# event/base.py
# Copyright (C) 2005-2025 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: https://www.opensource.org/licenses/mit-license.php
"""Base implementation classes.
The public-facing ``Events`` serves as the base class for an event interface;
its public attributes represent different kinds of events. These attributes
are mirrored onto a ``_Dispatch`` class, which serves as a container for
collections of listener functions. These collections are represented both
at the class level of a particular ``_Dispatch`` class as well as within
instances of ``_Dispatch``.
"""
from __future__ import annotations
import typing
from typing import Any
from typing import cast
from typing import Dict
from typing import Generic
from typing import Iterator
from typing import List
from typing import Mapping
from typing import MutableMapping
from typing import Optional
from typing import overload
from typing import Tuple
from typing import Type
from typing import Union
import weakref
from .attr import _ClsLevelDispatch
from .attr import _EmptyListener
from .attr import _InstanceLevelDispatch
from .attr import _JoinedListener
from .registry import _ET
from .registry import _EventKey
from .. import util
from ..util.typing import Literal
_registrars: MutableMapping[str, List[Type[_HasEventsDispatch[Any]]]] = (
util.defaultdict(list)
)
def _is_event_name(name: str) -> bool:
# _sa_event prefix is special to support internal-only event names.
# most event names are just plain method names that aren't
# underscored.
return (
not name.startswith("_") and name != "dispatch"
) or name.startswith("_sa_event")
class _UnpickleDispatch:
"""Serializable callable that re-generates an instance of
:class:`_Dispatch` given a particular :class:`.Events` subclass.
"""
def __call__(self, _instance_cls: Type[_ET]) -> _Dispatch[_ET]:
for cls in _instance_cls.__mro__:
if "dispatch" in cls.__dict__:
return cast(
"_Dispatch[_ET]", cls.__dict__["dispatch"].dispatch
)._for_class(_instance_cls)
else:
raise AttributeError("No class with a 'dispatch' member present.")
class _DispatchCommon(Generic[_ET]):
__slots__ = ()
_instance_cls: Optional[Type[_ET]]
def _join(self, other: _DispatchCommon[_ET]) -> _JoinedDispatcher[_ET]:
raise NotImplementedError()
def __getattr__(self, name: str) -> _InstanceLevelDispatch[_ET]:
raise NotImplementedError()
@property
def _events(self) -> Type[_HasEventsDispatch[_ET]]:
raise NotImplementedError()
class _Dispatch(_DispatchCommon[_ET]):
"""Mirror the event listening definitions of an Events class with
listener collections.
Classes which define a "dispatch" member will return a
non-instantiated :class:`._Dispatch` subclass when the member
is accessed at the class level. When the "dispatch" member is
accessed at the instance level of its owner, an instance
of the :class:`._Dispatch` class is returned.
A :class:`._Dispatch` class is generated for each :class:`.Events`
class defined, by the :meth:`._HasEventsDispatch._create_dispatcher_class`
method. The original :class:`.Events` classes remain untouched.
This decouples the construction of :class:`.Events` subclasses from
the implementation used by the event internals, and allows
inspecting tools like Sphinx to work in an unsurprising
way against the public API.
"""
# "active_history" is an ORM case we add here. ideally a better
# system would be in place for ad-hoc attributes.
__slots__ = "_parent", "_instance_cls", "__dict__", "_empty_listeners"
_active_history: bool
_empty_listener_reg: MutableMapping[
Type[_ET], Dict[str, _EmptyListener[_ET]]
] = weakref.WeakKeyDictionary()
_empty_listeners: Dict[str, _EmptyListener[_ET]]
_event_names: List[str]
_instance_cls: Optional[Type[_ET]]
_joined_dispatch_cls: Type[_JoinedDispatcher[_ET]]
_events: Type[_HasEventsDispatch[_ET]]
"""reference back to the Events class.
Bidirectional against _HasEventsDispatch.dispatch
"""
def __init__(
self,
parent: Optional[_Dispatch[_ET]],
instance_cls: Optional[Type[_ET]] = None,
):
self._parent = parent
self._instance_cls = instance_cls
if instance_cls:
assert parent is not None
try:
self._empty_listeners = self._empty_listener_reg[instance_cls]
except KeyError:
self._empty_listeners = self._empty_listener_reg[
instance_cls
] = {
ls.name: _EmptyListener(ls, instance_cls)
for ls in parent._event_descriptors
}
else:
self._empty_listeners = {}
def __getattr__(self, name: str) -> _InstanceLevelDispatch[_ET]:
# Assign EmptyListeners as attributes on demand
# to reduce startup time for new dispatch objects.
try:
ls = self._empty_listeners[name]
except KeyError:
raise AttributeError(name)
else:
setattr(self, ls.name, ls)
return ls
@property
def _event_descriptors(self) -> Iterator[_ClsLevelDispatch[_ET]]:
for k in self._event_names:
# Yield _ClsLevelDispatch related
# to relevant event name.
yield getattr(self, k)
def _listen(self, event_key: _EventKey[_ET], **kw: Any) -> None:
return self._events._listen(event_key, **kw)
def _for_class(self, instance_cls: Type[_ET]) -> _Dispatch[_ET]:
return self.__class__(self, instance_cls)
def _for_instance(self, instance: _ET) -> _Dispatch[_ET]:
instance_cls = instance.__class__
return self._for_class(instance_cls)
def _join(self, other: _DispatchCommon[_ET]) -> _JoinedDispatcher[_ET]:
"""Create a 'join' of this :class:`._Dispatch` and another.
This new dispatcher will dispatch events to both
:class:`._Dispatch` objects.
"""
assert "_joined_dispatch_cls" in self.__class__.__dict__
return self._joined_dispatch_cls(self, other)
def __reduce__(self) -> Union[str, Tuple[Any, ...]]:
return _UnpickleDispatch(), (self._instance_cls,)
def _update(
self, other: _Dispatch[_ET], only_propagate: bool = True
) -> None:
"""Populate from the listeners in another :class:`_Dispatch`
object."""
for ls in other._event_descriptors:
if isinstance(ls, _EmptyListener):
continue
getattr(self, ls.name).for_modify(self)._update(
ls, only_propagate=only_propagate
)
def _clear(self) -> None:
for ls in self._event_descriptors:
ls.for_modify(self).clear()
def _remove_dispatcher(cls: Type[_HasEventsDispatch[_ET]]) -> None:
for k in cls.dispatch._event_names:
_registrars[k].remove(cls)
if not _registrars[k]:
del _registrars[k]
class _HasEventsDispatch(Generic[_ET]):
_dispatch_target: Optional[Type[_ET]]
"""class which will receive the .dispatch collection"""
dispatch: _Dispatch[_ET]
"""reference back to the _Dispatch class.
Bidirectional against _Dispatch._events
"""
if typing.TYPE_CHECKING:
def __getattr__(self, name: str) -> _InstanceLevelDispatch[_ET]: ...
def __init_subclass__(cls) -> None:
"""Intercept new Event subclasses and create associated _Dispatch
classes."""
cls._create_dispatcher_class(cls.__name__, cls.__bases__, cls.__dict__)
@classmethod
def _accept_with(
cls, target: Union[_ET, Type[_ET]], identifier: str
) -> Optional[Union[_ET, Type[_ET]]]:
raise NotImplementedError()
@classmethod
def _listen(
cls,
event_key: _EventKey[_ET],
*,
propagate: bool = False,
insert: bool = False,
named: bool = False,
asyncio: bool = False,
) -> None:
raise NotImplementedError()
@staticmethod
def _set_dispatch(
klass: Type[_HasEventsDispatch[_ET]],
dispatch_cls: Type[_Dispatch[_ET]],
) -> _Dispatch[_ET]:
# This allows an Events subclass to define additional utility
# methods made available to the target via
# "self.dispatch._events.<utilitymethod>"
# @staticmethod to allow easy "super" calls while in a metaclass
# constructor.
klass.dispatch = dispatch_cls(None)
dispatch_cls._events = klass
return klass.dispatch
@classmethod
def _create_dispatcher_class(
cls, classname: str, bases: Tuple[type, ...], dict_: Mapping[str, Any]
) -> None:
"""Create a :class:`._Dispatch` class corresponding to an
:class:`.Events` class."""
# there's all kinds of ways to do this,
# i.e. make a Dispatch class that shares the '_listen' method
# of the Event class, this is the straight monkeypatch.
if hasattr(cls, "dispatch"):
dispatch_base = cls.dispatch.__class__
else:
dispatch_base = _Dispatch
event_names = [k for k in dict_ if _is_event_name(k)]
dispatch_cls = cast(
"Type[_Dispatch[_ET]]",
type(
"%sDispatch" % classname,
(dispatch_base,),
{"__slots__": event_names},
),
)
dispatch_cls._event_names = event_names
dispatch_inst = cls._set_dispatch(cls, dispatch_cls)
for k in dispatch_cls._event_names:
setattr(dispatch_inst, k, _ClsLevelDispatch(cls, dict_[k]))
_registrars[k].append(cls)
for super_ in dispatch_cls.__bases__:
if issubclass(super_, _Dispatch) and super_ is not _Dispatch:
for ls in super_._events.dispatch._event_descriptors:
setattr(dispatch_inst, ls.name, ls)
dispatch_cls._event_names.append(ls.name)
if getattr(cls, "_dispatch_target", None):
dispatch_target_cls = cls._dispatch_target
assert dispatch_target_cls is not None
if (
hasattr(dispatch_target_cls, "__slots__")
and "_slots_dispatch" in dispatch_target_cls.__slots__
):
dispatch_target_cls.dispatch = slots_dispatcher(cls)
else:
dispatch_target_cls.dispatch = dispatcher(cls)
klass = type(
"Joined%s" % dispatch_cls.__name__,
(_JoinedDispatcher,),
{"__slots__": event_names},
)
dispatch_cls._joined_dispatch_cls = klass
# establish pickle capability by adding it to this module
globals()[klass.__name__] = klass
class _JoinedDispatcher(_DispatchCommon[_ET]):
"""Represent a connection between two _Dispatch objects."""
__slots__ = "local", "parent", "_instance_cls"
local: _DispatchCommon[_ET]
parent: _DispatchCommon[_ET]
_instance_cls: Optional[Type[_ET]]
def __init__(
self, local: _DispatchCommon[_ET], parent: _DispatchCommon[_ET]
):
self.local = local
self.parent = parent
self._instance_cls = self.local._instance_cls
def __reduce__(self) -> Any:
return (self.__class__, (self.local, self.parent))
def __getattr__(self, name: str) -> _JoinedListener[_ET]:
# Assign _JoinedListeners as attributes on demand
# to reduce startup time for new dispatch objects.
ls = getattr(self.local, name)
jl = _JoinedListener(self.parent, ls.name, ls)
setattr(self, ls.name, jl)
return jl
def _listen(self, event_key: _EventKey[_ET], **kw: Any) -> None:
return self.parent._listen(event_key, **kw)
@property
def _events(self) -> Type[_HasEventsDispatch[_ET]]:
return self.parent._events
class Events(_HasEventsDispatch[_ET]):
"""Define event listening functions for a particular target type."""
@classmethod
def _accept_with(
cls, target: Union[_ET, Type[_ET]], identifier: str
) -> Optional[Union[_ET, Type[_ET]]]:
def dispatch_is(*types: Type[Any]) -> bool:
return all(isinstance(target.dispatch, t) for t in types)
def dispatch_parent_is(t: Type[Any]) -> bool:
parent = cast("_JoinedDispatcher[_ET]", target.dispatch).parent
while isinstance(parent, _JoinedDispatcher):
parent = cast("_JoinedDispatcher[_ET]", parent).parent
return isinstance(parent, t)
# Mapper, ClassManager, Session override this to
# also accept classes, scoped_sessions, sessionmakers, etc.
if hasattr(target, "dispatch"):
if (
dispatch_is(cls.dispatch.__class__)
or dispatch_is(type, cls.dispatch.__class__)
or (
dispatch_is(_JoinedDispatcher)
and dispatch_parent_is(cls.dispatch.__class__)
)
):
return target
return None
@classmethod
def _listen(
cls,
event_key: _EventKey[_ET],
*,
propagate: bool = False,
insert: bool = False,
named: bool = False,
asyncio: bool = False,
) -> None:
event_key.base_listen(
propagate=propagate, insert=insert, named=named, asyncio=asyncio
)
@classmethod
def _remove(cls, event_key: _EventKey[_ET]) -> None:
event_key.remove()
@classmethod
def _clear(cls) -> None:
cls.dispatch._clear()
class dispatcher(Generic[_ET]):
"""Descriptor used by target classes to
deliver the _Dispatch class at the class level
and produce new _Dispatch instances for target
instances.
"""
def __init__(self, events: Type[_HasEventsDispatch[_ET]]):
self.dispatch = events.dispatch
self.events = events
@overload
def __get__(
self, obj: Literal[None], cls: Type[Any]
) -> Type[_Dispatch[_ET]]: ...
@overload
def __get__(self, obj: Any, cls: Type[Any]) -> _DispatchCommon[_ET]: ...
def __get__(self, obj: Any, cls: Type[Any]) -> Any:
if obj is None:
return self.dispatch
disp = self.dispatch._for_instance(obj)
try:
obj.__dict__["dispatch"] = disp
except AttributeError as ae:
raise TypeError(
"target %r doesn't have __dict__, should it be "
"defining _slots_dispatch?" % (obj,)
) from ae
return disp
class slots_dispatcher(dispatcher[_ET]):
def __get__(self, obj: Any, cls: Type[Any]) -> Any:
if obj is None:
return self.dispatch
if hasattr(obj, "_slots_dispatch"):
return obj._slots_dispatch
disp = self.dispatch._for_instance(obj)
obj._slots_dispatch = disp
return disp
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