.
Other attributes may exist, but they are private and must not be
inspected or relied on.
@ -459,6 +459,9 @@ Module contents
p = Point(0, y=1.5, z=2.0)
In a single dataclass, it is an error to specify more than one
field whose type is :const:`KW_ONLY`.
.. exception:: FrozenInstanceError
Raised when an implicitly defined :meth:`__setattr__` or
@ -582,9 +585,12 @@ Re-ordering of keyword-only parameters in :meth:`__init__`
After the parameters needed for :meth:`__init__` are computed, any
keyword-only parameters are moved to come after all regular
(non-keyword-only) parameters. In this example, ``Base.y``,
``Base.w``, and ``D.t`` are keyword-only fields, and ``Base.x`` and
``D.z`` are regular fields::
(non-keyword-only) parameters. This is a requirement of how
keyword-only parameters are implemented in Python: they must come
after non-keyword-only parameters.
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:
@ -666,14 +672,15 @@ Mutable default values
assert D().x is D().x
This has the same issue as the original example using class ``C``.
That is, two instances of class ``D`` that do not specify a value for
``x`` when creating a class instance will share the same copy of
``x``. Because dataclasses just use normal Python class creation
they also share this behavior. There is no general way for Data
Classes to detect this condition. Instead, dataclasses will raise a
:exc:`TypeError` if it detects a default parameter of type ``list``,
``dict``, or ``set``. This is a partial solution, but it does protect
against many common errors.
That is, two instances of class ``D`` that do not specify a value
for ``x`` when creating a class instance will share the same copy
of ``x``. Because dataclasses just use normal Python class
creation they also share this behavior. There is no general way
for Data Classes to detect this condition. Instead, the
:func:`dataclass` decorator will raise a :exc:`TypeError` if it
detects a default parameter of type ``list``, ``dict``, or ``set``.
This is a partial solution, but it does protect against many common
errors.
Using default factory functions is a way to create new instances of
mutable types as default values for fields::