dispatcher.py

import weakref
try:
    set
except NameError:
    from sets import Set as set # Python 2.3 fallback

from django.dispatch import saferef

WEAKREF_TYPES = (weakref.ReferenceType, saferef.BoundMethodWeakref)

def _make_id(target):
    if hasattr(target, 'im_func'):
        return (id(target.im_self), id(target.im_func))
    return id(target)

class Signal(object):
    """Base class for all signals
    
    Internal attributes:
        receivers -- { receriverkey (id) : weakref(receiver) }
    """
    
    def __init__(self, providing_args=None):
        """providing_args -- A list of the arguments this signal can pass along in
                       a send() call.
        """
        self.receivers = []
        if providing_args is None:
            providing_args = []
        self.providing_args = set(providing_args)

    def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
        """Connect receiver to sender for signal
    
        receiver -- a function or an instance method which is to
            receive signals.  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 must be able to accept keyword arguments.

            If receivers have a dispatch_uid attribute, the receiver will
              not be added if another receiver already exists with that
              dispatch_uid.

        sender -- the sender to which the receiver should respond
            Must either be of type Signal, or None to receive events
            from any sender.

        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.
        
        dispatch_uid -- an identifier used to uniquely identify a particular
            instance of a receiver. This will usually be a string, though it
            may be anything hashable.

        returns None
        """
        from django.conf import settings
        
        # If DEBUG is on, check that we got a good receiver
        if settings.DEBUG:
            import inspect
            assert callable(receiver), "Signal receivers must be callable."
            
            # Check for **kwargs
            # Not all callables are inspectable with getargspec, so we'll
            # try a couple different ways but in the end fall back on assuming
            # it is -- we don't want to prevent registration of valid but weird
            # callables.
            try:
                argspec = inspect.getargspec(receiver)
            except TypeError:
                try:
                    argspec = inspect.getargspec(receiver.__call__)
                except (TypeError, AttributeError):
                    argspec = None
            if argspec:
                assert argspec[2] is not None, \
                    "Signal receivers must accept keyword arguments (**kwargs)."
        
        if dispatch_uid:
            lookup_key = (dispatch_uid, _make_id(sender))
        else:
            lookup_key = (_make_id(receiver), _make_id(sender))

        if weak:
            receiver = saferef.safeRef(receiver, onDelete=self._remove_receiver)

        for r_key, _ in self.receivers:
            if r_key == lookup_key:
                break
        else:
            self.receivers.append((lookup_key, receiver))

    def disconnect(self, receiver=None, sender=None, weak=True, dispatch_uid=None):
        """Disconnect receiver from sender for signal
    
        receiver -- the registered receiver to disconnect. May be none if
            dispatch_uid is specified.
        sender -- the registered sender to disconnect
        weak -- the weakref state to disconnect
        dispatch_uid -- the unique identifier of the receiver to disconnect
    
        disconnect reverses the process of connect.

        If weak references are used, disconnect need not be called.
          The receiver will be remove from dispatch automatically.

        returns None
        """

        if dispatch_uid:
            lookup_key = (dispatch_uid, _make_id(sender))
        else:
            lookup_key = (_make_id(receiver), _make_id(sender))
        
        for index in xrange(len(self.receivers)):
            (r_key, _) = self.receivers[index]
            if r_key == lookup_key:
                del self.receivers[index]

    def send(self, sender, **named):
        """Send signal from sender to all connected receivers.

        sender -- the sender of the signal
            Either a specific object or None.
    
        named -- named arguments which will be passed to receivers.

        Returns 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.
        """

        responses = []
        if not self.receivers:
            return responses

        for receiver in self._live_receivers(_make_id(sender)):
            response = receiver(signal=self, sender=sender, **named)
            responses.append((receiver, response))
        return responses

    def send_robust(self, sender, **named):
        """Send signal from sender to all connected receivers catching errors

        sender -- the sender of the signal
            Can be any python object (normally one registered with
            a connect if you actually want something to occur).

        named -- named arguments which will be passed to receivers.
            These arguments must be a subset of the argument names
            defined in providing_args.

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

        if any receiver raises an error (specifically any subclass of Exception),
        the error instance is returned as the result for that receiver.
        """

        responses = []
        if not self.receivers:
            return responses

        # Call each receiver with whatever arguments it can accept.
        # Return a list of tuple pairs [(receiver, response), ... ].
        for receiver in self._live_receivers(_make_id(sender)):
            try:
                response = receiver(signal=self, sender=sender, **named)
            except Exception, err:
                responses.append((receiver, err))
            else:
                responses.append((receiver, response))
        return responses

    def _live_receivers(self, senderkey):
        """Filter sequence of receivers to get resolved, live receivers

        This checks for weak references
        and resolves them, then returning only live
        receivers.
        """
        none_senderkey = _make_id(None)

        for (receiverkey, r_senderkey), receiver in self.receivers:
            if r_senderkey == none_senderkey or r_senderkey == senderkey:
                if isinstance(receiver, WEAKREF_TYPES):
                    # Dereference the weak reference.
                    receiver = receiver()
                    if receiver is not None:
                        yield receiver
                else:
                    yield receiver

    def _remove_receiver(self, receiver):
        """Remove dead receivers from connections."""

        to_remove = []
        for key, connected_receiver in self.receivers:
            if connected_receiver == receiver:
                to_remove.append(key)
        for key in to_remove:
            for idx, (r_key, _) in enumerate(self.receivers):
                if r_key == key:
                    del self.receivers[idx]