# documentação Django 2.0.8

# Código fonte para django.dispatch.dispatcher

    import threading
    import weakref

    from django.utils.inspect import func_accepts_kwargs

    def _make_id(target):
        if hasattr(target, '__func__'):
            return (id(target.__self__), id(target.__func__))
        return id(target)

    NONE_ID = _make_id(None)

    # A marker for caching
    NO_RECEIVERS = object()

    class Signal:
        """
        Base class for all signals

        Internal attributes:

            receivers
                { receiverkey (id) : weakref(receiver) }
        """
        def __init__(self, providing_args=None, use_caching=False):
            """
            Create a new signal.

            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)
            self.lock = threading.Lock()
            self.use_caching = use_caching
            # For convenience we create empty caches even if they are not used.
            # A note about caching: if use_caching is defined, then for each
            # distinct sender we cache the receivers that sender has in
            # 'sender_receivers_cache'. The cache is cleaned when .connect() or
            # .disconnect() is called and populated on send().
            self.sender_receivers_cache = weakref.WeakKeyDictionary() if use_caching else {}
            self._dead_receivers = False

        def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
            """
            Connect receiver to sender for signal.

            Arguments:

                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 referenceable.

                    Receivers must be able to accept keyword arguments.

                    If a receiver is connected with a dispatch_uid argument, it
                    will not be added if another receiver was already connected
                    with that dispatch_uid.

                sender
                    The sender to which the receiver should respond. Must either be
                    a Python object, 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.
            """
            from django.conf import settings

            # If DEBUG is on, check that we got a good receiver
            if settings.configured and settings.DEBUG:
                assert callable(receiver), "Signal receivers must be callable."

                # Check for **kwargs
                if not func_accepts_kwargs(receiver):
                    raise ValueError("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:
                ref = weakref.ref
                receiver_object = receiver
                # Check for bound methods
                if hasattr(receiver, '__self__') and hasattr(receiver, '__func__'):
                    ref = weakref.WeakMethod
                    receiver_object = receiver.__self__
                receiver = ref(receiver)
                weakref.finalize(receiver_object, self._remove_receiver)

            with self.lock:
                self._clear_dead_receivers()
                for r_key, _ in self.receivers:
                    if r_key == lookup_key:
                        break
                else:
                    self.receivers.append((lookup_key, receiver))
                self.sender_receivers_cache.clear()

        def disconnect(self, receiver=None, sender=None, dispatch_uid=None):
            """
            Disconnect receiver from sender for signal.

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

            Arguments:

                receiver
                    The registered receiver to disconnect. May be none if
                    dispatch_uid is specified.

                sender
                    The registered sender to disconnect

                dispatch_uid
                    the unique identifier of the receiver to disconnect
            """
            if dispatch_uid:
                lookup_key = (dispatch_uid, _make_id(sender))
            else:
                lookup_key = (_make_id(receiver), _make_id(sender))

            disconnected = False
            with self.lock:
                self._clear_dead_receivers()
                for index in range(len(self.receivers)):
                    (r_key, _) = self.receivers[index]
                    if r_key == lookup_key:
                        disconnected = True
                        del self.receivers[index]
                        break
                self.sender_receivers_cache.clear()
            return disconnected

        def has_listeners(self, sender=None):
            return bool(self._live_receivers(sender))

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

            If any receiver raises an error, the error propagates back through send,
            terminating the dispatch loop. So it's possible that all receivers
            won't be called if an error is raised.

            Arguments:

                sender
                    The sender of the signal. Either a specific object or None.

                named
                    Named arguments which will be passed to receivers.

            Return a list of tuple pairs [(receiver, response), ... ].
            """
            if not self.receivers or self.sender_receivers_cache.get(sender) is NO_RECEIVERS:
                return []

            return [
                (receiver, receiver(signal=self, sender=sender, **named))
                for receiver in self._live_receivers(sender)
            ]

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

            Arguments:

                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), ... ].

            If any receiver raises an error (specifically any subclass of
            Exception), return the error instance as the result for that receiver.
            """
            if not self.receivers or self.sender_receivers_cache.get(sender) is NO_RECEIVERS:
                return []

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

        def _clear_dead_receivers(self):
            # Note: caller is assumed to hold self.lock.
            if self._dead_receivers:
                self._dead_receivers = False
                new_receivers = []
                for r in self.receivers:
                    if isinstance(r[1], weakref.ReferenceType) and r[1]() is None:
                        continue
                    new_receivers.append(r)
                self.receivers = new_receivers

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

            This checks for weak references and resolves them, then returning only
            live receivers.
            """
            receivers = None
            if self.use_caching and not self._dead_receivers:
                receivers = self.sender_receivers_cache.get(sender)
                # We could end up here with NO_RECEIVERS even if we do check this case in
                # .send() prior to calling _live_receivers() due to concurrent .send() call.
                if receivers is NO_RECEIVERS:
                    return []
            if receivers is None:
                with self.lock:
                    self._clear_dead_receivers()
                    senderkey = _make_id(sender)
                    receivers = []
                    for (receiverkey, r_senderkey), receiver in self.receivers:
                        if r_senderkey == NONE_ID or r_senderkey == senderkey:
                            receivers.append(receiver)
                    if self.use_caching:
                        if not receivers:
                            self.sender_receivers_cache[sender] = NO_RECEIVERS
                        else:
                            # Note, we must cache the weakref versions.
                            self.sender_receivers_cache[sender] = receivers
            non_weak_receivers = []
            for receiver in receivers:
                if isinstance(receiver, weakref.ReferenceType):
                    # Dereference the weak reference.
                    receiver = receiver()
                    if receiver is not None:
                        non_weak_receivers.append(receiver)
                else:
                    non_weak_receivers.append(receiver)
            return non_weak_receivers

        def _remove_receiver(self, receiver=None):
            # Mark that the self.receivers list has dead weakrefs. If so, we will
            # clean those up in connect, disconnect and _live_receivers while
            # holding self.lock. Note that doing the cleanup here isn't a good
            # idea, _remove_receiver() will be called as side effect of garbage
            # collection, and so the call can happen while we are already holding
            # self.lock.
            self._dead_receivers = True

    def receiver(signal, **kwargs):
        """
        A decorator for connecting receivers to signals. Used by passing in the
        signal (or list of signals) and keyword arguments to connect::

            @receiver(post_save, sender=MyModel)
            def signal_receiver(sender, **kwargs):
                ...

            @receiver([post_save, post_delete], sender=MyModel)
            def signals_receiver(sender, **kwargs):
                ...
        """
        def _decorator(func):
            if isinstance(signal, (list, tuple)):
                for s in signal:
                    s.connect(func, **kwargs)
            else:
                signal.connect(func, **kwargs)
            return func
        return _decorator