A class decorator that adds `dunder
<https://wiki.python.org/moin/DunderAlias>`_\ -methods according to the
specified attributes using `attr.ib` or the *these* argument.
:param these: A dictionary of name to `attr.ib` mappings. This is
useful to avoid the definition of your attributes within the class body
because you can't (e.g. if you want to add ``__repr__`` methods to
Django models) or don't want to.
If *these* is not ``None``, ``attrs`` will *not* search the class body
for attributes and will *not* remove any attributes from it.
If *these* is an ordered dict (`dict` on Python 3.6+,
`collections.OrderedDict` otherwise), the order is deduced from
the order of the attributes inside *these*. Otherwise the order
of the definition of the attributes is used.
:type these: `dict` of `str` to `attr.ib`
:param str repr_ns: When using nested classes, there's no way in Python 2
to automatically detect that. Therefore it's possible to set the
namespace explicitly for a more meaningful ``repr`` output.
:param bool auto_detect: Instead of setting the *init*, *repr*, *eq*,
*order*, and *hash* arguments explicitly, assume they are set to
``True`` **unless any** of the involved methods for one of the
arguments is implemented in the *current* class (i.e. it is *not*
inherited from some base class).
So for example by implementing ``__eq__`` on a class yourself,
``attrs`` will deduce ``eq=False`` and will create *neither*
``__eq__`` *nor* ``__ne__`` (but Python classes come with a sensible
``__ne__`` by default, so it *should* be enough to only implement
``__eq__`` in most cases).
.. warning::
If you prevent ``attrs`` from creating the ordering methods for you
(``order=False``, e.g. by implementing ``__le__``), it becomes
*your* responsibility to make sure its ordering is sound. The best
way is to use the `functools.total_ordering` decorator.
Passing ``True`` or ``False`` to *init*, *repr*, *eq*, *order*,
*cmp*, or *hash* overrides whatever *auto_detect* would determine.
*auto_detect* requires Python 3. Setting it ``True`` on Python 2 raises
a `PythonTooOldError`.
:param bool repr: Create a ``__repr__`` method with a human readable
representation of ``attrs`` attributes..
:param bool str: Create a ``__str__`` method that is identical to
``__repr__``. This is usually not necessary except for
`Exception`\ s.
:param Optional[bool] eq: If ``True`` or ``None`` (default), add ``__eq__``
and ``__ne__`` methods that check two instances for equality.
They compare the instances as if they were tuples of their ``attrs``
attributes if and only if the types of both classes are *identical*!
:param Optional[bool] order: If ``True``, add ``__lt__``, ``__le__``,
``__gt__``, and ``__ge__`` methods that behave like *eq* above and
allow instances to be ordered. If ``None`` (default) mirror value of
*eq*.
:param Optional[bool] cmp: Setting *cmp* is equivalent to setting *eq*
and *order* to the same value. Must not be mixed with *eq* or *order*.
:param Optional[bool] hash: If ``None`` (default), the ``__hash__`` method
is generated according how *eq* and *frozen* are set.
1. If *both* are True, ``attrs`` will generate a ``__hash__`` for you.
2. If *eq* is True and *frozen* is False, ``__hash__`` will be set to
None, marking it unhashable (which it is).
3. If *eq* is False, ``__hash__`` will be left untouched meaning the
``__hash__`` method of the base class will be used (if base class is
``object``, this means it will fall back to id-based hashing.).
Although not recommended, you can decide for yourself and force
``attrs`` to create one (e.g. if the class is immutable even though you
didn't freeze it programmatically) by passing ``True`` or not. Both of
these cases are rather special and should be used carefully.
See our documentation on `hashing`, Python's documentation on
`object.__hash__`, and the `GitHub issue that led to the default \
behavior <https://github.com/python-attrs/attrs/issues/136>`_ for more
details.
:param bool init: Create a ``__init__`` method that initializes the
``attrs`` attributes. Leading underscores are stripped for the argument
name. If a ``__attrs_pre_init__`` method exists on the class, it will
be called before the class is initialized. If a ``__attrs_post_init__``
method exists on the class, it will be called after the class is fully
initialized.
If ``init`` is ``False``, an ``__attrs_init__`` method will be
injected instead. This allows you to define a custom ``__init__``
method that can do pre-init work such as ``super().__init__()``,
and then call ``__attrs_init__()`` and ``__attrs_post_init__()``.
:param bool slots: Create a `slotted class <slotted classes>` that's more
memory-efficient. Slotted classes are generally superior to the default
dict classes, but have some gotchas you should know about, so we
encourage you to read the `glossary entry <slotted classes>`.
:param bool frozen: Make instances immutable after initialization. If
someone attempts to modify a frozen instance,
`attr.exceptions.FrozenInstanceError` is raised.
.. note::
1. This is achieved by installing a custom ``__setattr__`` method
on your class, so you can't implement your own.
2. True immutability is impossible in Python.
3. This *does* have a minor a runtime performance `impact
<how-frozen>` when initializing new instances. In other words:
``__init__`` is slightly slower with ``frozen=True``.
4. If a class is frozen, you cannot modify ``self`` in
``__attrs_post_init__`` or a self-written ``__init__``. You can
circumvent that limitation by using
``object.__setattr__(self, "attribute_name", value)``.
5. Subclasses of a frozen class are frozen too.
:param bool weakref_slot: Make instances weak-referenceable. This has no
effect unless ``slots`` is also enabled.
:param bool auto_attribs: If ``True``, collect `PEP 526`_-annotated
attributes (Python 3.6 and later only) from the class body.
In this case, you **must** annotate every field. If ``attrs``
encounters a field that is set to an `attr.ib` but lacks a type
annotation, an `attr.exceptions.UnannotatedAttributeError` is
raised. Use ``field_name: typing.Any = attr.ib(...)`` if you don't
want to set a type.
If you assign a value to those attributes (e.g. ``x: int = 42``), that
value becomes the default value like if it were passed using
``attr.ib(default=42)``. Passing an instance of `Factory` also
works as expected in most cases (see warning below).
Attributes annotated as `typing.ClassVar`, and attributes that are
neither annotated nor set to an `attr.ib` are **ignored**.
.. warning::
For features that use the attribute name to create decorators (e.g.
`validators <validators>`), you still *must* assign `attr.ib` to
them. Otherwise Python will either not find the name or try to use
the default value to call e.g. ``validator`` on it.
These errors can be quite confusing and probably the most common bug
report on our bug tracker.
.. _`PEP 526`: https://www.python.org/dev/peps/pep-0526/
:param bool kw_only: Make all attributes keyword-only (Python 3+)
in the generated ``__init__`` (if ``init`` is ``False``, this
parameter is ignored).
:param bool cache_hash: Ensure that the object's hash code is computed
only once and stored on the object. If this is set to ``True``,
hashing must be either explicitly or implicitly enabled for this
class. If the hash code is cached, avoid any reassignments of
fields involved in hash code computation or mutations of the objects
those fields point to after object creation. If such changes occur,
the behavior of the object's hash code is undefined.
:param bool auto_exc: If the class subclasses `BaseException`
(which implicitly includes any subclass of any exception), the
following happens to behave like a well-behaved Python exceptions
class:
- the values for *eq*, *order*, and *hash* are ignored and the
instances compare and hash by the instance's ids (N.B. ``attrs`` will
*not* remove existing implementations of ``__hash__`` or the equality
methods. It just won't add own ones.),
- all attributes that are either passed into ``__init__`` or have a
default value are additionally available as a tuple in the ``args``
attribute,
- the value of *str* is ignored leaving ``__str__`` to base classes.
:param bool collect_by_mro: Setting this to `True` fixes the way ``attrs``
collects attributes from base classes. The default behavior is
incorrect in certain cases of multiple inheritance. It should be on by
default but is kept off for backward-compatability.
See issue `#428 <https://github.com/python-attrs/attrs/issues/428>`_ for
more details.
:param Optional[bool] getstate_setstate:
.. note::
This is usually only interesting for slotted classes and you should
probably just set *auto_detect* to `True`.
If `True`, ``__getstate__`` and
``__setstate__`` are generated and attached to the class. This is
necessary for slotted classes to be pickleable. If left `None`, it's
`True` by default for slotted classes and ``False`` for dict classes.
If *auto_detect* is `True`, and *getstate_setstate* is left `None`,
and **either** ``__getstate__`` or ``__setstate__`` is detected directly
on the class (i.e. not inherited), it is set to `False` (this is usually
what you want).
:param on_setattr: A callable that is run whenever the user attempts to set
an attribute (either by assignment like ``i.x = 42`` or by using
`setattr` like ``setattr(i, "x", 42)``). It receives the same arguments
as validators: the instance, the attribute that is being modified, and
the new value.
If no exception is raised, the attribute is set to the return value of
the callable.
If a list of callables is passed, they're automatically wrapped in an
`attr.setters.pipe`.
:param Optional[callable] field_transformer:
A function that is called with the original class object and all
fields right before ``attrs`` finalizes the class. You can use
this, e.g., to automatically add converters or validators to
fields based on their types. See `transform-fields` for more details.
.. versionadded:: 16.0.0 *slots*
.. versionadded:: 16.1.0 *frozen*
.. versionadded:: 16.3.0 *str*
.. versionadded:: 16.3.0 Support for ``__attrs_post_init__``.
.. versionchanged:: 17.1.0
*hash* supports ``None`` as value which is also the default now.
.. versionadded:: 17.3.0 *auto_attribs*
.. versionchanged:: 18.1.0
If *these* is passed, no attributes are deleted from the class body.
.. versionchanged:: 18.1.0 If *these* is ordered, the order is retained.
.. versionadded:: 18.2.0 *weakref_slot*
.. deprecated:: 18.2.0
``__lt__``, ``__le__``, ``__gt__``, and ``__ge__`` now raise a
`DeprecationWarning` if the classes compared are subclasses of
each other. ``__eq`` and ``__ne__`` never tried to compared subclasses
to each other.
.. versionchanged:: 19.2.0
``__lt__``, ``__le__``, ``__gt__``, and ``__ge__`` now do not consider
subclasses comparable anymore.
.. versionadded:: 18.2.0 *kw_only*
.. versionadded:: 18.2.0 *cache_hash*
.. versionadded:: 19.1.0 *auto_exc*
.. deprecated:: 19.2.0 *cmp* Removal on or after 2021-06-01.
.. versionadded:: 19.2.0 *eq* and *order*
.. versionadded:: 20.1.0 *auto_detect*
.. versionadded:: 20.1.0 *collect_by_mro*
.. versionadded:: 20.1.0 *getstate_setstate*
.. versionadded:: 20.1.0 *on_setattr*
.. versionadded:: 20.3.0 *field_transformer*
.. versionchanged:: 21.1.0
``init=False`` injects ``__attrs_init__``
.. versionchanged:: 21.1.0 Support for ``__attrs_pre_init__``
.. versionchanged:: 21.1.0 *cmp* undeprecated