path: root/django/dispatch/dispatcher.py

"""Multiple-producer-multiple-consumer signal-dispatching

dispatcher is the core of the PyDispatcher system,
providing the primary API and the core logic for the
system.

Module attributes of note:

    Any -- Singleton used to signal either "Any Sender" or
        "Any Signal".  See documentation of the _Any class.
    Anonymous -- Singleton used to signal "Anonymous Sender"
        See documentation of the _Anonymous class.

Internal attributes:
    WEAKREF_TYPES -- tuple of types/classes which represent
        weak references to receivers, and thus must be de-
        referenced on retrieval to retrieve the callable
        object
    connections -- { senderkey (id) : { signal : [receivers...]}}
    senders -- { senderkey (id) : weakref(sender) }
        used for cleaning up sender references on sender
        deletion
    sendersBack -- { receiverkey (id) : [senderkey (id)...] }
        used for cleaning up receiver references on receiver
        deletion, (considerably speeds up the cleanup process
        vs. the original code.)
"""
import types, weakref
from django.dispatch import saferef, robustapply, errors

__author__ = "Patrick K. O'Brien <pobrien@orbtech.com>"
__cvsid__ = "$Id: dispatcher.py,v 1.9 2005/09/17 04:55:57 mcfletch Exp $"
__version__ = "$Revision: 1.9 $"[11:-2]


class _Parameter:
    """Used to represent default parameter values."""
    def __repr__(self):
        return self.__class__.__name__

class _Any(_Parameter):
    """Singleton used to signal either "Any Sender" or "Any Signal"

    The Any object can be used with connect, disconnect,
    send, or sendExact to signal that the parameter given
    Any should react to all senders/signals, not just
    a particular sender/signal.
    """
Any = _Any()

class _Anonymous(_Parameter):
    """Singleton used to signal "Anonymous Sender"

    The Anonymous object is used to signal that the sender
    of a message is not specified (as distinct from being
    "any sender").  Registering callbacks for Anonymous
    will only receive messages sent without senders.  Sending
    with anonymous will only send messages to those receivers
    registered for Any or Anonymous.

    Note:
        The default sender for connect is Any, while the
        default sender for send is Anonymous.  This has
        the effect that if you do not specify any senders
        in either function then all messages are routed
        as though there was a single sender (Anonymous)
        being used everywhere.
    """
Anonymous = _Anonymous()

WEAKREF_TYPES = (weakref.ReferenceType, saferef.BoundMethodWeakref)

connections = {}
senders = {}
sendersBack = {}


def connect(receiver, signal=Any, sender=Any, weak=True):
    """Connect receiver to sender for signal

    receiver -- a callable Python object which is to receive
        messages/signals/events.  Receivers must be hashable
        objects.

        if weak is True, then receiver must be weak-referencable
        (more precisely saferef.safeRef() must be able to create
        a reference to the receiver).
    
        Receivers are fairly flexible in their specification,
        as the machinery in the robustApply module takes care
        of most of the details regarding figuring out appropriate
        subsets of the sent arguments to apply to a given
        receiver.

        Note:
            if receiver is itself a weak reference (a callable),
            it will be de-referenced by the system's machinery,
            so *generally* weak references are not suitable as
            receivers, though some use might be found for the
            facility whereby a higher-level library passes in
            pre-weakrefed receiver references.

    signal -- the signal to which the receiver should respond
    
        if Any, receiver will receive any signal from the
        indicated sender (which might also be Any, but is not
        necessarily Any).
        
        Otherwise must be a hashable Python object other than
        None (DispatcherError raised on None).
        
    sender -- the sender to which the receiver should respond
    
        if Any, receiver will receive the indicated signals
        from any sender.
        
        if Anonymous, receiver will only receive indicated
        signals from send/sendExact which do not specify a
        sender, or specify Anonymous explicitly as the sender.

        Otherwise can be any python object.
        
    weak -- whether to use weak references to the receiver
        By default, the module will attempt to use weak
        references to the receiver objects.  If this parameter
        is false, then strong references will be used.

    returns None, may raise DispatcherTypeError
    """
    if signal is None:
        raise errors.DispatcherTypeError(
            'Signal cannot be None (receiver=%r sender=%r)'%( receiver,sender)
        )
    if weak:
        receiver = saferef.safeRef(receiver, onDelete=_removeReceiver)
    senderkey = id(sender)

    signals = connections.setdefault(senderkey, {})

    # Keep track of senders for cleanup.
    # Is Anonymous something we want to clean up?
    if sender not in (None, Anonymous, Any):
        def remove(object, senderkey=senderkey):
            _removeSender(senderkey=senderkey)
        # Skip objects that can not be weakly referenced, which means
        # they won't be automatically cleaned up, but that's too bad.
        try:
            weakSender = weakref.ref(sender, remove)
            senders[senderkey] = weakSender
        except:
            pass
        
    receiverID = id(receiver)
    # get current set, remove any current references to
    # this receiver in the set, including back-references
    if signals.has_key(signal):
        receivers = signals[signal]
        _removeOldBackRefs(senderkey, signal, receiver, receivers)
    else:
        receivers = signals[signal] = []
    try:
        current = sendersBack.get( receiverID )
        if current is None:
            sendersBack[ receiverID ] = current = []
        if senderkey not in current:
            current.append(senderkey)
    except:
        pass

    receivers.append(receiver)



def disconnect(receiver, signal=Any, sender=Any, weak=True):
    """Disconnect receiver from sender for signal

    receiver -- the registered receiver to disconnect
    signal -- the registered signal to disconnect
    sender -- the registered sender to disconnect
    weak -- the weakref state to disconnect

    disconnect reverses the process of connect,
    the semantics for the individual elements are
    logically equivalent to a tuple of
    (receiver, signal, sender, weak) used as a key
    to be deleted from the internal routing tables.
    (The actual process is slightly more complex
    but the semantics are basically the same).

    Note:
        Using disconnect is not required to cleanup
        routing when an object is deleted, the framework
        will remove routes for deleted objects
        automatically.  It's only necessary to disconnect
        if you want to stop routing to a live object.
        
    returns None, may raise DispatcherTypeError or
        DispatcherKeyError
    """
    if signal is None:
        raise errors.DispatcherTypeError(
            'Signal cannot be None (receiver=%r sender=%r)'%( receiver,sender)
        )
    if weak: receiver = saferef.safeRef(receiver)
    senderkey = id(sender)
    try:
        signals = connections[senderkey]
        receivers = signals[signal]
    except KeyError:
        raise errors.DispatcherKeyError(
            """No receivers found for signal %r from sender %r""" %(
                signal,
                sender
            )
        )
    try:
        # also removes from receivers
        _removeOldBackRefs(senderkey, signal, receiver, receivers)
    except ValueError:
        raise errors.DispatcherKeyError(
            """No connection to receiver %s for signal %s from sender %s""" %(
                receiver,
                signal,
                sender
            )
        )
    _cleanupConnections(senderkey, signal)

def getReceivers( sender = Any, signal = Any ):
    """Get list of receivers from global tables

    This utility function allows you to retrieve the
    raw list of receivers from the connections table
    for the given sender and signal pair.

    Note:
        there is no guarantee that this is the actual list
        stored in the connections table, so the value
        should be treated as a simple iterable/truth value
        rather than, for instance a list to which you
        might append new records.

    Normally you would use liveReceivers( getReceivers( ...))
    to retrieve the actual receiver objects as an iterable
    object.
    """
    existing = connections.get(id(sender))
    if existing is not None:
        return existing.get(signal, [])
    return []

def liveReceivers(receivers):
    """Filter sequence of receivers to get resolved, live receivers

    This is a generator which will iterate over
    the passed sequence, checking for weak references
    and resolving them, then returning all live
    receivers.
    """
    for receiver in receivers:
        if isinstance( receiver, WEAKREF_TYPES):
            # Dereference the weak reference.
            receiver = receiver()
            if receiver is not None:
                yield receiver
        else:
            yield receiver



def getAllReceivers( sender = Any, signal = Any ):
    """Get list of all receivers from global tables

    This gets all dereferenced receivers which should receive
    the given signal from sender, each receiver should
    be produced only once by the resulting generator
    """
    receivers = {}
    # Get receivers that receive *this* signal from *this* sender.
    # Add receivers that receive *any* signal from *this* sender.
    # Add receivers that receive *this* signal from *any* sender.
    # Add receivers that receive *any* signal from *any* sender.
    l = []
    i = id(sender)
    if i in connections:
        sender_receivers = connections[i]
        if signal in sender_receivers:
            l.extend(sender_receivers[signal])
        if signal is not Any and Any in sender_receivers:
            l.extend(sender_receivers[Any])

    if sender is not Any:
        i = id(Any)
        if i in connections:
            sender_receivers = connections[i]
            if sender_receivers is not None:
                if signal in sender_receivers:
                    l.extend(sender_receivers[signal])
                if signal is not Any and Any in sender_receivers:
                    l.extend(sender_receivers[Any])

    for receiver in l:
        try:
            if not receiver in receivers:
                if isinstance(receiver, WEAKREF_TYPES):
                    receiver = receiver()
                    # this should only (rough guess) be possible if somehow, deref'ing
                    # triggered a wipe.
                    if receiver is None:
                        continue
                receivers[receiver] = 1
                yield receiver
        except TypeError:
            # dead weakrefs raise TypeError on hash...
            pass

def send(signal=Any, sender=Anonymous, *arguments, **named):
    """Send signal from sender to all connected receivers.
    
    signal -- (hashable) signal value, see connect for details

    sender -- the sender of the signal
    
        if Any, only receivers registered for Any will receive
        the message.

        if Anonymous, only receivers registered to receive
        messages from Anonymous or Any will receive the message

        Otherwise can be any python object (normally one
        registered with a connect if you actually want
        something to occur).

    arguments -- positional arguments which will be passed to
        *all* receivers. Note that this may raise TypeErrors
        if the receivers do not allow the particular arguments.
        Note also that arguments are applied before named
        arguments, so they should be used with care.

    named -- named arguments which will be filtered according
        to the parameters of the receivers to only provide those
        acceptable to the receiver.

    Return a list of tuple pairs [(receiver, response), ... ]

    if any receiver raises an error, the error propagates back
    through send, terminating the dispatch loop, so it is quite
    possible to not have all receivers called if a raises an
    error.
    """
    # Call each receiver with whatever arguments it can accept.
    # Return a list of tuple pairs [(receiver, response), ... ].
    responses = []
    for receiver in getAllReceivers(sender, signal):
        response = robustapply.robustApply(
            receiver,
            signal=signal,
            sender=sender,
            *arguments,
            **named
        )
        responses.append((receiver, response))
    return responses


def sendExact( signal=Any, sender=Anonymous, *arguments, **named ):
    """Send signal only to those receivers registered for exact message

    sendExact allows for avoiding Any/Anonymous registered
    handlers, sending only to those receivers explicitly
    registered for a particular signal on a particular
    sender.
    """
    responses = []
    for receiver in liveReceivers(getReceivers(sender, signal)):
        response = robustapply.robustApply(
            receiver,
            signal=signal,
            sender=sender,
            *arguments,
            **named
        )
        responses.append((receiver, response))
    return responses
    

def _removeReceiver(receiver):
    """Remove receiver from connections."""
    if not sendersBack:
        # During module cleanup the mapping will be replaced with None
        return False
    backKey = id(receiver)
    for senderkey in sendersBack.get(backKey,()):
        try:
            signals = connections[senderkey].keys()
        except KeyError,err:
            pass
        else:
            for signal in signals:
                try:
                    receivers = connections[senderkey][signal]
                except KeyError:
                    pass
                else:
                    try:
                        receivers.remove( receiver )
                    except Exception, err:
                        pass
                _cleanupConnections(senderkey, signal)
    try:
        del sendersBack[ backKey ]
    except KeyError:
        pass
            
def _cleanupConnections(senderkey, signal):
    """Delete any empty signals for senderkey. Delete senderkey if empty."""
    try:
        receivers = connections[senderkey][signal]
    except:
        pass
    else:
        if not receivers:
            # No more connected receivers. Therefore, remove the signal.
            try:
                signals = connections[senderkey]
            except KeyError:
                pass
            else:
                del signals[signal]
                if not signals:
                    # No more signal connections. Therefore, remove the sender.
                    _removeSender(senderkey)

def _removeSender(senderkey):
    """Remove senderkey from connections."""
    _removeBackrefs(senderkey)

    connections.pop(senderkey, None)
    senders.pop(senderkey, None)


def _removeBackrefs( senderkey):
    """Remove all back-references to this senderkey"""
    for receiver_list in connections.pop(senderkey, {}).values():
        for receiver in receiver_list:
            _killBackref( receiver, senderkey )


def _removeOldBackRefs(senderkey, signal, receiver, receivers):
    """Kill old sendersBack references from receiver

    This guards against multiple registration of the same
    receiver for a given signal and sender leaking memory
    as old back reference records build up.

    Also removes old receiver instance from receivers
    """
    try:
        index = receivers.index(receiver)
        # need to scan back references here and remove senderkey
    except ValueError:
        return False
    else:
        oldReceiver = receivers[index]
        del receivers[index]
        found = 0
        signals = connections.get(signal)
        if signals is not None:
            for sig,recs in connections.get(signal,{}).iteritems():
                if sig != signal:
                    for rec in recs:
                        if rec is oldReceiver:
                            found = 1
                            break
        if not found:
            _killBackref( oldReceiver, senderkey )
            return True
        return False
        
        
def _killBackref( receiver, senderkey ):
    """Do the actual removal of back reference from receiver to senderkey"""
    receiverkey = id(receiver)
    receivers_list = sendersBack.get( receiverkey, () )
    while senderkey in receivers_list:
        try:
            receivers_list.remove( senderkey )
        except:
            break
    if not receivers_list:
        try:
            del sendersBack[ receiverkey ]
        except KeyError:
            pass
    return True