API

pydispatch.dispatch module

class Dispatcher(*args, **kwargs)

Bases: object

Core class used to enable all functionality in the library

Interfaces with Event and Property objects upon instance creation.

Events can be created by calling register_event() or by the subclass definition:

class Foo(Dispatcher):
    _events_ = ['awesome_event', 'on_less_awesome_event']

Once defined, an event can be dispatched to listeners by calling emit().

bind(**kwargs)

Subscribes to events or to Property updates

Keyword arguments are used with the Event or Property names as keys and the callbacks as values:

class Foo(Dispatcher):
    name = Property()

foo = Foo()

foo.bind(name=my_listener.on_foo_name_changed)
foo.bind(name=other_listener.on_name,
         value=other_listener.on_value)

The callbacks are stored as weak references and their order is not maintained relative to the order of binding.

emission_lock(name)

Holds emission of events and dispatches the last event on release

The context manager returned will store the last event data called by emit() and prevent callbacks until it exits. On exit, it will dispatch the last event captured (if any):

class Foo(Dispatcher):
    _events_ = ['my_event']

def on_my_event(value):
    print(value)

foo = Foo()
foo.bind(my_event=on_my_event)

with foo.emission_lock('my_event'):
    foo.emit('my_event', 1)
    foo.emit('my_event', 2)

>>> 2

Parameters:name (str) – The name of the Event or Property Returns:A context manager to be used by the with statement.

If available, this will also be an async context manager to be used with the async with statement (see PEP 492).

Note

The context manager is re-entrant, meaning that multiple calls to this method within nested context scopes are possible.

emit(name, *args, **kwargs)

Dispatches an event to any subscribed listeners

Note

If a listener returns False, the event will stop dispatching to other listeners. Any other return value is ignored.

Parameters:

• name (str) – The name of the Event to dispatch
• *args (Optional) – Positional arguments to be sent to listeners
• **kwargs (Optional) – Keyword arguments to be sent to listeners

register_event(*names)

Registers new events after instance creation

Parameters:*names (str) – Name or names of the events to register

unbind(*args)

Unsubscribes from events or Property updates

Multiple arguments can be given. Each of which can be either the method that was used for the original call to bind() or an instance object.

If an instance of an object is supplied, any previously bound Events and Properties will be ‘unbound’.

class Event(name)

Bases: object

Holds references to event names and subscribed listeners

This is used internally by Dispatcher.

__call__(*args, **kwargs)

Dispatches the event to listeners

Called by emit()

pydispatch.properties module

Properties

Property objects can be defined on subclasses of Dispatcher to create instance attributes that act as events when their values change:

from pydispatch import Dispatcher, Property

class Foo(Dispatcher):
    name = Property()
    value = Property()
def __str__(self):
    return self.__class__.__name__

class Listener(object):
    def on_foo_name(self, instance, value, **kwargs):
        print("{}'s name is {}".format(instance, value))
    def on_foo_value(self, instance, value, **kwargs):
        print('{} = {}'.format(instance, value))

foo_obj = Foo()
listener_obj = Listener()

foo_obj.bind(name=listener_obj.on_foo_name, value=listener_obj.on_foo_value)

foo_obj.name = 'bar'
# Foo's name is bar

foo_obj.value = 42
# Foo = 42

Type checking is not enforced, so values can be any valid python type. Values are however checked for equality to avoid dispatching events for no reason. If custom objects are used as values, they must be able to support equality checking. In most cases, this will be handled automatically.

class Property(default=None)

Bases: object

Defined on the class level to create an observable attribute

Parameters:default (Optional) – If supplied, this will be the default value of the Property for all instances of the class. Otherwise None

name

str – The name of the Property as defined in the class definition. This will match the attribute name for the Dispatcher instance.

_on_change(obj, old, value, **kwargs)

Called internally to emit changes from the instance object

The keyword arguments here will be passed to callbacks through the instance object’s emit() method.

Keyword Arguments:  

• property – The Property instance. This is useful if multiple properties are bound to the same callback. The attribute name
• keys (optional) – If the Property is a container type (ListProperty or DictProperty), the changes may be found here. This is not implemented for nested containers and will only be available for operations that do not alter the size of the container.

class ListProperty(default=None, copy_on_change=False)

Bases: pydispatch.properties.Property

Property with a list type value

Parameters:

• default (Optional) – If supplied, this will be the default value of the Property for all instances of the class. Otherwise None
• copy_on_change (bool, optional) – If True, the list will be copied when contents are modified. This can be useful for observing the original state of the list from within callbacks. The copied (original) state will be available from the keyword argument ‘old’. The default if False (for performance and memory reasons).

Changes to the contents of the list are able to be observed through ObservableList.

class DictProperty(default=None, copy_on_change=False)

Bases: pydispatch.properties.Property

Property with a dict type value

Parameters:

• default (Optional) – If supplied, this will be the default value of the Property for all instances of the class. Otherwise None
• copy_on_change (bool, optional) – If True, the dict will be copied when contents are modified. This can be useful for observing the original state of the dict from within callbacks. The copied (original) state will be available from the keyword argument ‘old’. The default if False (for performance and memory reasons).

Changes to the contents of the dict are able to be observed through ObservableDict.