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A small implementation of signals, inspired by a snippet of Django signal API client code seen in a blog post. Signals are first-class objects and each manages its own receivers and message emission. The :func:`signal` function provides singleton behavior for named signals. """ from warnings import warn from weakref import WeakValueDictionary from blinker._utilities import ( WeakTypes, contextmanager, defaultdict, hashable_identity, lazy_property, reference, symbol, ) ANY = symbol('ANY') ANY.__doc__ = 'Token for "any sender".' ANY_ID = 0 class Signal(object): """A notification emitter.""" #: An :obj:`ANY` convenience synonym, allows ``Signal.ANY`` #: without an additional import. ANY = ANY @lazy_property def receiver_connected(self): """Emitted after each :meth:`connect`. The signal sender is the signal instance, and the :meth:`connect` arguments are passed through: *receiver*, *sender*, and *weak*. .. versionadded:: 1.2 """ return Signal(doc="Emitted after a receiver connects.") @lazy_property def receiver_disconnected(self): """Emitted after :meth:`disconnect`. The sender is the signal instance, and the :meth:`disconnect` arguments are passed through: *receiver* and *sender*. Note, this signal is emitted **only** when :meth:`disconnect` is called explicitly. The disconnect signal can not be emitted by an automatic disconnect (due to a weakly referenced receiver or sender going out of scope), as the receiver and/or sender instances are no longer available for use at the time this signal would be emitted. An alternative approach is available by subscribing to :attr:`receiver_connected` and setting up a custom weakref cleanup callback on weak receivers and senders. .. versionadded:: 1.2 """ return Signal(doc="Emitted after a receiver disconnects.") def __init__(self, doc=None): """ :param doc: optional. If provided, will be assigned to the signal's __doc__ attribute. """ if doc: self.__doc__ = doc #: A mapping of connected receivers. #: #: The values of this mapping are not meaningful outside of the #: internal :class:`Signal` implementation, however the boolean value #: of the mapping is useful as an extremely efficient check to see if #: any receivers are connected to the signal. self.receivers = {} self._by_receiver = defaultdict(set) self._by_sender = defaultdict(set) self._weak_senders = {} def connect(self, receiver, sender=ANY, weak=True): """Connect *receiver* to signal events sent by *sender*. :param receiver: A callable. Will be invoked by :meth:`send` with `sender=` as a single positional argument and any \*\*kwargs that were provided to a call to :meth:`send`. :param sender: Any object or :obj:`ANY`, defaults to ``ANY``. Restricts notifications delivered to *receiver* to only those :meth:`send` emissions sent by *sender*. If ``ANY``, the receiver will always be notified. A *receiver* may be connected to multiple *sender* values on the same Signal through multiple calls to :meth:`connect`. :param weak: If true, the Signal will hold a weakref to *receiver* and automatically disconnect when *receiver* goes out of scope or is garbage collected. Defaults to True. """ receiver_id = hashable_identity(receiver) if weak: receiver_ref = reference(receiver, self._cleanup_receiver) receiver_ref.receiver_id = receiver_id else: receiver_ref = receiver if sender is ANY: sender_id = ANY_ID else: sender_id = hashable_identity(sender) self.receivers.setdefault(receiver_id, receiver_ref) self._by_sender[sender_id].add(receiver_id) self._by_receiver[receiver_id].add(sender_id) del receiver_ref if sender is not ANY and sender_id not in self._weak_senders: # wire together a cleanup for weakref-able senders try: sender_ref = reference(sender, self._cleanup_sender) sender_ref.sender_id = sender_id except TypeError: pass else: self._weak_senders.setdefault(sender_id, sender_ref) del sender_ref # broadcast this connection. if receivers raise, disconnect. if ('receiver_connected' in self.__dict__ and self.receiver_connected.receivers): try: self.receiver_connected.send(self, receiver=receiver, sender=sender, weak=weak) except: self.disconnect(receiver, sender) raise if receiver_connected.receivers and self is not receiver_connected: try: receiver_connected.send(self, receiver_arg=receiver, sender_arg=sender, weak_arg=weak) except: self.disconnect(receiver, sender) raise return receiver def connect_via(self, sender, weak=False): """Connect the decorated function as a receiver for *sender*. :param sender: Any object or :obj:`ANY`. The decorated function will only receive :meth:`send` emissions sent by *sender*. If ``ANY``, the receiver will always be notified. A function may be decorated multiple times with differing *sender* values. :param weak: If true, the Signal will hold a weakref to the decorated function and automatically disconnect when *receiver* goes out of scope or is garbage collected. Unlike :meth:`connect`, this defaults to False. The decorated function will be invoked by :meth:`send` with `sender=` as a single positional argument and any \*\*kwargs that were provided to the call to :meth:`send`. .. versionadded:: 1.1 """ def decorator(fn): self.connect(fn, sender, weak) return fn return decorator @contextmanager def connected_to(self, receiver, sender=ANY): """Execute a block with the signal temporarily connected to *receiver*. :param receiver: a receiver callable :param sender: optional, a sender to filter on This is a context manager for use in the ``with`` statement. It can be useful in unit tests. *receiver* is connected to the signal for the duration of the ``with`` block, and will be disconnected automatically when exiting the block: .. testsetup:: from __future__ import with_statement from blinker import Signal on_ready = Signal() receiver = lambda sender: None .. testcode:: with on_ready.connected_to(receiver): # do stuff on_ready.send(123) .. versionadded:: 1.1 """ self.connect(receiver, sender=sender, weak=False) try: yield None except: self.disconnect(receiver) raise else: self.disconnect(receiver) def temporarily_connected_to(self, receiver, sender=ANY): """An alias for :meth:`connected_to`. :param receiver: a receiver callable :param sender: optional, a sender to filter on .. versionadded:: 0.9 .. versionchanged:: 1.1 Renamed to :meth:`connected_to`. ``temporarily_connected_to`` was deprecated in 1.2 and will be removed in a subsequent version. """ warn("temporarily_connected_to is deprecated; " "use connected_to instead.", DeprecationWarning) return self.connected_to(receiver, sender) def send(self, *sender, **kwargs): """Emit this signal on behalf of *sender*, passing on \*\*kwargs. Returns a list of 2-tuples, pairing receivers with their return value. The ordering of receiver notification is undefined. :param \*sender: Any object or ``None``. If omitted, synonymous with ``None``. Only accepts one positional argument. :param \*\*kwargs: Data to be sent to receivers. """ # Using '*sender' rather than 'sender=None' allows 'sender' to be # used as a keyword argument- i.e. it's an invisible name in the # function signature. if len(sender) == 0: sender = None elif len(sender) > 1: raise TypeError('send() accepts only one positional argument, ' '%s given' % len(sender)) else: sender = sender[0] if not self.receivers: return [] else: return [(receiver, receiver(sender, **kwargs)) for receiver in self.receivers_for(sender)] def has_receivers_for(self, sender): """True if there is probably a receiver for *sender*. Performs an optimistic check only. Does not guarantee that all weakly referenced receivers are still alive. See :meth:`receivers_for` for a stronger search. """ if not self.receivers: return False if self._by_sender[ANY_ID]: return True if sender is ANY: return False return hashable_identity(sender) in self._by_sender def receivers_for(self, sender): """Iterate all live receivers listening for *sender*.""" # TODO: test receivers_for(ANY) if self.receivers: sender_id = hashable_identity(sender) if sender_id in self._by_sender: ids = (self._by_sender[ANY_ID] | self._by_sender[sender_id]) else: ids = self._by_sender[ANY_ID].copy() for receiver_id in ids: receiver = self.receivers.get(receiver_id) if receiver is None: continue if isinstance(receiver, WeakTypes): strong = receiver() if strong is None: self._disconnect(receiver_id, ANY_ID) continue receiver = strong yield receiver def disconnect(self, receiver, sender=ANY): """Disconnect *receiver* from this signal's events. :param receiver: a previously :meth:`connected` callable :param sender: a specific sender to disconnect from, or :obj:`ANY` to disconnect from all senders. Defaults to ``ANY``. """ if sender is ANY: sender_id = ANY_ID else: sender_id = hashable_identity(sender) receiver_id = hashable_identity(receiver) self._disconnect(receiver_id, sender_id) if ('receiver_disconnected' in self.__dict__ and self.receiver_disconnected.receivers): self.receiver_disconnected.send(self, receiver=receiver, sender=sender) def _disconnect(self, receiver_id, sender_id): if sender_id == ANY_ID: if self._by_receiver.pop(receiver_id, False): for bucket in self._by_sender.values(): bucket.discard(receiver_id) self.receivers.pop(receiver_id, None) else: self._by_sender[sender_id].discard(receiver_id) self._by_receiver[receiver_id].discard(sender_id) def _cleanup_receiver(self, receiver_ref): """Disconnect a receiver from all senders.""" self._disconnect(receiver_ref.receiver_id, ANY_ID) def _cleanup_sender(self, sender_ref): """Disconnect all receivers from a sender.""" sender_id = sender_ref.sender_id assert sender_id != ANY_ID self._weak_senders.pop(sender_id, None) for receiver_id in self._by_sender.pop(sender_id, ()): self._by_receiver[receiver_id].discard(sender_id) def _cleanup_bookkeeping(self): """Prune unused sender/receiver bookeeping. Not threadsafe. Connecting & disconnecting leave behind a small amount of bookeeping for the receiver and sender values. Typical workloads using Blinker, for example in most web apps, Flask, CLI scripts, etc., are not adversely affected by this bookkeeping. With a long-running Python process performing dynamic signal routing with high volume- e.g. connecting to function closures, "senders" are all unique object instances, and doing all of this over and over- you may see memory usage will grow due to extraneous bookeeping. (An empty set() for each stale sender/receiver pair.) This method will prune that bookeeping away, with the caveat that such pruning is not threadsafe. The risk is that cleanup of a fully disconnected receiver/sender pair occurs while another thread is connecting that same pair. If you are in the highly dynamic, unique receiver/sender situation that has lead you to this method, that failure mode is perhaps not a big deal for you. """ for mapping in (self._by_sender, self._by_receiver): for _id, bucket in list(mapping.items()): if not bucket: mapping.pop(_id, None) def _clear_state(self): """Throw away all signal state. Useful for unit tests.""" self._weak_senders.clear() self.receivers.clear() self._by_sender.clear() self._by_receiver.clear() receiver_connected = Signal("""\ Sent by a :class:`Signal` after a receiver connects. :argument: the Signal that was connected to :keyword receiver_arg: the connected receiver :keyword sender_arg: the sender to connect to :keyword weak_arg: true if the connection to receiver_arg is a weak reference .. deprecated:: 1.2 As of 1.2, individual signals have their own private :attr:`~Signal.receiver_connected` and :attr:`~Signal.receiver_disconnected` signals with a slightly simplified call signature. This global signal is planned to be removed in 1.6. """) class NamedSignal(Signal): """A named generic notification emitter.""" def __init__(self, name, doc=None): Signal.__init__(self, doc) #: The name of this signal. self.name = name def __repr__(self): base = Signal.__repr__(self) return "%s; %r>" % (base[:-1], self.name) class Namespace(dict): """A mapping of signal names to signals.""" def signal(self, name, doc=None): """Return the :class:`NamedSignal` *name*, creating it if required. Repeated calls to this function will return the same signal object. """ try: return self[name] except KeyError: return self.setdefault(name, NamedSignal(name, doc)) class WeakNamespace(WeakValueDictionary): """A weak mapping of signal names to signals. Automatically cleans up unused Signals when the last reference goes out of scope. This namespace implementation exists for a measure of legacy compatibility with Blinker <= 1.2, and may be dropped in the future. .. versionadded:: 1.3 """ def signal(self, name, doc=None): """Return the :class:`NamedSignal` *name*, creating it if required. Repeated calls to this function will return the same signal object. """ try: return self[name] except KeyError: return self.setdefault(name, NamedSignal(name, doc)) signal = Namespace().signal