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# event/base.py
# Copyright (C) 2005-2021 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://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 absolute_import
import weakref
from .attr import _ClsLevelDispatch
from .attr import _EmptyListener
from .attr import _JoinedListener
from .. import util
_registrars = util.defaultdict(list)
def _is_event_name(name):
# _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(object):
"""Serializable callable that re-generates an instance of
:class:`_Dispatch` given a particular :class:`.Events` subclass.
"""
def __call__(self, _instance_cls):
for cls in _instance_cls.__mro__:
if "dispatch" in cls.__dict__:
return cls.__dict__["dispatch"].dispatch._for_class(
_instance_cls
)
else:
raise AttributeError("No class with a 'dispatch' member present.")
class _Dispatch(object):
"""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 :func:`._create_dispatcher_class` function.
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.
"""
# In one ORM edge case, an attribute is added to _Dispatch,
# so __dict__ is used in just that case and potentially others.
__slots__ = "_parent", "_instance_cls", "__dict__", "_empty_listeners"
_empty_listener_reg = weakref.WeakKeyDictionary()
def __init__(self, parent, instance_cls=None):
self._parent = parent
self._instance_cls = instance_cls
if instance_cls:
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):
# 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):
for k in self._event_names:
# Yield _ClsLevelDispatch related
# to relevant event name.
yield getattr(self, k)
@property
def _listen(self):
return self._events._listen
def _for_class(self, instance_cls):
return self.__class__(self, instance_cls)
def _for_instance(self, instance):
instance_cls = instance.__class__
return self._for_class(instance_cls)
def _join(self, other):
"""Create a 'join' of this :class:`._Dispatch` and another.
This new dispatcher will dispatch events to both
:class:`._Dispatch` objects.
"""
if "_joined_dispatch_cls" not in self.__class__.__dict__:
cls = type(
"Joined%s" % self.__class__.__name__,
(_JoinedDispatcher,),
{"__slots__": self._event_names},
)
self.__class__._joined_dispatch_cls = cls
return self._joined_dispatch_cls(self, other)
def __reduce__(self):
return _UnpickleDispatch(), (self._instance_cls,)
def _update(self, other, only_propagate=True):
"""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):
for ls in self._event_descriptors:
ls.for_modify(self).clear()
class _EventMeta(type):
"""Intercept new Event subclasses and create
associated _Dispatch classes."""
def __init__(cls, classname, bases, dict_):
_create_dispatcher_class(cls, classname, bases, dict_)
type.__init__(cls, classname, bases, dict_)
def _create_dispatcher_class(cls, classname, bases, dict_):
"""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 = 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):
cls._dispatch_target.dispatch = dispatcher(cls)
def _remove_dispatcher(cls):
for k in cls.dispatch._event_names:
_registrars[k].remove(cls)
if not _registrars[k]:
del _registrars[k]
class Events(util.with_metaclass(_EventMeta, object)):
"""Define event listening functions for a particular target type."""
@staticmethod
def _set_dispatch(cls, dispatch_cls):
# 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.
cls.dispatch = dispatch_cls(None)
dispatch_cls._events = cls
return cls.dispatch
@classmethod
def _accept_with(cls, target):
def dispatch_is(*types):
return all(isinstance(target.dispatch, t) for t in types)
def dispatch_parent_is(t):
return isinstance(target.dispatch.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
@classmethod
def _listen(cls, event_key, propagate=False, insert=False, named=False):
event_key.base_listen(propagate=propagate, insert=insert, named=named)
@classmethod
def _remove(cls, event_key):
event_key.remove()
@classmethod
def _clear(cls):
cls.dispatch._clear()
class _JoinedDispatcher(object):
"""Represent a connection between two _Dispatch objects."""
__slots__ = "local", "parent", "_instance_cls"
def __init__(self, local, parent):
self.local = local
self.parent = parent
self._instance_cls = self.local._instance_cls
def __getattr__(self, name):
# 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
@property
def _listen(self):
return self.parent._listen
@property
def _events(self):
return self.parent._events
class dispatcher(object):
"""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):
self.dispatch = events.dispatch
self.events = events
def __get__(self, obj, cls):
if obj is None:
return self.dispatch
obj.__dict__["dispatch"] = disp = self.dispatch._for_instance(obj)
return disp
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