Basic Usage

Simple subject/observer

events.Handler is the base class of all eventize handlers (“on_get”, “before”...)

It is a simple callable list of functions wich receive the argument (of type events.Event) you’ve passed when calling your Handler object.

As a list an Handler support common methods “append”, “remove”, “prepend”, “insert”, “extend”, “empty”..., “__setitem__”, plus some syntactic sugar like “__iadd__” (+=) for append and “__isub__” (-=) for remove.

You can stop event propagation by raising an events.StopPropagation exception which store exception message in “Event.messages” by default.

You can hook event propagation by overriding methods “before_propagation” and “after_propagation” or dynamically change Handler behaviour at creation by passing visitors (object with a method “visit”) see events.EventType visitor for example.

An handler can build its proper events of the class defined in Handler.event_type when calling Handler.make_event (just create and returns an event instanciated with given arguments) or Handler.notify (create event with make_event and propagates it)

You can add conditional handlers by using method “when” or restrict the current handler execution by passing “condition” kwarg argument to constructor. Conditions can be chained with methods “do” or “then” (aliases of “append”)

Each time you trigger an event, it is stored in You can empty past events by calling “clear_events” or all (events and callbacks) with “clear”.

from import Handler
from eventize.typing import Visitor

# a condition
def is_string(event):
  return isinstance(event.content, str)

# an observer
def titlecase(event):
  event.content = event.content.title()

# A visitor
class WeirdVisitor(Visitor):
  def visit(self, handler):
    # it add/prepend observer "save_default"

  def save_default(self, event):
    self.default = event.content

my_visitor = WeirdVisitor()
handler = Handler(titlecase, my_visitor, condition=is_string)

# An Handler is a callable list
assert isinstance(handler, list)
assert callable(handler)

# handler contains 2 callbacks:
assert len(handler) == 2
assert titlecase in handler
assert my_visitor.save_default in handler
# it removes titlecase
handler -= titlecase
assert titlecase not in handler
# it adds titlecase
handler += titlecase

# Create an event with attribute content and trigger it
event1 = handler.notify(content="a string") # each kwarg is converted in event attribute

# result of first observer (added by WeirdVisitor)
assert my_visitor.default == "a string"
# result of all observers (i.e. titlecase here)
assert event1.content == "A String"

# if event.content is not a string propagation is stopped
# these 2 lines are sames as notify
event2 = handler.make_event(content=1234)

assert len( == 2
assert == (event1, event2)
expected_message = "Condition '%s' for event 'Event' return False" % id(is_string)
assert event2.messages[0] == expected_message

# we remove all past events:
assert len( == 0

# we remove all observers and events:
assert len(handler) == 0

is_a_name = lambda event: event.content == "a name"
# create a new subhandler with a condition:
event1 = handler.notify(content="a name")
event2 = handler.notify(content="a string")
# only "a name" is titlecased
assert event1.content == "A Name"
assert event2.content == "a string"

# save_default is called only for event1:
assert my_visitor.default == "a name"

Observe a method

To observe a method, you can:
  • declare your method at class level with “Method” and a function as first argument
  • decorate a method with “Method”
  • use functions “handle”, “before” or “after” on class or object callable attribute with type of event in the optionalthird argument (recommended)

Method events objects are of type BeforeEvent and AfterEvent y default.

They have 4 main attributes:
  • “subject”: the object instance where event happens
  • “name”: the method name of the object instance
  • “args”: the tuple of passed args
  • “kwargs”: the dict of named args
from eventize import before, after
from eventize.method import BeforeEvent, AfterEvent

class Observed(object):
  def __init__(self):
    self.valid = False

  def is_valid(self, *args):
    return self.valid

  def not_valid(self, event):
    assert == "is_valid" # (event subject name)
    assert event.subject == self
    self.valid = not self.valid

class Logger(list):
  def log_before(self, event):
    assert type(event) is BeforeEvent
    self.append(self.message('before %s'  %, *event.args, is_valid=event.subject.valid))

  def log_after(self, event):
    assert type(event) is AfterEvent
    self.append(self.message('after %s' %, *event.args, is_valid=event.subject.valid))

  def message(self, event_name, *args, **kwargs):
    return "%s called with args: '%s', current:'%s'" % (event_name, args, kwargs['is_valid'])

args_have_permute = lambda event: 'permute' in event.args

my_object = Observed()
my_logs = Logger()

before_is_valid = before(my_object, 'is_valid')
before_is_valid += my_logs.log_before
after(my_object, 'is_valid').do(my_logs.log_after)

assert my_object.is_valid() is False
assert my_object.is_valid('permute') is True

assert my_logs == [
  my_logs.message('before is_valid', is_valid=False),
  my_logs.message('after is_valid', is_valid=False),
  my_logs.message('before is_valid', 'permute', is_valid=False),
  my_logs.message('after is_valid', 'permute', is_valid=True),

Observe an attribute

“Attribute” is like “Method”, to observe it you can:
  • declare your attribute at class level with “Attribute” and an optionnal default value as first argument
  • decorate an existing attribute with “Attribute”
  • use functions “handle”, “on_get”, “on_change”, “on_set”, “on_del” on class or object attribute with the type of event on the third argument (recommended)

Attribute events objects are of type OnGetEvent, OnChangeEvent, OnSetEvent, OnDelEvent.

They have 3 main attributes:
  • “subject”: the object instance where event happens
  • “name”: the attribute name of the object instance
  • “value”: the attribute value if set

In addition each kwarg is added to event as an attribute. (like “content” in ex 0)

from eventize import handle, on_get, Attribute
from eventize.attribute import OnGetEvent, OnGetHandler

class Validator(object):
  def __init__(self, is_valid):
    self.valid = is_valid
  def __call__(self):
    return self.valid

class Observed(object):
  validate = Validator(False)

class Logger(list):
  def log_get(self, event):
    assert type(event) is OnGetEvent, "Get event of type %s" % type(event)
    self.append(self.message('on_get',, event.value()))
  def log_change(self, event):
    self.append(self.message('on_change',, event.value()))
  def log_set(self, event):
    self.append(self.message('on_set',, event.value()))
  def log_del(self, event):
    self.append(self.message('on_del',, event.value()))

  def message(self, event_name, attr_name, value):
    return "'%s' called for attribute '%s', with value '%s'" % (event_name, attr_name, value)

my_object = Observed()
my_logs = Logger()
# Adding observers at object instance:
my_object_validate = handle(my_object, 'validate')
my_object_validate.on_get += my_logs.log_get
my_object_validate.on_change += my_logs.log_change
my_object_validate.on_set += my_logs.log_set
my_object_validate.on_del += my_logs.log_del

# Adding observers for all objects of class Observed
Observed_validate = handle(Observed, 'validate')
Observed_validate.on_get += my_logs.log_get
Observed_validate.on_change += my_logs.log_change
Observed_validate.on_set += my_logs.log_set
Observed_validate.on_del += my_logs.log_del

# Trigger on_get event
is_valid = getattr(my_object, 'validate')  # same as my_object.validate
# check if default value is False as defined in class
assert is_valid() == False, '[error] Default value was not set'
# Trigger on_set event
setattr(my_object, 'validate', Validator(True))  # # same as my_object.validate = Validator(True)
# Trigger on_del event
delattr(my_object, 'validate') # same as del my_object.validate

assert my_logs == [
  my_logs.message('on_get', 'validate', False),  # Called at class level
  my_logs.message('on_get', 'validate', False),  # Called at instance level
  my_logs.message('on_set', 'validate', True),   # Called at class level
  my_logs.message('on_set', 'validate', True),   # Called at instance level
  my_logs.message('on_change', 'validate', True),   # Called at class level
  my_logs.message('on_change', 'validate', True),   # Called at instance level
  my_logs.message('on_del', 'validate', True),   # Called at class level
  my_logs.message('on_del', 'validate', True),   # Called at instance level

# You can use your own events types
class OnGetCall(OnGetEvent):
  def returns(self):
    return self.value()

# and override Attribute or Method types
class CallAttr(Attribute):
  # must be redefined otherwise callbacks are appended to class Attribute
  # see example 3 for callbacks inheritance
  on_get = OnGetHandler()

my_object = Observed()
# third argument permits to set new type of attribute
on_get_validate = on_get(my_object, 'validate', CallAttr)
# set event type
on_get_validate.event_type = OnGetCall

assert isinstance(Observed.validate, CallAttr)

# OnGetCall Event returns my_object.validate()
assert my_object.validate is False
assert len(on_get_validate) == 0, "Expect my_object.validate.on_get has no callbacks"

## Defining observers at Attribute level:
# an observer
def set_to_true(event):
  assert type(event) == OnGetCall
  event.value = Validator(True)

# All objects with CallAttr attribute will call set_to_true
CallAttr.on_get += set_to_true

# set_to_true change value and check if event is of type OnGetCall
assert my_object.validate is True

# remove all callbacks and events at descriptor, class and instance level
handle(my_object, 'validate').clear_all()

assert len(CallAttr.on_get) == 0