-:mod:`dataclasses` --- Data Classes
-===================================
+:mod:`!dataclasses` --- Data Classes
+====================================
.. module:: dataclasses
:synopsis: Generate special methods on user-defined classes.
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
.. 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 <special method>` 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 <variable 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
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
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
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__`,
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).
- ``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.
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.
- ``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
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:
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
.. 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 <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::
.. 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.
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:
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)
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)
``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 <dataclass>`.
If ``module`` is defined, the ``__module__`` attribute
of the dataclass is set to that value.
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::
: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
.. 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.
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
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::
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 <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::
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 <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
Init-only variables
-------------------
-Another place where :func:`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
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
----------------
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 <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 <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.
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
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):
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
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
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 <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.
Fields that are assigned :ref:`descriptor objects <descriptors>` 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 <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
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