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Add events to object methods and attributes

Project description

Add events to object methods and attributes.

Events are triggered at 3 levels in order:
  • Descriptor Class: for all Attribute or Method types (from version 0.3)
  • Descriptor Instance: for all classes that have an Attribute or a Method
  • Object instance: for the given object attribute value or method

Methods support events “before” and “after” Attributes support events: “on_get”, “on_set”, “on_del”

Events can be triggered conditionaly within arguments or user condition.

Since version 0.3, observers defined at Descriptor Instance level are preserved in descriptor container class children, providing inheritance. see example 4

[!] Take Care Api has changed from version 3.1.

I have to apologize the first design idea of setting properties “on_get”, “on_set”, “on_del”, “before” and “after” directly on class/object attributes/methods was stupid.

From version 0.4 you will not be able to access these properties in object instances.

So please start using “handle”, “on_get”, “on_set”, “on_del”, “before” and “after” functions, like described in this documentation.

Table of Contents


Install it from pypi:

pip install eventize

or from sources:

git clone
cd eventize
python install


Example 1 - observe a method:

For compatibility with older versions there is a decorator named “ObservedMethod” wich returns a method with “before” and “after” properties. Function “handle” provides a similar result except you can specify the handler type as optionnal third argment. It’s simpler to use directly functions “handle”, “before”, and “after” as shown here.

from eventize import before, after
from import Expect

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

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

  def not_valid(self, event):
    # can do:
    # event.subject.valid = not event.subject.valid
    # equivalent to
    self.valid = not self.valid

class Logger(list):
  def log_before(self, event):
    self.append(self.message('before', *event.args, is_valid=event.subject.valid))

  def log_after(self, event):
    self.append(self.message('after', *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'])

my_object = Observed()
my_logs = Logger()
called_with_permute = Expect.arg('permute')

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

Example 2 - observe an attribute:

Like for methods, you can still use “ObservedAttribute” to declare directly an attribute (see ex. 4) or to decorate an attribute. New api at version 0.3.1, provides “handle”, “on_get”, “on_set” and “on_del” functions to add events on attributes. As I had to provide ‘on_set’, ‘on_get’, ‘on_del’ on object instance observed attributes, each times you were setting an observed attribute, its value was replaced by a wrapper which causes matters for constants like booleans or None (ex 3). This behaviour will be removed soon (version 0.4) so prefer use new api which will hide all this mecanic.

from eventize import handle
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):
    self.append(self.message('on_get',, event.value.valid))
  def log_set(self, event):
    self.append(self.message('on_set',, event.value.valid))
  def log_del(self, event):
    self.append(self.message('on_del',, event.value.valid))

  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()
# Note: order matter here !
my_object_validate = handle(my_object, 'validate')
my_object_validate.on_del += my_logs.log_del
my_object_validate.on_set += my_logs.log_set
my_object_validate.on_get += my_logs.log_get

Observed_validate = handle(Observed, 'validate')
Observed_validate.on_set += my_logs.log_set
Observed_validate.on_del += my_logs.log_del
Observed_validate.on_get += my_logs.log_get

assert my_object.validate() == False, 'Default value was not set'
setattr(my_object, 'validate', Validator(True))
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_del', 'validate', True),   # Called at class level
  my_logs.message('on_del', 'validate', True),   # Called at instance level

Example 3 - observe an attribute for non overridable types:

Note (will change soon):

If can’t set attributes (when setattr fails for on_get) to Attribute value

-> Handler try to subtype value.

If value can’t be subtyped (for non overridable type like None, Booleans…)

-> Handler returns value as is.

This means you can’t call on_get, on_set, or on_del on instance.

Yet, you can do this at class level, with handler conditional method ‘when’

For more information about Expect and how it functions have a look at inxpect package:

from eventize import on_set
from import Expect

class Observed(object):
  valid = False

class Logger(list):
  def log_set(self, event):
    self.append(self.message('on_set',, event.value))

  def log_set_error(self, event):
    self.append(self.message('on_set_error',, 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()
other_object = Observed()
my_logs = Logger()

dont_change_value = lambda event: setattr(event, 'value', event.subject.valid)
value_is_not_bool = Expect.value.type_is_not(bool)
subject_is_my_object = Expect.subject(my_object)

getting_my_object = on_set(Observed, 'valid').when(subject_is_my_object)
getting_my_object += my_logs.log_set  # (1)
getting_my_object.when(value_is_not_bool).do(my_logs.log_set_error).then(dont_change_value)  # (2)

my_object.valid = True  # (1)
my_object.valid = None  # (2)
other_object.valid = True  # Trigger no event
other_object.valid = None  # Trigger no event

assert my_object.valid == True  # (2) -> dont_change_value

assert my_logs == [
  my_logs.message('on_set', 'valid', True),
  my_logs.message('on_set', 'valid', None),
  my_logs.message('on_set_error', 'valid', None),

Example 4 - 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 make 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.

Here we’ll see only how observers inheritance is done.

from eventize.attribute import Attribute, AttributeHandler, AttributeSubject

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 = AttributeHandler(validate_string)

@AttributeSubject  # Bind handlers to the class -> this is the way inheritance is done
class NameAttribute(StringAttribute):
  on_set = AttributeHandler(titlecase)

class Person(object):
  name = NameAttribute('doe')

john = Person()

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

assert validation_fails
assert == 'Doe'  # Name is auto magically set in title case

Example 5 - Choose your handler:

Illustrate the use of the third optionnal argument of “handle”, “on_get”, “on_set”, “on_del”, “before” and “after”

from eventize.method import Method, MethodHandler
from eventize import before

def first_arg_is_string(event):
    if isinstance(event.args[0], type('')): return
    raise TypeError("First arg must be a string!")

def titlecase(event):
    # args are a tuple
    args = list(event.args)
    args[0] = args[0].title()
    event.args = tuple(args)

class FirstArgIsStringMethod(Method):
    before = MethodHandler(first_arg_is_string)

class Person(object):
    def __init__(self, name):

    def set_name(self, name): = name

# calling before with FirstArgIsStringMethod
before(Person, 'set_name', FirstArgIsStringMethod).do(titlecase)

validation_fails = False
except TypeError:
    validation_fails = True

john = Person("john doe")

assert validation_fails, "Validation should fail"
assert == 'John Doe'  # Name is auto magically 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

Launch test:

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

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