text: # event/base.py
  # Copyright (C) 2005-2020 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):
      return not name.startswith("_") and name != "dispatch"
  
  
  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
  