Blinker Documentation — Blinker

# Blinker Documentation

Blinker provides fast & simple object-to-object and broadcast signaling for Python objects.

The core of Blinker is quite small but provides powerful features:

> - a global registry of named signals
> - anonymous signals
> - custom name registries
> - permanently or temporarily connected receivers
> - automatically disconnected receivers via weak referencing
> - sending arbitrary data payloads
> - collecting return values from signal receivers
> - thread safety

Blinker was written by Jason Kirtand and is provided under the MIT License. The library supports Python 2.4 or later; Python 3.0 or later; or Jython 2.5 or later; or PyPy 1.6 or later.

## Decoupling With Named Signals

Named signals are created with `signal()`:

    >>> from blinker import signal
    >>> initialized = signal('initialized')
    >>> initialized is signal('initialized')
    True

Every call to `signal('name')` returns the same signal object, allowing unconnected parts of code (different modules, plugins, anything) to all use the same signal without requiring any code sharing or special imports.

## Subscribing to Signals

`Signal.connect()` registers a function to be invoked each time the signal is emitted. Connected functions are always passed the object that caused the signal to be emitted.

    >>> def subscriber(sender):
    ...     print("Got a signal sent by %r" % sender)
    ...
    >>> ready = signal('ready')
    >>> ready.connect(subscriber)
    <function subscriber at 0x...>

## Emitting Signals

Code producing events of interest can `Signal.send()` notifications to all connected receivers.

Below, a simple `Processor` class emits a `ready` signal when it’s about to process something, and `complete` when it is done. It passes `self` to the `send()` method, signifying that that particular instance was responsible for emitting the signal.

    >>> class Processor:
    ...    def __init__(self, name):
    ...        self.name = name
    ...
    ...    def go(self):
    ...        ready = signal('ready')
    ...        ready.send(self)
    ...        print("Processing.")
    ...        complete = signal('complete')
    ...        complete.send(self)
    ...
    ...    def __repr__(self):
    ...        return '<Processor %s>' % self.name
    ...
    >>> processor_a = Processor('a')
    >>> processor_a.go()
    Got a signal sent by <Processor a>
    Processing.

Notice the `complete` signal in `go()`? No receivers have connected to `complete` yet, and that’s a-ok. Calling `send()` on a signal with no receivers will result in no notifications being sent, and these no-op sends are optimized to be as inexpensive as possible.

## Subscribing to Specific Senders

The default connection to a signal invokes the receiver function when any sender emits it. The `Signal.connect()` function accepts an optional argument to restrict the subscription to one specific sending object:

    >>> def b_subscriber(sender):
    ...     print("Caught signal from processor_b.")
    ...     assert sender.name == 'b'
    ...
    >>> processor_b = Processor('b')
    >>> ready.connect(b_subscriber, sender=processor_b)
    <function b_subscriber at 0x...>

This function has been subscribed to `ready` but only when sent by `processor_b`:

    >>> processor_a.go()
    Got a signal sent by <Processor a>
    Processing.
    >>> processor_b.go()
    Got a signal sent by <Processor b>
    Caught signal from processor_b.
    Processing.

## Sending and Receiving Data Through Signals

Additional keyword arguments can be passed to `send()`. These will in turn be passed to the connected functions:

    >>> send_data = signal('send-data')
    >>> @send_data.connect
    ... def receive_data(sender, **kw):
    ...     print("Caught signal from %r, data %r" % (sender, kw))
    ...     return 'received!'
    ...
    >>> result = send_data.send('anonymous', abc=123)
    Caught signal from 'anonymous', data {'abc': 123}

The return value of `send()` collects the return values of each connected function as a list of (`receiver function`, `return value`) pairs:

    >>> result
    [(<function receive_data at 0x...>, 'received!')]

## Anonymous Signals

Signals need not be named. The `Signal` constructor creates a unique signal each time it is invoked. For example, an alternative implementation of the Processor from above might provide the processing signals as class attributes:

    >>> from blinker import Signal
    >>> class AltProcessor:
    ...    on_ready = Signal()
    ...    on_complete = Signal()
    ...
    ...    def __init__(self, name):
    ...        self.name = name
    ...
    ...    def go(self):
    ...        self.on_ready.send(self)
    ...        print("Alternate processing.")
    ...        self.on_complete.send(self)
    ...
    ...    def __repr__(self):
    ...        return '<AltProcessor %s>' % self.name
    ...

## `connect` as a Decorator

You may have noticed the return value of `connect()` in the console output in the sections above. This allows `connect` to be used as a decorator on functions:

    >>> apc = AltProcessor('c')
    >>> @apc.on_complete.connect
    ... def completed(sender):
    ...     print "AltProcessor %s completed!" % sender.name
    ...
    >>> apc.go()
    Alternate processing.
    AltProcessor c completed!

While convenient, this form unfortunately does not allow the `sender` or `weak` arguments to be customized for the connected function. For this, `connect_via()` can be used:

    >>> dice_roll = signal('dice_roll')
    >>> @dice_roll.connect_via(1)
    ... @dice_roll.connect_via(3)
    ... @dice_roll.connect_via(5)
    ... def odd_subscriber(sender):
    ...     print("Observed dice roll %r." % sender)
    ...
    >>> result = dice_roll.send(3)
    Observed dice roll 3.

## Optimizing Signal Sending

Signals are optimized to send very quickly, whether receivers are connected or not. If the keyword data to be sent with a signal is expensive to compute, it can be more efficient to check to see if any receivers are connected first by testing the `receivers` property:

    >>> bool(signal('ready').receivers)
    True
    >>> bool(signal('complete').receivers)
    False
    >>> bool(AltProcessor.on_complete.receivers)
    True

Checking for a receiver listening for a particular sender is also possible:

    >>> signal('ready').has_receivers_for(processor_a)
    True

## Documenting Signals

Both named and anonymous signals can be passed a `doc` argument at construction to set the pydoc help text for the signal. This documentation will be picked up by most documentation generators (such as sphinx) and is nice for documenting any additional data parameters that will be sent down with the signal.

See the documentation of the `receiver_connected` built-in signal for an example.

## API Documentation

All public API members can (and should) be imported from `blinker`:

    from blinker import ANY, signal

### Basic Signals

`blinker.base.` `ANY` = ANY

  Token for “any sender”.

`blinker.base.` `receiver_connected` = <blinker.base.Signal object at 0x1041516d0>

  Sent by a `Signal` after a receiver connects.

  Argument : | the Signal that was connected to
  Parameters: | receiver_arg – the connected receiver sender_arg – the sender to connect to weak_arg – true if the connection to receiver_arg is a weak reference

  Deprecated since version 1.2.

  As of 1.2, individual signals have their own private `receiver_connected` and `receiver_disconnected` signals with a slightly simplified call signature. This global signal is planned to be removed in 1.6.

class `blinker.base.` `Signal`(doc=None)

  A notification emitter.

  Parameters: | doc – optional. If provided, will be assigned to the signal’s __doc__ attribute.

  `ANY` = ANY

    An `ANY` convenience synonym, allows `Signal.ANY` without an additional import.

  `receiver_connected`

    Emitted after each `connect()`.

    The signal sender is the signal instance, and the `connect()` arguments are passed through: receiver, sender, and weak.

    New in version 1.2.

  `receiver_disconnected`

    Emitted after `disconnect()`.

    The sender is the signal instance, and the `disconnect()` arguments are passed through: receiver and sender.

    Note, this signal is emitted only when `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 `receiver_connected` and setting up a custom weakref cleanup callback on weak receivers and senders.

    New in version 1.2.

  `receivers` = None

    A mapping of connected receivers.

    The values of this mapping are not meaningful outside of the internal `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.

  `connect`(receiver, sender=ANY, weak=True)

    Connect receiver to signal events sent by sender.

    Parameters: | receiver – A callable. Will be invoked by `send()` with sender= as a single positional argument and any **kwargs that were provided to a call to `send()`. sender – Any object or `ANY`, defaults to `ANY`. Restricts notifications delivered to receiver to only those `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 `connect()`. 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.

  `connect_via`(sender, weak=False)

    Connect the decorated function as a receiver for sender.

    Parameters: | sender – Any object or `ANY`. The decorated function will only receive `send()` emissions sent by sender. If `ANY`, the receiver will always be notified. A function may be decorated multiple times with differing sender values. 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 `connect()`, this defaults to False.

    The decorated function will be invoked by `send()` with
      sender= as a single positional argument and any **kwargs that were provided to the call to `send()`.

    New in version 1.1.

  `connected_to`(*args, **kwds)

    Execute a block with the signal temporarily connected to receiver.

    Parameters: | receiver – a receiver callable 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:

        with on_ready.connected_to(receiver):
           # do stuff
           on_ready.send(123)

    New in version 1.1.

  `disconnect`(receiver, sender=ANY)

    Disconnect receiver from this signal’s events.

    Parameters: | receiver – a previously `connected` callable sender – a specific sender to disconnect from, or `ANY` to disconnect from all senders. Defaults to `ANY`.

  `has_receivers_for`(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 `receivers_for()` for a stronger search.

  `receivers_for`(sender)

    Iterate all live receivers listening for sender.

  `send`(*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.

    Parameters: | *sender – Any object or `None`. If omitted, synonymous with `None`. Only accepts one positional argument. **kwargs – Data to be sent to receivers.

  `temporarily_connected_to`(receiver, sender=ANY)

    An alias for `connected_to()`.

    Parameters: | receiver – a receiver callable sender – optional, a sender to filter on

    New in version 0.9.

    Changed in version 1.1: Renamed to `connected_to()`. `temporarily_connected_to` was deprecated in 1.2 and will be removed in a subsequent version.

### Named Signals

`blinker.base.` `signal`(name, doc=None)

  Return the `NamedSignal` name, creating it if required.

  Repeated calls to this function will return the same signal object. Signals are created in a global `Namespace`.

class `blinker.base.` `NamedSignal`(name, doc=None)

  Bases: `blinker.base.Signal`

  A named generic notification emitter.

  `name` = None

    The name of this signal.

class `blinker.base.` `Namespace`

  Bases: `dict`

  A mapping of signal names to signals.

  `signal`(name, doc=None)

    Return the `NamedSignal` name, creating it if required.

    Repeated calls to this function will return the same signal object.

class `blinker.base.` `WeakNamespace`(*args, **kw)

  Bases: `weakref.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.

  New in version 1.3.

  `signal`(name, doc=None)

    Return the `NamedSignal` name, creating it if required.

    Repeated calls to this function will return the same signal object.

# Blinker Changelog

## Version 1.4

Released July 23, 2015

- Verified Python 3.4 support (no changes needed)
- Additional bookkeeping cleanup for non-ANY connections at disconnect time.
- Added Signal._cleanup_bookeeping() to prune stale bookkeeping on demand

## Version 1.3

Released July 3, 2013

- The global signal stash behind blinker.signal() is now backed by a regular name-to-Signal dictionary. Previously, weak references were held in the mapping and ephermal usage in code like `signal('foo').connect(...)` could have surprising program behavior depending on import order of modules.
- blinker.Namespace is now built on a regular dict. Use blinker.WeakNamespace for the older, weak-referencing behavior.
- Signal.connect(‘text-sender’) uses an alterate hashing strategy to avoid sharp edges in text identity.

## Version 1.2

Released October 26, 2011

- Added Signal.receiver_connected and Signal.receiver_disconnected per-Signal signals.
- Deprecated the global ‘receiver_connected’ signal.
- Verified Python 3.2 support (no changes needed!)

## Version 1.1

Released July 21, 2010

- Added `@signal.connect_via(sender)` decorator
- Added `signal.connected_to` shorthand name for the `temporarily_connected_to` context manager.

## Version 1.0

Released March 28, 2010

- Python 3.0 and 3.1 compatibility

## Version 0.9

Released February 26, 2010

- Added `Signal.temporarily_connected_to` context manager
- Docs! Sphinx docs, project web site.

## Version 0.8

Released February 14, 2010

- Initial release
- Extracted from flatland.util.signals
- Added Python 2.4 compatibility
- Added nearly functional Python 3.1 compatibility (everything except connecting to instance methods seems to work.)