596 lines
23 KiB
ReStructuredText
596 lines
23 KiB
ReStructuredText
:mod:`dataclasses` --- Data Classes
|
|
===================================
|
|
|
|
.. module:: dataclasses
|
|
:synopsis: Generate special methods on user-defined classes.
|
|
|
|
.. moduleauthor:: Eric V. Smith <eric@trueblade.com>
|
|
.. sectionauthor:: Eric V. Smith <eric@trueblade.com>
|
|
|
|
**Source code:** :source:`Lib/dataclasses.py`
|
|
|
|
--------------
|
|
|
|
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:`557`.
|
|
|
|
The member variables to use in these generated methods are defined
|
|
using :pep:`526` type annotations. For example this code::
|
|
|
|
from dataclasses import dataclass
|
|
|
|
@dataclass
|
|
class InventoryItem:
|
|
'''Class for keeping track of an item in inventory.'''
|
|
name: str
|
|
unit_price: float
|
|
quantity_on_hand: int = 0
|
|
|
|
def total_cost(self) -> float:
|
|
return self.unit_price * self.quantity_on_hand
|
|
|
|
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
|
|
self.unit_price = unit_price
|
|
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.
|
|
|
|
.. versionadded:: 3.7
|
|
|
|
Module-level decorators, classes, and functions
|
|
-----------------------------------------------
|
|
|
|
.. decorator:: dataclass(*, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False)
|
|
|
|
This function is a :term:`decorator` that is used to add generated
|
|
:term:`special method`\s to classes, as described below.
|
|
|
|
The :func:`dataclass` decorator examines the class to find
|
|
``field``\s. A ``field`` is defined as 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.
|
|
|
|
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 class, described below. If any of the added methods already
|
|
exist on the class, the behavior depends on the parameter, as documented
|
|
below. The decorator returns the same class that is called on; no new
|
|
class is created.
|
|
|
|
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::
|
|
|
|
@dataclass
|
|
class C:
|
|
...
|
|
|
|
@dataclass()
|
|
class C:
|
|
...
|
|
|
|
@dataclass(init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False)
|
|
class C:
|
|
...
|
|
|
|
The parameters to :func:`dataclass` are:
|
|
|
|
- ``init``: If true (the default), a :meth:`__init__` method will be
|
|
generated.
|
|
|
|
If the class already defines :meth:`__init__`, this parameter is
|
|
ignored.
|
|
|
|
- ``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)``.
|
|
|
|
If the class already defines :meth:`__repr__`, this parameter is
|
|
ignored.
|
|
|
|
- ``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.
|
|
|
|
If the class already defines :meth:`__eq__`, this parameter is
|
|
ignored.
|
|
|
|
- ``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.
|
|
|
|
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:`__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 :func:`dataclass` decorator.
|
|
|
|
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.
|
|
|
|
If :meth:`__hash__` is not explicit 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.
|
|
|
|
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:`__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
|
|
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.
|
|
|
|
``field``\s may optionally specify a default value, using normal
|
|
Python syntax::
|
|
|
|
@dataclass
|
|
class C:
|
|
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
|
|
:meth:`__init__` method, which will be defined as::
|
|
|
|
def __init__(self, a: int, b: int = 0):
|
|
|
|
:exc:`TypeError` will be raised if a field without a default value
|
|
follows a field with a default value. This is true either when this
|
|
occurs in a single class, or as a result of class inheritance.
|
|
|
|
.. function:: field(*, default=MISSING, default_factory=MISSING, repr=True, hash=None, init=True, compare=True, metadata=None)
|
|
|
|
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::
|
|
|
|
@dataclass
|
|
class C:
|
|
mylist: List[int] = field(default_factory=list)
|
|
|
|
c = C()
|
|
c.mylist += [1, 2, 3]
|
|
|
|
As shown above, the ``MISSING`` value is a sentinel object used to
|
|
detect if the ``default`` and ``default_factory`` parameters are
|
|
provided. This sentinel is used because ``None`` is a valid value
|
|
for ``default``. No code should directly use the ``MISSING``
|
|
value.
|
|
|
|
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
|
|
replaces the normal position of the default value.
|
|
|
|
- ``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``.
|
|
|
|
- ``init``: If true (the default), this field is included as a
|
|
parameter to the generated :meth:`__init__` method.
|
|
|
|
- ``repr``: If true (the default), this field is included in the
|
|
string returned by the generated :meth:`__repr__` method.
|
|
|
|
- ``compare``: If true (the default), this field is included in the
|
|
generated equality and comparison methods (:meth:`__eq__`,
|
|
:meth:`__gt__`, et al.).
|
|
|
|
- ``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.
|
|
|
|
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.
|
|
|
|
- ``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.
|
|
|
|
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,
|
|
after::
|
|
|
|
@dataclass
|
|
class C:
|
|
x: int
|
|
y: int = field(repr=False)
|
|
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.
|
|
|
|
.. class:: Field
|
|
|
|
: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:
|
|
|
|
- ``name``: The name of the field.
|
|
|
|
- ``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.
|
|
|
|
Other attributes may exist, but they are private and must not be
|
|
inspected or relied on.
|
|
|
|
.. function:: fields(class_or_instance)
|
|
|
|
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``.
|
|
|
|
.. function:: asdict(instance, *, dict_factory=dict)
|
|
|
|
Converts the dataclass ``instance`` 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. For example::
|
|
|
|
@dataclass
|
|
class Point:
|
|
x: int
|
|
y: int
|
|
|
|
@dataclass
|
|
class C:
|
|
mylist: List[Point]
|
|
|
|
p = Point(10, 20)
|
|
assert asdict(p) == {'x': 10, 'y': 20}
|
|
|
|
c = C([Point(0, 0), Point(10, 4)])
|
|
assert asdict(c) == {'mylist': [{'x': 0, 'y': 0}, {'x': 10, 'y': 4}]}
|
|
|
|
Raises :exc:`TypeError` if ``instance`` is not a dataclass instance.
|
|
|
|
.. function:: astuple(instance, *, tuple_factory=tuple)
|
|
|
|
Converts the dataclass ``instance`` 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.
|
|
|
|
Continuing from the previous example::
|
|
|
|
assert astuple(p) == (10, 20)
|
|
assert astuple(c) == ([(0, 0), (10, 4)],)
|
|
|
|
Raises :exc:`TypeError` if ``instance`` 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)
|
|
|
|
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``, and ``frozen`` have
|
|
the same meaning as they do in :func:`dataclass`.
|
|
|
|
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::
|
|
|
|
C = make_dataclass('C',
|
|
[('x', int),
|
|
'y',
|
|
('z', int, field(default=5))],
|
|
namespace={'add_one': lambda self: self.x + 1})
|
|
|
|
Is equivalent to::
|
|
|
|
@dataclass
|
|
class C:
|
|
x: int
|
|
y: 'typing.Any'
|
|
z: int = 5
|
|
|
|
def add_one(self):
|
|
return self.x + 1
|
|
|
|
.. function:: replace(instance, **changes)
|
|
|
|
Creates a new object of the same type of ``instance``, replacing
|
|
fields with values from ``changes``. If ``instance`` is not a Data
|
|
Class, raises :exc:`TypeError`. If values in ``changes`` do not
|
|
specify fields, raises :exc:`TypeError`.
|
|
|
|
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.
|
|
|
|
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__`.
|
|
|
|
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
|
|
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.
|
|
|
|
.. function:: is_dataclass(class_or_instance)
|
|
|
|
Return ``True`` if its parameter is a dataclass or an instance of one,
|
|
otherwise return ``False``.
|
|
|
|
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)``::
|
|
|
|
def is_dataclass_instance(obj):
|
|
return is_dataclass(obj) and not isinstance(obj, type)
|
|
|
|
Post-init processing
|
|
--------------------
|
|
|
|
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.
|
|
|
|
Among other uses, this allows for initializing field values that
|
|
depend on one or more other fields. For example::
|
|
|
|
@dataclass
|
|
class C:
|
|
a: float
|
|
b: float
|
|
c: float = field(init=False)
|
|
|
|
def __post_init__(self):
|
|
self.c = self.a + self.b
|
|
|
|
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.
|
|
|
|
Class variables
|
|
---------------
|
|
|
|
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.
|
|
|
|
Init-only variables
|
|
-------------------
|
|
|
|
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.
|
|
|
|
For example, suppose a field will be initialized from a database, if a
|
|
value is not provided when creating the class::
|
|
|
|
@dataclass
|
|
class C:
|
|
i: int
|
|
j: int = None
|
|
database: InitVar[DatabaseType] = None
|
|
|
|
def __post_init__(self, database):
|
|
if self.j is None and database is not None:
|
|
self.j = database.lookup('j')
|
|
|
|
c = C(10, database=my_database)
|
|
|
|
In this case, :func:`fields` will return :class:`Field` objects for ``i`` and
|
|
``j``, but not for ``database``.
|
|
|
|
Frozen instances
|
|
----------------
|
|
|
|
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.
|
|
|
|
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__`.
|
|
|
|
Inheritance
|
|
-----------
|
|
|
|
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::
|
|
|
|
@dataclass
|
|
class Base:
|
|
x: Any = 15.0
|
|
y: int = 0
|
|
|
|
@dataclass
|
|
class C(Base):
|
|
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 generated :meth:`__init__` method for ``C`` will look like::
|
|
|
|
def __init__(self, x: int = 15, y: int = 0, z: int = 10):
|
|
|
|
Default factory functions
|
|
-------------------------
|
|
|
|
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:`__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.
|
|
|
|
Mutable default values
|
|
----------------------
|
|
|
|
Python stores default member variable values in class attributes.
|
|
Consider this example, not using dataclasses::
|
|
|
|
class C:
|
|
x = []
|
|
def add(self, element):
|
|
self.x.append(element)
|
|
|
|
o1 = C()
|
|
o2 = C()
|
|
o1.add(1)
|
|
o2.add(2)
|
|
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.
|
|
|
|
Using dataclasses, *if* this code was valid::
|
|
|
|
@dataclass
|
|
class D:
|
|
x: List = []
|
|
def add(self, element):
|
|
self.x += element
|
|
|
|
it would generate code similar to::
|
|
|
|
class D:
|
|
x = []
|
|
def __init__(self, x=x):
|
|
self.x = x
|
|
def add(self, element):
|
|
self.x += element
|
|
|
|
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.
|
|
|
|
Using default factory functions is a way to create new instances of
|
|
mutable types as default values for fields::
|
|
|
|
@dataclass
|
|
class D:
|
|
x: list = field(default_factory=list)
|
|
|
|
assert D().x is not D().x
|
|
|
|
Exceptions
|
|
----------
|
|
|
|
.. exception:: FrozenInstanceError
|
|
|
|
Raised when an implicitly defined :meth:`__setattr__` or
|
|
:meth:`__delattr__` is called on a dataclass which was defined with
|
|
``frozen=True``.
|