--------------
This module provides a decorator and functions for automatically
-adding generated :term:`special method`\s such as :meth:`~object.__init__` and
+adding generated :term:`special methods <special method>` such as :meth:`~object.__init__` and
:meth:`~object.__repr__` to user-defined classes. It was originally described
in :pep:`557`.
self.quantity_on_hand = quantity_on_hand
Note that this method is automatically added to the class: it is not
-directly specified in the ``InventoryItem`` definition shown above.
+directly specified in the :class:`!InventoryItem` definition shown above.
.. versionadded:: 3.7
The parameters to ``@dataclass`` are:
- - ``init``: If true (the default), a :meth:`~object.__init__` method will be
+ - *init*: If true (the default), a :meth:`~object.__init__` method will be
generated.
If the class already defines :meth:`!__init__`, this parameter is
ignored.
- - ``repr``: If true (the default), a :meth:`~object.__repr__` method will be
+ - *repr*: If true (the default), a :meth:`~object.__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
If the class already defines :meth:`!__repr__`, this parameter is
ignored.
- - ``eq``: If true (the default), an :meth:`~object.__eq__` method will be
+ - *eq*: If true (the default), an :meth:`~object.__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.
If the class already defines :meth:`!__eq__`, this parameter is
ignored.
- - ``order``: If true (the default is ``False``), :meth:`~object.__lt__`,
+ - *order*: If true (the default is ``False``), :meth:`~object.__lt__`,
:meth:`~object.__le__`, :meth:`~object.__gt__`, and :meth:`~object.__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
+ identical type. If *order* is true and *eq* is false, a
:exc:`ValueError` is raised.
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.
+ - *unsafe_hash*: If ``False`` (the default), a :meth:`~object.__hash__` method
+ is generated according to how *eq* and *frozen* are set.
: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 ``@dataclass`` decorator.
+ the *eq* and *frozen* flags in the ``@dataclass`` decorator.
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
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 ``@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,
+ 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:`!__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).
- - ``frozen``: If true (the default is ``False``), assigning to fields will
+ - *frozen*: If true (the default is ``False``), assigning to fields will
generate an exception. This emulates read-only frozen instances. If
:meth:`~object.__setattr__` or :meth:`~object.__delattr__` is defined in the class, then
:exc:`TypeError` is raised. See the discussion below.
- - ``match_args``: If true (the default is ``True``), the
- ``__match_args__`` tuple will be created from the list of
+ - *match_args*: If true (the default is ``True``), the
+ :attr:`~object.__match_args__` tuple will be created from the list of
parameters to the generated :meth:`~object.__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.
+ :attr:`!__match_args__` is already defined in the class, then
+ :attr:`!__match_args__` will not be generated.
.. versionadded:: 3.10
- - ``kw_only``: If true (the default value is ``False``), then all
+ - *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:`~object.__init__`
parameter generated from a keyword-only field must be specified
.. versionadded:: 3.10
- - ``slots``: If true (the default is ``False``), :attr:`~object.__slots__` attribute
+ - *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:`!__slots__` is already defined in the class, then :exc:`TypeError`
is raised.
.. versionadded:: 3.10
.. versionchanged:: 3.11
- If a field name is already included in the ``__slots__``
- of a base class, it will not be included in the generated ``__slots__``
+ If a field name is already included in the :attr:`!__slots__`
+ of a base class, it will not be included in the generated :attr:`!__slots__`
to prevent :ref:`overriding them <datamodel-note-slots>`.
- Therefore, do not use ``__slots__`` to retrieve the field names of a
+ Therefore, do not use :attr:`!__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.
+ base class :attr:`!__slots__` may be any iterable, but *not* an iterator.
- - ``weakref_slot``: If true (the default is ``False``), add a slot
+ - *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``.
a: int # 'a' has no default value
b: int = 0 # assign a default value for 'b'
- In this example, both ``a`` and ``b`` will be included in the added
+ In this example, both :attr:`!a` and :attr:`!b` will be included in the added
:meth:`~object.__init__` method, which will be defined as::
def __init__(self, a: int, b: int = 0):
The parameters to :func:`!field` are:
- - ``default``: If provided, this will be the default value for this
+ - *default*: If provided, this will be the default value for this
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
+ - *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``.
+ error to specify both *default* and *default_factory*.
- - ``init``: If true (the default), this field is included as a
+ - *init*: If true (the default), this field is included as a
parameter to the generated :meth:`~object.__init__` method.
- - ``repr``: If true (the default), this field is included in the
+ - *repr*: If true (the default), this field is included in the
string returned by the generated :meth:`~object.__repr__` method.
- - ``hash``: This can be a bool or ``None``. If true, this field is
+ - *hash*: This can be a bool or ``None``. If true, this field is
included in the generated :meth:`~object.__hash__` method. If ``None`` (the
- default), use the value of ``compare``: this would normally be
+ 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.
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.
- - ``compare``: If true (the default), this field is included in the
+ - *compare*: If true (the default), this field is included in the
generated equality and comparison methods (:meth:`~object.__eq__`,
:meth:`~object.__gt__`, et al.).
- - ``metadata``: This can be a mapping or None. None is treated as
+ - *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
Multiple third-parties can each have their own key, to use as a
namespace in the metadata.
- - ``kw_only``: If true, this field will be marked as keyword-only.
+ - *kw_only*: If true, this field will be marked as keyword-only.
This is used when the generated :meth:`~object.__init__` method's
parameters are computed.
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
+ replaced by the specified *default* value. If *default* is not
provided, then the class attribute will be deleted. The intent is
that after the :func:`@dataclass <dataclass>` decorator runs, the class
attributes will all contain the default values for the fields, just
z: int = field(repr=False, default=10)
t: int = 20
- 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.
+ The class attribute :attr:`!C.z` will be ``10``, the class attribute
+ :attr:`!C.t` will be ``20``, and the class attributes :attr:`!C.x` and
+ :attr:`!C.y` will not be set.
.. class:: Field
module-level method (see below). Users should never instantiate a
:class:`!Field` object directly. Its documented attributes are:
- - ``name``: The name of the field.
- - ``type``: The type of the field.
- - ``default``, ``default_factory``, ``init``, ``repr``, ``hash``,
- ``compare``, ``metadata``, and ``kw_only`` have the identical
+ - :attr:`!name`: The name of the field.
+ - :attr:`!type`: The type of the field.
+ - :attr:`!default`, :attr:`!default_factory`, :attr:`!init`, :attr:`!repr`, :attr:`!hash`,
+ :attr:`!compare`, :attr:`!metadata`, and :attr:`!kw_only` have the identical
meaning and values as they do in the :func:`field` function.
Other attributes may exist, but they are private and must not be
.. function:: asdict(obj, *, dict_factory=dict)
- Converts the dataclass ``obj`` to a dict (by using the
- factory function ``dict_factory``). Each dataclass is converted
+ 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`.
{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)
- Converts the dataclass ``obj`` to a tuple (by using the
- factory function ``tuple_factory``). Each dataclass is converted
+ 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`.
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)
- 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
+ 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
+ :data:`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 <dataclass>`.
- If ``module`` is defined, the ``__module__`` attribute
+ If *module* is defined, the :attr:`!__module__` attribute
of the dataclass is set to that value.
By default, it is set to the module name of the caller.
This function is not strictly required, because any Python
- mechanism for creating a new class with ``__annotations__`` can
- then apply the ``@dataclass`` function to convert that class to
+ mechanism for creating a new class with :attr:`!__annotations__` can
+ then apply the :func:`@dataclass <dataclass>` function to convert that class to
a dataclass. This function is provided as a convenience. For
example::
.. function:: replace(obj, /, **changes)
- 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 keys in ``changes`` are not
+ 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 keys in *changes* are not
field names of the given dataclass, raises :exc:`TypeError`.
The newly returned object is created by calling the :meth:`~object.__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
+ 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.
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
+ :func:`!replace` (or similarly named) method which handles instance
copying.
Dataclass instances are also supported by generic function :func:`copy.replace`.
.. function:: __post_init__()
When defined on the class, it will be called by the generated
- :meth:`~object.__init__`, normally as ``self.__post_init__()``.
+ :meth:`~object.__init__`, normally as :meth:`!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
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
+:data:`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.
c = C(10, database=my_database)
-In this case, :func:`fields` will return :class:`Field` objects for ``i`` and
-``j``, but not for ``database``.
+In this case, :func:`fields` will return :class:`Field` objects for :attr:`!i` and
+:attr:`!j`, but not for :attr:`!database`.
.. _dataclasses-frozen:
z: int = 10
x: int = 15
-The final list of fields is, in order, ``x``, ``y``, ``z``. The final
-type of ``x`` is ``int``, as specified in class ``C``.
+The final list of fields is, in order, :attr:`!x`, :attr:`!y`, :attr:`!z`. The final
+type of :attr:`!x` is :class:`int`, as specified in class :class:`!C`.
-The generated :meth:`~object.__init__` method for ``C`` will look like::
+The generated :meth:`~object.__init__` method for :class:`!C` will look like::
def __init__(self, x: int = 15, y: int = 0, z: int = 10):
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::
+In this example, :attr:`!Base.y`, :attr:`!Base.w`, and :attr:`!D.t` are keyword-only
+fields, and :attr:`!Base.x` and :attr:`!D.z` are regular fields::
@dataclass
class Base:
z: int = 10
t: int = field(kw_only=True, default=0)
-The generated :meth:`!__init__` method for ``D`` will look like::
+The generated :meth:`!__init__` method for :class:`!D` will look like::
def __init__(self, x: Any = 15.0, z: int = 10, *, y: int = 0, w: int = 1, t: int = 0):
Default factory functions
-------------------------
-If a :func:`field` specifies a ``default_factory``, it is called with
+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::
mylist: list = field(default_factory=list)
If a field is excluded from :meth:`~object.__init__` (using ``init=False``)
-and the field also specifies ``default_factory``, then the default
+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.
assert o1.x == [1, 2]
assert o1.x is o2.x
-Note that the two instances of class ``C`` share the same class
-variable ``x``, as expected.
+Note that the two instances of class :class:`!C` share the same class
+variable :attr:`!x`, as expected.
Using dataclasses, *if* this code was valid::
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
+This has the same issue as the original example using class :class:`!C`.
+That is, two instances of class :class:`!D` that do not specify a value
+for :attr:`!x` when creating a class instance will share the same copy
+of :attr:`!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 <dataclass>` decorator will raise a :exc:`ValueError` if it
assert D().x is not D().x
.. versionchanged:: 3.11
- Instead of looking for and disallowing objects of type ``list``,
- ``dict``, or ``set``, unhashable objects are now not allowed as
+ Instead of looking for and disallowing objects of type :class:`list`,
+ :class:`dict`, or :class:`set`, unhashable objects are now not allowed as
default values. Unhashability is used to approximate
mutability.
projects / thirdparty / Python / cpython.git / commitdiff
