diff --git a/Doc/library/dataclasses.rst b/Doc/library/dataclasses.rst index c612c138fc6..61b2263339d 100644 --- a/Doc/library/dataclasses.rst +++ b/Doc/library/dataclasses.rst @@ -1,5 +1,5 @@ -:mod:`dataclasses` --- Data Classes -=================================== +:mod:`!dataclasses` --- Data Classes +==================================== .. module:: dataclasses :synopsis: Generate special methods on user-defined classes. @@ -31,7 +31,7 @@ using :pep:`526` type annotations. For example, this code:: def total_cost(self) -> float: return self.unit_price * self.quantity_on_hand -will add, among other things, a :meth:`~object.__init__` that looks like:: +will add, among other things, a :meth:`!__init__` that looks like:: def __init__(self, name: str, unit_price: float, quantity_on_hand: int = 0): self.name = name @@ -49,26 +49,26 @@ Module contents .. decorator:: dataclass(*, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False, match_args=True, kw_only=False, slots=False, weakref_slot=False) This function is a :term:`decorator` that is used to add generated - :term:`special method`\s to classes, as described below. + :term:`special methods ` to classes, as described below. - The :func:`dataclass` decorator examines the class to find + The ``@dataclass`` decorator examines the class to find ``field``\s. A ``field`` is defined as a class variable that has a :term:`type annotation `. With two - exceptions described below, nothing in :func:`dataclass` + exceptions described below, nothing in ``@dataclass`` examines the type specified in the variable annotation. The order of the fields in all of the generated methods is the order in which they appear in the class definition. - The :func:`dataclass` decorator will add various "dunder" methods to + The ``@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 created. - If :func:`dataclass` is used just as a simple decorator with no parameters, + If ``@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 + signature. That is, these three uses of ``@dataclass`` are equivalent:: @dataclass @@ -84,12 +84,12 @@ Module contents class C: ... - The parameters to :func:`dataclass` are: + The parameters to ``@dataclass`` are: - ``init``: If true (the default), a :meth:`~object.__init__` method will be generated. - If the class already defines :meth:`~object.__init__`, this parameter is + If the class already defines :meth:`!__init__`, this parameter is ignored. - ``repr``: If true (the default), a :meth:`~object.__repr__` method will be @@ -99,7 +99,7 @@ Module contents are not included. For example: ``InventoryItem(name='widget', unit_price=3.0, quantity_on_hand=10)``. - If the class already defines :meth:`~object.__repr__`, this parameter is + If the class already defines :meth:`!__repr__`, this parameter is ignored. - ``eq``: If true (the default), an :meth:`~object.__eq__` method will be @@ -107,7 +107,7 @@ Module contents of its fields, in order. Both instances in the comparison must be of the identical type. - If the class already defines :meth:`~object.__eq__`, this parameter is + If the class already defines :meth:`!__eq__`, this parameter is ignored. - ``order``: If true (the default is ``False``), :meth:`~object.__lt__`, @@ -117,43 +117,43 @@ Module contents identical type. If ``order`` is true and ``eq`` is false, a :exc:`ValueError` is raised. - If the class already defines any of :meth:`~object.__lt__`, - :meth:`~object.__le__`, :meth:`~object.__gt__`, or :meth:`~object.__ge__`, then + If the class already defines any of :meth:`!__lt__`, + :meth:`!__le__`, :meth:`!__gt__`, or :meth:`!__ge__`, then :exc:`TypeError` is raised. - ``unsafe_hash``: If ``False`` (the default), a :meth:`~object.__hash__` method is generated according to how ``eq`` and ``frozen`` are set. - :meth:`~object.__hash__` is used by built-in :meth:`hash()`, and when objects are + :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:`~object.__hash__` implies that instances of the class are immutable. + :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:`~object.__eq__`, and the values of - the ``eq`` and ``frozen`` flags in the :func:`dataclass` decorator. + intent, the existence and behavior of :meth:`!__eq__`, and the values of + the ``eq`` and ``frozen`` flags in the ``@dataclass`` decorator. - By default, :func:`dataclass` will not implicitly add a :meth:`~object.__hash__` + By default, ``@dataclass`` will not implicitly add a :meth:`~object.__hash__` method unless it is safe to do so. Neither will it add or change an - existing explicitly defined :meth:`~object.__hash__` method. Setting the class + existing explicitly defined :meth:`!__hash__` method. Setting the class attribute ``__hash__ = None`` has a specific meaning to Python, as - described in the :meth:`~object.__hash__` documentation. + described in the :meth:`!__hash__` documentation. - If :meth:`~object.__hash__` is not explicitly defined, or if it is set to ``None``, - then :func:`dataclass` *may* add an implicit :meth:`~object.__hash__` method. - Although not recommended, you can force :func:`dataclass` to create a - :meth:`~object.__hash__` method with ``unsafe_hash=True``. This might be the case + If :meth:`!__hash__` is not explicitly defined, or if it is set to ``None``, + then ``@dataclass`` *may* add an implicit :meth:`!__hash__` method. + Although not recommended, you can force ``@dataclass`` to create a + :meth:`!__hash__` method with ``unsafe_hash=True``. This might be the case if your class is logically immutable but can still be mutated. This is a specialized use case and should be considered carefully. - Here are the rules governing implicit creation of a :meth:`~object.__hash__` - method. Note that you cannot both have an explicit :meth:`~object.__hash__` + 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`. - If ``eq`` and ``frozen`` are both true, by default :func:`dataclass` will - generate a :meth:`~object.__hash__` method for you. If ``eq`` is true and - ``frozen`` is false, :meth:`~object.__hash__` will be set to ``None``, marking it + If ``eq`` and ``frozen`` are both true, by default ``@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:`~object.__hash__` will be left untouched meaning the :meth:`~object.__hash__` + :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). @@ -165,7 +165,7 @@ Module contents - ``match_args``: If true (the default is ``True``), the ``__match_args__`` tuple will be created from the list of parameters to the generated :meth:`~object.__init__` method (even if - :meth:`~object.__init__` is not generated, see above). If false, or 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. @@ -175,7 +175,7 @@ Module contents fields will be marked as keyword-only. If a field is marked as keyword-only, then the only effect is that the :meth:`~object.__init__` parameter generated from a keyword-only field must be specified - with a keyword when :meth:`~object.__init__` is called. There is no + 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` section. @@ -184,7 +184,7 @@ Module contents - ``slots``: If true (the default is ``False``), :attr:`~object.__slots__` attribute will be generated and new class will be returned instead of the original one. - If :attr:`~object.__slots__` is already defined in the class, then :exc:`TypeError` + If :attr:`!__slots__` is already defined in the class, then :exc:`TypeError` is raised. .. versionadded:: 3.10 @@ -229,7 +229,7 @@ Module contents 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:: + with a call to the provided :func:`!field` function. For example:: @dataclass class C: @@ -243,10 +243,10 @@ Module contents used because ``None`` is a valid value for some parameters with a distinct meaning. No code should directly use the :const:`MISSING` value. - The parameters to :func:`field` are: + The parameters to :func:`!field` are: - ``default``: If provided, this will be the default value for this - field. This is needed because the :meth:`field` call itself + field. This is needed because the :func:`!field` call itself replaces the normal position of the default value. - ``default_factory``: If provided, it must be a zero-argument @@ -293,10 +293,10 @@ Module contents .. versionadded:: 3.10 If the default value of a field is specified by a call to - :func:`field()`, then the class attribute for this field will be + :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 + 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, after:: @@ -314,10 +314,10 @@ Module contents .. class:: Field - :class:`Field` objects describe each defined field. These objects + :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: + :class:`!Field` object directly. Its documented attributes are: - ``name``: The name of the field. - ``type``: The type of the field. @@ -343,7 +343,7 @@ Module contents lists, and tuples are recursed into. Other objects are copied with :func:`copy.deepcopy`. - Example of using :func:`asdict` on nested dataclasses:: + Example of using :func:`!asdict` on nested dataclasses:: @dataclass class Point: @@ -364,7 +364,7 @@ Module contents dict((field.name, getattr(obj, field.name)) for field in fields(obj)) - :func:`asdict` raises :exc:`TypeError` if ``obj`` is not a dataclass + :func:`!asdict` raises :exc:`TypeError` if ``obj`` is not a dataclass instance. .. function:: astuple(obj, *, tuple_factory=tuple) @@ -384,7 +384,7 @@ Module contents tuple(getattr(obj, field.name) for field in dataclasses.fields(obj)) - :func:`astuple` raises :exc:`TypeError` if ``obj`` is not a dataclass + :func:`!astuple` raises :exc:`TypeError` if ``obj`` is not a dataclass instance. .. function:: make_dataclass(cls_name, fields, *, bases=(), namespace=None, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False, match_args=True, kw_only=False, slots=False, weakref_slot=False, module=None) @@ -397,7 +397,7 @@ Module contents ``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`. + the same meaning as they do in :func:`@dataclass `. If ``module`` is defined, the ``__module__`` attribute of the dataclass is set to that value. @@ -405,7 +405,7 @@ Module contents 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 + then apply the ``@dataclass`` function to convert that class to a dataclass. This function is provided as a convenience. For example:: @@ -438,15 +438,15 @@ Module contents :meth:`__post_init__`, if present, is also called. 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:`~object.__init__` and :meth:`__post_init__`. + specified on the call to :func:`!replace` so that they can be passed to + :meth:`!__init__` and :meth:`__post_init__`. 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. Be forewarned about how ``init=False`` fields work during a call to - :func:`replace`. They are not copied from the source object, but + :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 @@ -475,11 +475,11 @@ Module contents .. data:: KW_ONLY A sentinel value used as a type annotation. Any fields after a - pseudo-field with the type of :const:`KW_ONLY` are marked as + 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 + :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 + :const:`!KW_ONLY` field. Keyword-only fields signify :meth:`~object.__init__` parameters that must be specified as keywords when the class is instantiated. @@ -495,7 +495,7 @@ 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`. + field whose type is :const:`!KW_ONLY`. .. versionadded:: 3.10 @@ -515,9 +515,9 @@ Post-init processing When defined on the class, it will be called by the generated :meth:`~object.__init__`, normally 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:`~object.__init__` method is generated, then - :meth:`__post_init__` will not automatically be called. + 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. Among other uses, this allows for initializing field values that depend on one or more other fields. For example:: @@ -531,8 +531,8 @@ Post-init processing def __post_init__(self): self.c = self.a + self.b -The :meth:`~object.__init__` method generated by :func:`dataclass` does not call base -class :meth:`~object.__init__` methods. If the base class has an :meth:`~object.__init__` method +The :meth:`~object.__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:: @@ -548,18 +548,18 @@ that has to be called, it is common to call this method in a def __post_init__(self): super().__init__(self.side, self.side) -Note, however, that in general the dataclass-generated :meth:`~object.__init__` methods +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. See the section below on init-only variables for ways to pass -parameters to :meth:`__post_init__`. Also see the warning about how +parameters to :meth:`!__post_init__`. Also see the warning about how :func:`replace` handles ``init=False`` fields. Class variables --------------- -One of the few places where :func:`dataclass` actually inspects the type +One of the few 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 @@ -570,7 +570,7 @@ module-level :func:`fields` function. Init-only variables ------------------- -Another place where :func:`dataclass` inspects a type annotation is to +Another 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 @@ -602,19 +602,19 @@ Frozen instances ---------------- It is not possible to create truly immutable Python objects. However, -by passing ``frozen=True`` to the :meth:`dataclass` decorator you can +by passing ``frozen=True`` to the :func:`@dataclass ` decorator you can emulate immutability. In that case, dataclasses will add :meth:`~object.__setattr__` and :meth:`~object.__delattr__` methods to the class. These methods will raise a :exc:`FrozenInstanceError` when invoked. There is a tiny performance penalty when using ``frozen=True``: :meth:`~object.__init__` cannot use simple assignment to initialize fields, and -must use :meth:`!object.__setattr__`. +must use :meth:`!__setattr__`. Inheritance ----------- -When the dataclass is being created by the :meth:`dataclass` decorator, +When the dataclass is being created by the :func:`@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. @@ -641,8 +641,8 @@ The generated :meth:`~object.__init__` method for ``C`` will look like:: def __init__(self, x: int = 15, y: int = 0, z: int = 10): -Re-ordering of keyword-only parameters in :meth:`~object.__init__` ------------------------------------------------------------------- +Re-ordering of keyword-only parameters in :meth:`!__init__` +----------------------------------------------------------- After the parameters needed for :meth:`~object.__init__` are computed, any keyword-only parameters are moved to come after all regular @@ -665,7 +665,7 @@ fields, and ``Base.x`` and ``D.z`` are regular fields:: z: int = 10 t: int = field(kw_only=True, default=0) -The generated :meth:`~object.__init__` method for ``D`` will look like:: +The generated :meth:`!__init__` method for ``D`` will look like:: def __init__(self, x: Any = 15.0, z: int = 10, *, y: int = 0, w: int = 1, t: int = 0): @@ -674,7 +674,7 @@ 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 maintained in the -re-ordered :meth:`~object.__init__` parameter list. +re-ordered :meth:`!__init__` parameter list. Default factory functions @@ -689,7 +689,7 @@ example, to create a new instance of a list, use:: If a field is excluded from :meth:`~object.__init__` (using ``init=False``) and the field also specifies ``default_factory``, then the default factory function will always be called from the generated -:meth:`~object.__init__` function. This happens because there is no other +:meth:`!__init__` function. This happens because there is no other way to give the field an initial value. Mutable default values @@ -738,7 +738,7 @@ 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:`ValueError` if it +:func:`@dataclass ` decorator will raise a :exc:`ValueError` 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. @@ -764,15 +764,17 @@ Descriptor-typed fields Fields that are assigned :ref:`descriptor objects ` as their default value have the following special behaviors: -* The value for the field passed to the dataclass's ``__init__`` method is - passed to the descriptor's ``__set__`` method rather than overwriting the +* The value for the field passed to the dataclass's :meth:`~object.__init__` method is + passed to the descriptor's :meth:`~object.__set__` method rather than overwriting the descriptor object. + * Similarly, when getting or setting the field, the descriptor's - ``__get__`` or ``__set__`` method is called rather than returning or + :meth:`~object.__get__` or :meth:`!__set__` method is called rather than returning or overwriting the descriptor object. -* 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 + +* To determine whether a field contains a default value, :func:`@dataclass ` + will call the descriptor's :meth:`!__get__` method using its class access + form: ``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