path: root/lib/sqlalchemy/event.py

# sqlalchemy/event.py
# Copyright (C) 2005-2013 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 event API."""

from . import util, exc
from itertools import chain
import weakref

CANCEL = util.symbol('CANCEL')
NO_RETVAL = util.symbol('NO_RETVAL')


def listen(target, identifier, fn, *args, **kw):
    """Register a listener function for the given target.

    e.g.::

        from sqlalchemy import event
        from sqlalchemy.schema import UniqueConstraint

        def unique_constraint_name(const, table):
            const.name = "uq_%s_%s" % (
                table.name,
                list(const.columns)[0].name
            )
        event.listen(
                UniqueConstraint,
                "after_parent_attach",
                unique_constraint_name)

    """

    for evt_cls in _registrars[identifier]:
        tgt = evt_cls._accept_with(target)
        if tgt is not None:
            tgt.dispatch._listen(tgt, identifier, fn, *args, **kw)
            return
    raise exc.InvalidRequestError("No such event '%s' for target '%s'" %
                                (identifier, target))


def listens_for(target, identifier, *args, **kw):
    """Decorate a function as a listener for the given target + identifier.

    e.g.::

        from sqlalchemy import event
        from sqlalchemy.schema import UniqueConstraint

        @event.listens_for(UniqueConstraint, "after_parent_attach")
        def unique_constraint_name(const, table):
            const.name = "uq_%s_%s" % (
                table.name,
                list(const.columns)[0].name
            )
    """
    def decorate(fn):
        listen(target, identifier, fn, *args, **kw)
        return fn
    return decorate


def remove(target, identifier, fn):
    """Remove an event listener.

    Note that some event removals, particularly for those event dispatchers
    which create wrapper functions and secondary even listeners, may not yet
    be supported.

    """
    for evt_cls in _registrars[identifier]:
        for tgt in evt_cls._accept_with(target):
            tgt.dispatch._remove(identifier, tgt, fn)
            return

_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, _parent_cls):
        for cls in _parent_cls.__mro__:
            if 'dispatch' in cls.__dict__:
                return cls.__dict__['dispatch'].dispatch_cls(_parent_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.

    """

    def __init__(self, _parent_cls):
        self._parent_cls = _parent_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.

        Once constructed, the joined dispatch will respond to new events
        added to this dispatcher, but may not be aware of events
        added to the other dispatcher after creation of the join.  This is
        currently for performance reasons so that both dispatchers need
        not be "evaluated" fully on each call.

        """
        if '_joined_dispatch_cls' not in self.__class__.__dict__:
            cls = type(
                    "Joined%s" % self.__class__.__name__,
                    (_JoinedDispatcher, self.__class__), {}
                )
            for ls in _event_descriptors(self):
                setattr(cls, ls.name, _JoinedDispatchDescriptor(ls.name))

            self.__class__._joined_dispatch_cls = cls
        return self._joined_dispatch_cls(self, other)

    def __reduce__(self):
        return _UnpickleDispatch(), (self._parent_cls, )

    def _update(self, other, only_propagate=True):
        """Populate from the listeners in another :class:`_Dispatch`
            object."""

        for ls in _event_descriptors(other):
            getattr(self, ls.name).\
                for_modify(self)._update(ls, only_propagate=only_propagate)

    @util.hybridmethod
    def _clear(self):
        for attr in dir(self):
            if _is_event_name(attr):
                getattr(self, attr).for_modify(self).clear()


def _event_descriptors(target):
    return [getattr(target, k) for k in dir(target) if _is_event_name(k)]


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_)
        return 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.
    dispatch_base = getattr(cls, 'dispatch', _Dispatch)
    cls.dispatch = dispatch_cls = type("%sDispatch" % classname,
                                        (dispatch_base, ), {})
    dispatch_cls._listen = cls._listen

    for k in dict_:
        if _is_event_name(k):
            setattr(dispatch_cls, k, _DispatchDescriptor(dict_[k]))
            _registrars[k].append(cls)


def _remove_dispatcher(cls):
    for k in dir(cls):
        if _is_event_name(k):
            _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."""

    @classmethod
    def _accept_with(cls, target):
        # Mapper, ClassManager, Session override this to
        # also accept classes, scoped_sessions, sessionmakers, etc.
        if hasattr(target, 'dispatch') and (
                    isinstance(target.dispatch, cls.dispatch) or \
                    isinstance(target.dispatch, type) and \
                    issubclass(target.dispatch, cls.dispatch)
                ):
            return target
        else:
            return None

    @classmethod
    def _listen(cls, target, identifier, fn, propagate=False, insert=False):
        if insert:
            getattr(target.dispatch, identifier).\
                    for_modify(target.dispatch).insert(fn, target, propagate)
        else:
            getattr(target.dispatch, identifier).\
                    for_modify(target.dispatch).append(fn, target, propagate)

    @classmethod
    def _remove(cls, target, identifier, fn):
        getattr(target.dispatch, identifier).remove(fn, target)

    @classmethod
    def _clear(cls):
        cls.dispatch._clear()


class _DispatchDescriptor(object):
    """Class-level attributes on :class:`._Dispatch` classes."""

    def __init__(self, fn):
        self.__name__ = fn.__name__
        self.__doc__ = fn.__doc__
        self._clslevel = weakref.WeakKeyDictionary()
        self._empty_listeners = weakref.WeakKeyDictionary()

    def _contains(self, cls, evt):
        return cls in self._clslevel and \
            evt in self._clslevel[cls]

    def insert(self, obj, target, propagate):
        assert isinstance(target, type), \
                "Class-level Event targets must be classes."
        stack = [target]
        while stack:
            cls = stack.pop(0)
            stack.extend(cls.__subclasses__())
            if cls is not target and cls not in self._clslevel:
                self.update_subclass(cls)
            else:
                if cls not in self._clslevel:
                    self._clslevel[cls] = []
                self._clslevel[cls].insert(0, obj)

    def append(self, obj, target, propagate):
        assert isinstance(target, type), \
                "Class-level Event targets must be classes."

        stack = [target]
        while stack:
            cls = stack.pop(0)
            stack.extend(cls.__subclasses__())
            if cls is not target and cls not in self._clslevel:
                self.update_subclass(cls)
            else:
                if cls not in self._clslevel:
                    self._clslevel[cls] = []
                self._clslevel[cls].append(obj)

    def update_subclass(self, target):
        if target not in self._clslevel:
            self._clslevel[target] = []
        clslevel = self._clslevel[target]
        for cls in target.__mro__[1:]:
            if cls in self._clslevel:
                clslevel.extend([
                    fn for fn
                    in self._clslevel[cls]
                    if fn not in clslevel
                ])

    def remove(self, obj, target):
        stack = [target]
        while stack:
            cls = stack.pop(0)
            stack.extend(cls.__subclasses__())
            if cls in self._clslevel:
                self._clslevel[cls].remove(obj)

    def clear(self):
        """Clear all class level listeners"""

        for dispatcher in self._clslevel.values():
            dispatcher[:] = []

    def for_modify(self, obj):
        """Return an event collection which can be modified.

        For _DispatchDescriptor at the class level of
        a dispatcher, this returns self.

        """
        return self

    def __get__(self, obj, cls):
        if obj is None:
            return self
        elif obj._parent_cls in self._empty_listeners:
            ret = self._empty_listeners[obj._parent_cls]
        else:
            self._empty_listeners[obj._parent_cls] = ret = \
                _EmptyListener(self, obj._parent_cls)
        # assigning it to __dict__ means
        # memoized for fast re-access.  but more memory.
        obj.__dict__[self.__name__] = ret
        return ret


class _EmptyListener(object):
    """Serves as a class-level interface to the events
    served by a _DispatchDescriptor, when there are no
    instance-level events present.

    Is replaced by _ListenerCollection when instance-level
    events are added.

    """
    def __init__(self, parent, target_cls):
        if target_cls not in parent._clslevel:
            parent.update_subclass(target_cls)
        self.parent = parent
        self.parent_listeners = parent._clslevel[target_cls]
        self.name = parent.__name__
        self.propagate = frozenset()
        self.listeners = ()

    def for_modify(self, obj):
        """Return an event collection which can be modified.

        For _EmptyListener at the instance level of
        a dispatcher, this generates a new
        _ListenerCollection, applies it to the instance,
        and returns it.

        """
        result = _ListenerCollection(self.parent, obj._parent_cls)
        if obj.__dict__[self.name] is self:
            obj.__dict__[self.name] = result
        return result

    def _needs_modify(self, *args, **kw):
        raise NotImplementedError("need to call for_modify()")

    exec_once = insert = append = remove = clear = _needs_modify

    def __call__(self, *args, **kw):
        """Execute this event."""

        for fn in self.parent_listeners:
            fn(*args, **kw)

    def __len__(self):
        return len(self.parent_listeners)

    def __iter__(self):
        return iter(self.parent_listeners)

    def __bool__(self):
        return bool(self.parent_listeners)

    __nonzero__ = __bool__


class _CompoundListener(object):
    _exec_once = False

    def exec_once(self, *args, **kw):
        """Execute this event, but only if it has not been
        executed already for this collection."""

        if not self._exec_once:
            self(*args, **kw)
            self._exec_once = True

    # I'm not entirely thrilled about the overhead here,
    # but this allows class-level listeners to be added
    # at any point.
    #
    # In the absense of instance-level listeners,
    # we stay with the _EmptyListener object when called
    # at the instance level.

    def __call__(self, *args, **kw):
        """Execute this event."""

        for fn in self.parent_listeners:
            fn(*args, **kw)
        for fn in self.listeners:
            fn(*args, **kw)

    def __len__(self):
        return len(self.parent_listeners) + len(self.listeners)

    def __iter__(self):
        return chain(self.parent_listeners, self.listeners)

    def __bool__(self):
        return bool(self.listeners or self.parent_listeners)

    __nonzero__ = __bool__

class _ListenerCollection(_CompoundListener):
    """Instance-level attributes on instances of :class:`._Dispatch`.

    Represents a collection of listeners.

    As of 0.7.9, _ListenerCollection is only first
    created via the _EmptyListener.for_modify() method.

    """

    def __init__(self, parent, target_cls):
        if target_cls not in parent._clslevel:
            parent.update_subclass(target_cls)
        self.parent_listeners = parent._clslevel[target_cls]
        self.name = parent.__name__
        self.listeners = []
        self.propagate = set()

    def for_modify(self, obj):
        """Return an event collection which can be modified.

        For _ListenerCollection at the instance level of
        a dispatcher, this returns self.

        """
        return self

    def _update(self, other, only_propagate=True):
        """Populate from the listeners in another :class:`_Dispatch`
            object."""

        existing_listeners = self.listeners
        existing_listener_set = set(existing_listeners)
        self.propagate.update(other.propagate)
        existing_listeners.extend([l for l
                                in other.listeners
                                if l not in existing_listener_set
                                and not only_propagate or l in self.propagate
                                ])

    def insert(self, obj, target, propagate):
        if obj not in self.listeners:
            self.listeners.insert(0, obj)
            if propagate:
                self.propagate.add(obj)

    def append(self, obj, target, propagate):
        if obj not in self.listeners:
            self.listeners.append(obj)
            if propagate:
                self.propagate.add(obj)

    def remove(self, obj, target):
        if obj in self.listeners:
            self.listeners.remove(obj)
            self.propagate.discard(obj)

    def clear(self):
        self.listeners[:] = []
        self.propagate.clear()


class _JoinedDispatcher(object):
    """Represent a connection between two _Dispatch objects."""

    def __init__(self, local, parent):
        self.local = local
        self.parent = parent
        self._parent_cls = local._parent_cls


class _JoinedDispatchDescriptor(object):
    def __init__(self, name):
        self.name = name

    def __get__(self, obj, cls):
        if obj is None:
            return self
        else:
            obj.__dict__[self.name] = ret = _JoinedListener(
                        obj.parent, self.name,
                        getattr(obj.local, self.name)
                    )
            return ret


class _JoinedListener(_CompoundListener):
    _exec_once = False

    def __init__(self, parent, name, local):
        self.parent = parent
        self.name = name
        self.local = local
        self.parent_listeners = self.local

        # fix .listeners for the parent.  This means
        # new events added to the parent won't be picked
        # up here.  Alternatively, the listeners can
        # be via @property to just return getattr(self.parent, self.name)
        # each time. less performant.
        self.listeners = list(getattr(self.parent, self.name))

    def for_modify(self, obj):
        self.local = self.parent_listeners = self.local.for_modify(obj)
        return self

    def insert(self, obj, target, propagate):
        self.local.insert(obj, target, propagate)

    def append(self, obj, target, propagate):
        self.local.append(obj, target, propagate)

    def remove(self, obj, target):
        self.local.remove(obj, target)

    def clear(self):
        raise NotImplementedError()


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_cls = events.dispatch
        self.events = events

    def __get__(self, obj, cls):
        if obj is None:
            return self.dispatch_cls
        obj.__dict__['dispatch'] = disp = self.dispatch_cls(cls)
        return disp