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#: library/dataclasses.rst:2
msgid ":mod:`dataclasses` --- Data Classes"
#: library/dataclasses.rst:10
msgid "**Source code:** :source:`Lib/dataclasses.py`"
#: library/dataclasses.rst:14
"This module provides a decorator and functions for automatically adding "
"generated :term:`special method`\\s such as :meth:`__init__` and :meth:"
"`__repr__` to user-defined classes.  It was originally described in :pep:"
#: library/dataclasses.rst:19
"The member variables to use in these generated methods are defined using :"
"pep:`526` type annotations.  For example, this code::"
#: library/dataclasses.rst:34
msgid "will add, among other things, a :meth:`__init__` that looks like::"
#: library/dataclasses.rst:41
"Note that this method is automatically added to the class: it is not "
"directly specified in the ``InventoryItem`` definition shown above."
#: library/dataclasses.rst:47
msgid "Module contents"
#: library/dataclasses.rst:51
"This function is a :term:`decorator` that is used to add generated :term:"
"`special method`\\s to classes, as described below."
#: library/dataclasses.rst:54
"The :func:`dataclass` decorator examines the class to find ``field``\\s.  A "
"``field`` is defined as a class variable that has a :term:`type annotation "
"<variable annotation>`.  With two exceptions described below, nothing in :"
"func:`dataclass` examines the type specified in the variable annotation."
#: library/dataclasses.rst:60
"The order of the fields in all of the generated methods is the order in "
"which they appear in the class definition."
#: library/dataclasses.rst:63
"The :func:`dataclass` decorator will add various \"dunder\" methods to the "
"class, described below.  If any of the added methods already exist in the "
"class, the behavior depends on the parameter, as documented below. The "
"decorator returns the same class that it is called on; no new class is "
#: library/dataclasses.rst:69
"If :func:`dataclass` is used just as a simple decorator with no parameters, "
"it acts as if it has the default values documented in this signature.  That "
"is, these three uses of :func:`dataclass` are equivalent::"
#: library/dataclasses.rst:87
msgid "The parameters to :func:`dataclass` are:"
#: library/dataclasses.rst:89
"``init``: If true (the default), a :meth:`__init__` method will be generated."
#: library/dataclasses.rst:92
"If the class already defines :meth:`__init__`, this parameter is ignored."
#: library/dataclasses.rst:95
"``repr``: If true (the default), a :meth:`__repr__` method will be "
"generated.  The generated repr string will have the class name and the name "
"and repr of each field, in the order they are defined in the class.  Fields "
"that are marked as being excluded from the repr are not included.  For "
"example: ``InventoryItem(name='widget', unit_price=3.0, "
"quantity_on_hand=10)``."
#: library/dataclasses.rst:102
"If the class already defines :meth:`__repr__`, this parameter is ignored."
#: library/dataclasses.rst:105
"``eq``: If true (the default), an :meth:`__eq__` method will be generated.  "
"This method compares the class as if it were a tuple of its fields, in "
"order.  Both instances in the comparison must be of the identical type."
#: library/dataclasses.rst:110
msgid "If the class already defines :meth:`__eq__`, this parameter is ignored."
#: library/dataclasses.rst:113
"``order``: If true (the default is ``False``), :meth:`__lt__`, :meth:"
"`__le__`, :meth:`__gt__`, and :meth:`__ge__` methods will be generated.  "
"These compare the class as if it were a tuple of its fields, in order.  Both "
"instances in the comparison must be of the identical type.  If ``order`` is "
"true and ``eq`` is false, a :exc:`ValueError` is raised."
#: library/dataclasses.rst:120
"If the class already defines any of :meth:`__lt__`, :meth:`__le__`, :meth:"
"`__gt__`, or :meth:`__ge__`, then :exc:`TypeError` is raised."
#: library/dataclasses.rst:124
"``unsafe_hash``: If ``False`` (the default), a :meth:`__hash__` method is "
"generated according to how ``eq`` and ``frozen`` are set."
#: library/dataclasses.rst:127
":meth:`__hash__` is used by built-in :meth:`hash()`, and when objects are "
"added to hashed collections such as dictionaries and sets.  Having a :meth:"
"`__hash__` implies that instances of the class are immutable. Mutability is "
"a complicated property that depends on the programmer's intent, the "
"existence and behavior of :meth:`__eq__`, and the values of the ``eq`` and "
"``frozen`` flags in the :func:`dataclass` decorator."
#: library/dataclasses.rst:134
"By default, :func:`dataclass` will not implicitly add a :meth:`__hash__` "
"method unless it is safe to do so.  Neither will it add or change an "
"existing explicitly defined :meth:`__hash__` method.  Setting the class "
"attribute ``__hash__ = None`` has a specific meaning to Python, as described "
"in the :meth:`__hash__` documentation."
#: library/dataclasses.rst:140
"If :meth:`__hash__` is not explicitly defined, or if it is set to ``None``, "
"then :func:`dataclass` *may* add an implicit :meth:`__hash__` method. "
"Although not recommended, you can force :func:`dataclass` to create a :meth:"
"`__hash__` method with ``unsafe_hash=True``. This might be the case if your "
"class is logically immutable but can nonetheless be mutated. This is a "
"specialized use case and should be considered carefully."
#: library/dataclasses.rst:147
"Here are the rules governing implicit creation of a :meth:`__hash__` "
"method.  Note that you cannot both have an explicit :meth:`__hash__` method "
"in your dataclass and set ``unsafe_hash=True``; this will result in a :exc:"
"`TypeError`."
#: library/dataclasses.rst:152
"If ``eq`` and ``frozen`` are both true, by default :func:`dataclass` will "
"generate a :meth:`__hash__` method for you.  If ``eq`` is true and "
"``frozen`` is false, :meth:`__hash__` will be set to ``None``, marking it "
"unhashable (which it is, since it is mutable).  If ``eq`` is false, :meth:"
"`__hash__` will be left untouched meaning the :meth:`__hash__` method of the "
"superclass will be used (if the superclass is :class:`object`, this means it "
"will fall back to id-based hashing)."
#: library/dataclasses.rst:160
"``frozen``: If true (the default is ``False``), assigning to fields will "
"generate an exception.  This emulates read-only frozen instances.  If :meth:"
"`__setattr__` or :meth:`__delattr__` is defined in the class, then :exc:"
"`TypeError` is raised.  See the discussion below."
#: library/dataclasses.rst:165
"``match_args``: If true (the default is ``True``), the ``__match_args__`` "
"tuple will be created from the list of parameters to the generated :meth:"
"`__init__` method (even if :meth:`__init__` is not generated, see above).  "
"If false, or if ``__match_args__`` is already defined in the class, then "
"``__match_args__`` will not be generated."
#: library/dataclasses.rst:174
"``kw_only``: If true (the default value is ``False``), then all fields will "
"be marked as keyword-only.  If a field is marked as keyword-only, then the "
"only effect 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 :const:`KW_ONLY` "
#: library/dataclasses.rst:185
"``slots``: If true (the default is ``False``), :attr:`__slots__` attribute "
"will be generated and new class will be returned instead of the original "
"one. If :attr:`__slots__` is already defined in the class, then :exc:"
"`TypeError` is raised."
#: library/dataclasses.rst:192
"If a field name is already included in the ``__slots__`` of a base class, it "
"will not be included in the generated ``__slots__`` to prevent :ref:"
"`overriding them <datamodel-note-slots>`. Therefore, do not use "
"``__slots__`` to retrieve the field names of a dataclass. Use :func:`fields` "
"instead. To be able to determine inherited slots, base class ``__slots__`` "
"may be any iterable, but *not* an iterator."
#: library/dataclasses.rst:202
"``weakref_slot``: If true (the default is ``False``), add a slot named "
"\"__weakref__\", which is required to make an instance weakref-able.  It is "
"an error to specify ``weakref_slot=True`` without also specifying "
"``slots=True``."
#: library/dataclasses.rst:209
"``field``\\s may optionally specify a default value, using normal Python "
#: library/dataclasses.rst:217
"In this example, both ``a`` and ``b`` will be included in the added :meth:"
"`__init__` method, which will be defined as::"
#: library/dataclasses.rst:222
":exc:`TypeError` will be raised if a field without a default value follows a "
"field with a default value.  This is true whether this occurs in a single "
"class, or as a result of class inheritance."
#: library/dataclasses.rst:228
"For common and simple use cases, no other functionality is required.  There "
"are, however, some dataclass features that require additional per-field "
"information.  To satisfy this need for additional information, you can "
"replace the default field value with a call to the provided :func:`field` "
"function.  For example::"
#: library/dataclasses.rst:241
"As shown above, the :const:`MISSING` value is a sentinel object used to "
"detect if some parameters are provided by the user. This sentinel is used "
"because ``None`` is a valid value for some parameters with a distinct "
"meaning.  No code should directly use the :const:`MISSING` value."
#: library/dataclasses.rst:246
msgid "The parameters to :func:`field` are:"
#: library/dataclasses.rst:248
"``default``: If provided, this will be the default value for this field.  "
"This is needed because the :meth:`field` call itself replaces the normal "
"position of the default value."
#: library/dataclasses.rst:252
"``default_factory``: If provided, it must be a zero-argument callable that "
"will be called when a default value is needed for this field.  Among other "
"purposes, this can be used to specify fields with mutable default values, as "
"discussed below.  It is an error to specify both ``default`` and "
"``default_factory``."
#: library/dataclasses.rst:258
"``init``: If true (the default), this field is included as a parameter to "
"the generated :meth:`__init__` method."
#: library/dataclasses.rst:261
"``repr``: If true (the default), this field is included in the string "
"returned by the generated :meth:`__repr__` method."
#: library/dataclasses.rst:264
"``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 the expected behavior.  A "
"field should be considered in the hash if it's used for comparisons.  "
"Setting this value to anything other than ``None`` is discouraged."
#: library/dataclasses.rst:271
"One possible reason to set ``hash=False`` but ``compare=True`` would be if a "
"field is expensive to compute a hash value for, that field is needed for "
"equality testing, and there are other 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."
#: library/dataclasses.rst:277
"``compare``: If true (the default), this field is included in the generated "
"equality and comparison methods (:meth:`__eq__`, :meth:`__gt__`, et al.)."
#: library/dataclasses.rst:281
"``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 on the :class:`Field` object. It is not used at all "
"by Data Classes, and is provided as a third-party extension mechanism. "
"Multiple third-parties can each have their own key, to use as a namespace in "
"the metadata."
#: library/dataclasses.rst:289
"``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."
#: library/dataclasses.rst:295
"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 provided, then the class attribute "
"will be deleted.  The intent is that after the :func:`dataclass` decorator "
"runs, the class attributes will all contain the default values for the "
"fields, just as if the default value itself were specified.  For example, "
#: library/dataclasses.rst:311
"The class attribute ``C.z`` will be ``10``, the class attribute ``C.t`` will "
"be ``20``, and the class attributes ``C.x`` and ``C.y`` will not be set."
#: library/dataclasses.rst:317
":class:`Field` objects describe each defined field. These objects are "
"created internally, and are returned by the :func:`fields` module-level "
"method (see below).  Users should never instantiate a :class:`Field` object "
"directly.  Its documented attributes are:"
#: library/dataclasses.rst:322
msgid "``name``: The name of the field."
#: library/dataclasses.rst:324
msgid "``type``: The type of the field."
#: library/dataclasses.rst:326
"``default``, ``default_factory``, ``init``, ``repr``, ``hash``, ``compare``, "
"``metadata``, and ``kw_only`` have the identical meaning and values as they "
"do in the :func:`field` function."
#: library/dataclasses.rst:330
"Other attributes may exist, but they are private and must not be inspected "
"or relied on."
#: library/dataclasses.rst:335
"Returns a tuple of :class:`Field` objects that define the fields for this "
"dataclass.  Accepts either a dataclass, or an instance of a dataclass. "
"Raises :exc:`TypeError` if not passed a dataclass or instance of one. Does "
"not return pseudo-fields which are ``ClassVar`` or ``InitVar``."
#: library/dataclasses.rst:342
"Converts the dataclass ``obj`` to a dict (by using the factory function "
"``dict_factory``).  Each dataclass is converted to a dict of its fields, as "
"``name: value`` pairs.  dataclasses, dicts, lists, and tuples are recursed "
"into.  Other objects are copied with :func:`copy.deepcopy`."
#: library/dataclasses.rst:348
msgid "Example of using :func:`asdict` on nested dataclasses::"
#: library/dataclasses.rst:385
msgid "To create a shallow copy, the following workaround may be used::"
#: library/dataclasses.rst:369
":func:`asdict` raises :exc:`TypeError` if ``obj`` is not a dataclass "
"instance."
#: library/dataclasses.rst:374
"Converts the dataclass ``obj`` to a tuple (by using the factory function "
"``tuple_factory``).  Each dataclass is converted to a tuple of its field "
"values.  dataclasses, dicts, lists, and tuples are recursed into. Other "
"objects are copied with :func:`copy.deepcopy`."
#: library/dataclasses.rst:380
msgid "Continuing from the previous example::"
#: library/dataclasses.rst:389
":func:`astuple` raises :exc:`TypeError` if ``obj`` is not a dataclass "
"instance."
#: library/dataclasses.rst:394
"Creates a new dataclass with name ``cls_name``, fields as defined in "
"``fields``, base classes as given in ``bases``, and initialized with a "
"namespace as given in ``namespace``.  ``fields`` is an iterable whose "
"elements are each either ``name``, ``(name, type)``, or ``(name, type, "
"Field)``.  If just ``name`` is supplied, ``typing.Any`` is used for "
"``type``.  The values of ``init``, ``repr``, ``eq``, ``order``, "
"``unsafe_hash``, ``frozen``, ``match_args``, ``kw_only``, ``slots``, and "
"``weakref_slot`` have the same meaning as they do in :func:`dataclass`."
#: library/dataclasses.rst:404
"This function is not strictly required, because any Python mechanism for "
"creating a new class with ``__annotations__`` can then apply the :func:"
"`dataclass` function to convert that class to a dataclass.  This function is "
"provided as a convenience.  For example::"
#: library/dataclasses.rst:416
msgid "Is equivalent to::"
#: library/dataclasses.rst:429
"Creates a new object of the same type as ``obj``, replacing fields with "
"values from ``changes``.  If ``obj`` is not a Data Class, raises :exc:"
"`TypeError`.  If values in ``changes`` do not specify fields, raises :exc:"
"`TypeError`."
#: library/dataclasses.rst:434
"The newly returned object is created by calling the :meth:`__init__` method "
"of the dataclass.  This ensures that :meth:`__post_init__`, if present, is "
"also called."
#: library/dataclasses.rst:438
"Init-only variables without default values, if any exist, must be specified "
"on the call to :func:`replace` so that they can be passed to :meth:"
"`__init__` and :meth:`__post_init__`."
#: library/dataclasses.rst:442
"It is an error for ``changes`` to contain any fields that are defined as "
"having ``init=False``.  A :exc:`ValueError` will be raised in this case."
#: library/dataclasses.rst:446
"Be forewarned about how ``init=False`` fields work during a call to :func:"
"`replace`.  They are not copied from the source object, but rather are "
"initialized in :meth:`__post_init__`, if they're initialized at all.  It is "
"expected that ``init=False`` fields will be rarely and judiciously used.  If "
"they are used, it might be wise to have alternate class constructors, or "
"perhaps a custom ``replace()`` (or similarly named) method which handles "
"instance copying."
#: library/dataclasses.rst:457
"Return ``True`` if its parameter is a dataclass or an instance of one, "
"otherwise return ``False``."
#: library/dataclasses.rst:460
"If you need to know if a class is an instance of a dataclass (and not a "
"dataclass itself), then add a further check for ``not isinstance(obj, "
"type)``::"
#: library/dataclasses.rst:469
msgid "A sentinel value signifying a missing default or default_factory."
#: library/dataclasses.rst:473
"A sentinel value used as a type annotation.  Any fields after a pseudo-field "
"with the type of :const:`KW_ONLY` are marked as keyword-only fields.  Note "
"that a pseudo-field of type :const:`KW_ONLY` is otherwise completely "
"ignored.  This includes the name of such a field.  By convention, a name of "
"``_`` is used for a :const:`KW_ONLY` field.  Keyword-only fields signify :"
"meth:`__init__` parameters that must be specified as keywords when the class "
"is instantiated."
#: library/dataclasses.rst:482
"In this example, the fields ``y`` and ``z`` will be marked as keyword-only "
#: library/dataclasses.rst:493
"In a single dataclass, it is an error to specify more than one field whose "
"type is :const:`KW_ONLY`."
#: library/dataclasses.rst:500
"Raised when an implicitly defined :meth:`__setattr__` or :meth:`__delattr__` "
"is called on a dataclass which was defined with ``frozen=True``. It is a "
"subclass of :exc:`AttributeError`."
#: library/dataclasses.rst:505
msgid "Post-init processing"
#: library/dataclasses.rst:507
"The generated :meth:`__init__` code will call a method named :meth:"
"`__post_init__`, if :meth:`__post_init__` is defined on the class.  It will "
"normally be called as ``self.__post_init__()``. However, if any ``InitVar`` "
"fields are defined, they will also be passed to :meth:`__post_init__` in the "
"order they were defined in the class.  If no :meth:`__init__` method is "
"generated, then :meth:`__post_init__` will not automatically be called."
#: library/dataclasses.rst:515
"Among other uses, this allows for initializing field values that depend on "
"one or more other fields.  For example::"
#: library/dataclasses.rst:527
"The :meth:`__init__` method generated by :func:`dataclass` does not call "
"base class :meth:`__init__` methods. If the base class has an :meth:"
"`__init__` method that has to be called, it is common to call this method in "
"a :meth:`__post_init__` method::"
#: library/dataclasses.rst:544
"Note, however, that in general the dataclass-generated :meth:`__init__` "
"methods don't need to be called, since the derived dataclass will take care "
"of initializing all fields of any base class that is a dataclass itself."
#: library/dataclasses.rst:548
"See the section below on init-only variables for ways to pass parameters to :"
"meth:`__post_init__`.  Also see the warning about how :func:`replace` "
"handles ``init=False`` fields."
#: library/dataclasses.rst:553
msgid "Class variables"
#: library/dataclasses.rst:555
"One of two places where :func:`dataclass` actually inspects the type of a "
"field is to determine if a field is a class variable as defined in :pep:"
"`526`.  It does this by checking if the type of the field is ``typing."
"ClassVar``.  If a field is a ``ClassVar``, it is excluded from consideration "
"as a field and is ignored by the dataclass mechanisms.  Such ``ClassVar`` "
"pseudo-fields are not returned by the module-level :func:`fields` function."
#: library/dataclasses.rst:564
msgid "Init-only variables"
#: library/dataclasses.rst:566
"The other place where :func:`dataclass` inspects a type annotation is to "
"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 is an "
"``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 module-level :func:"
"`fields` function.  Init-only fields are added as parameters to the "
"generated :meth:`__init__` method, and are passed to the optional :meth:"
"`__post_init__` method.  They are not otherwise used by dataclasses."
#: library/dataclasses.rst:576
"For example, suppose a field will be initialized from a database, if a value "
"is not provided when creating the class::"
#: library/dataclasses.rst:591
"In this case, :func:`fields` will return :class:`Field` objects for ``i`` "
"and ``j``, but not for ``database``."
#: library/dataclasses.rst:595
msgid "Frozen instances"
#: library/dataclasses.rst:597
"It is not possible to create truly immutable Python objects.  However, by "
"passing ``frozen=True`` to the :meth:`dataclass` decorator you can emulate "
"immutability.  In that case, dataclasses will add :meth:`__setattr__` and :"
"meth:`__delattr__` methods to the class.  These methods will raise a :exc:"
"`FrozenInstanceError` when invoked."
#: library/dataclasses.rst:603
"There is a tiny performance penalty when using ``frozen=True``: :meth:"
"`__init__` cannot use simple assignment to initialize fields, and must use :"
"meth:`object.__setattr__`."
#: library/dataclasses.rst:608
msgid "Inheritance"
#: library/dataclasses.rst:610
"When the dataclass is being created by the :meth:`dataclass` decorator, it "
"looks through all of the class's base classes in reverse MRO (that is, "
"starting at :class:`object`) and, for each dataclass that it finds, adds the "
"fields from that base class to an ordered mapping of fields. After all of "
"the base class fields are added, it adds its own fields to the ordered "
"mapping.  All of the generated methods will use this combined, calculated "
"ordered mapping of fields.  Because the fields are in insertion order, "
"derived classes override base classes.  An example::"
#: library/dataclasses.rst:630
"The final list of fields is, in order, ``x``, ``y``, ``z``.  The final type "
"of ``x`` is ``int``, as specified in class ``C``."
#: library/dataclasses.rst:633
msgid "The generated :meth:`__init__` method for ``C`` will look like::"
#: library/dataclasses.rst:638
msgid "Re-ordering of keyword-only parameters in :meth:`__init__`"
#: library/dataclasses.rst:640
"After the parameters needed for :meth:`__init__` are computed, any keyword-"
"only parameters are moved to come after all regular (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."
#: library/dataclasses.rst:646
"In this example, ``Base.y``, ``Base.w``, and ``D.t`` are keyword-only "
"fields, and ``Base.x`` and ``D.z`` are regular fields::"
#: library/dataclasses.rst:661
msgid "The generated :meth:`__init__` method for ``D`` will look like::"
#: library/dataclasses.rst:665
"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."
#: library/dataclasses.rst:669
"The relative ordering of keyword-only parameters is maintained in the re-"
"ordered :meth:`__init__` parameter list."
#: library/dataclasses.rst:674
msgid "Default factory functions"
#: library/dataclasses.rst:676
"If a :func:`field` specifies a ``default_factory``, it is called with zero "
"arguments when a default value for the field is needed.  For example, to "
"create a new instance of a list, use::"
#: library/dataclasses.rst:682
"If a field is excluded from :meth:`__init__` (using ``init=False``) and the "
"field also specifies ``default_factory``, then the default factory function "
"will always be called from the generated :meth:`__init__` function.  This "
"happens because there is no other way to give the field an initial value."
#: library/dataclasses.rst:689
msgid "Mutable default values"
#: library/dataclasses.rst:691
"Python stores default member variable values in class attributes. Consider "
"this example, not using dataclasses::"
#: library/dataclasses.rst:706
"Note that the two instances of class ``C`` share the same class variable "
"``x``, as expected."
#: library/dataclasses.rst:709
msgid "Using dataclasses, *if* this code was valid::"
#: library/dataclasses.rst:717
msgid "it would generate code similar to::"
#: library/dataclasses.rst:728
"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, the :func:`dataclass` decorator will raise a :exc:"
"`TypeError` if it detects an unhashable default parameter.  The assumption "
"is that if a value is unhashable, it is mutable.  This is a partial "
"solution, but it does protect against many common errors."
#: library/dataclasses.rst:739
"Using default factory functions is a way to create new instances of mutable "
"types as default values for fields::"
#: library/dataclasses.rst:748
"Instead of looking for and disallowing objects of type ``list``, ``dict``, "
"or ``set``, unhashable objects are now not allowed as default values.  "
"Unhashability is used to approximate mutability."
#: library/dataclasses.rst:755
msgid "Descriptor-typed fields"
#: library/dataclasses.rst:757
"Fields that are assigned :ref:`descriptor objects <descriptors>` as their "
"default value have the following special behaviors:"
#: library/dataclasses.rst:760
"The value for the field passed to the dataclass's ``__init__`` method is "
"passed to the descriptor's ``__set__`` method rather than overwriting the "
"descriptor object."
#: library/dataclasses.rst:763
"Similarly, when getting or setting the field, the descriptor's ``__get__`` "
"or ``__set__`` method is called rather than returning or overwriting the "
"descriptor object."
#: library/dataclasses.rst:766
"To determine whether a field contains a default value, ``dataclasses`` will "
"call the descriptor's ``__get__`` method using its class access form (i.e. "
"``descriptor.__get__(obj=None, type=cls)``.  If the descriptor returns a "
"value in this case, it will be used as the field's default. On the other "
"hand, if the descriptor raises :exc:`AttributeError` in this situation, no "
"default value will be provided for the field."
#: library/dataclasses.rst:801
"Note that if a field is annotated with a descriptor type, but is not "
"assigned a descriptor object as its default value, the field will act like a "
"normal field."
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