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Add events to object methods and attributes

Project description

Listen methods “before” and “after” events and attributes “on_get”, “on_change”, “on_set”, “on_del” events.

  • Easy to use subject/observer pattern
  • Conditionnal events within arguments, values, types… or user defined functions.
  • Attributes support events: “on_get”, “on_change”,”on_set”, “on_del”
  • Methods support events “before” and “after”
  • Precise callbacks inheritance (see Subject)
  • Statically and dynamically customizable (via inheritance, visitors, decorators…)
Can listen for events at 3 levels (by order of execution):
  • Descriptor Class: for all Attribute or Method types
  • Descriptor Instance: for all classes that have an Attribute or a Method
  • Object instance: for the given object attribute value or method

Table of Contents


Install it from pypi:

pip install eventize

or from sources:

git clone
cd eventize
python install


Example 0 - as a simple subject/observer pattern:

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”) You can dig eventize-expect to have a convenient wrapper to create conditions. It adds “” class (like eventize before 0.4.3) thanks to setuptools entry points.

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

def titlecase(event):
  event.content = event.content.title()

class WeirdVisitor(object):
  def visit(self, handler):

  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 remove titlecase
handler -= titlecase
assert titlecase not in handler
# it adds titlecase
handler += titlecase

# Create event with attribute content and trigger it
event1 = handler.notify(content="a string")

assert my_visitor.default == "a string"
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) # it's a tuple since 0.4.1
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 callbacks 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"

Example 1 - 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.

They have 4 main attributes:
  • “subject”: the object instance where event happens
  • “name”: the attribute name of object instance
  • “args”: the list 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),

Example 2 - 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 te 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 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()
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

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

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

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 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

Example 3 - Observers inheritance:

Descriptors in python don’t know their owner until a getter is called. Yet, as they help to define classes, it could be interesting to bind them to their class at class creation.

It’s the aim of Subject decorator. A Subject is a class that contains descriptors handlers (on_get, before…)

Subject makes 2 things:
  • it makes children handlers inheriting their parent handlers observers (parent handlers are found by their attribute name).
  • it calls method handler.bind (if exists) with the owner class as an argument while class is declared.

You can create your own subjects with “events.Subject([descriptor_type1, […]])”.

from eventize import Attribute
from eventize.attribute import Subject, OnSetHandler

def validate_string(event):
  if isinstance(event.value, type('')): return

  message = "%s.%s must be a string!" % (type(event.subject).__name__,
  raise TypeError(message)

def titlecase(event):
  event.value = event.value.title()

class StringAttribute(Attribute):
  on_set = OnSetHandler(validate_string)

# Subject == events.Subject(OnGetHandler, OnSetHandler, OnChangeDescriptor, OnDelDescriptor)
@Subject  # Bind handlers to the class
class Name(StringAttribute):
  on_set = OnSetHandler(titlecase)

class Person(object):
  name = Name('john doe')

john = Person()

validation_fails = False
try: = 0x007
except TypeError:
  validation_fails = True

assert validation_fails, "Validation should fail"
assert == 'John Doe'  # Name is set in title case


Your feedback, code review, improvements or bugs, and help to document is appreciated. You can contact me by mail: apieum [at] gmail [dot] com

Test recommended requirements:

pip install -r dev-requirements.txt

Sometimes –spec-color doesn’t function. Uninstall nosespec and nosecolor then reinstall nosecolor and nosespec separatly in this order (nosecolor first).

Launch tests:

git clone
cd eventize
nosetests --with-spec --spec-color ./eventize
# or with watch
# nosetests --with-spec --spec-color --with-watch ./eventize

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