Dynamic on_enter/exit_* methods

[TheMysteriousX]

The documentation says:

# Callbacks can also be added after initialization using 
# the dynamically added on_enter_ and on_exist_ methods.

I haven't looked into this in detail yet but a quick scan doesn't show a route through the code that creates on_enter/on_exit methods for each state after init has run.

Has this ever worked?

>>> from transitions import State, Machine
>>>
>>> class Matter(object):
...     def say_hello(self): print("hello, new state!")
...     def say_goodbye(self): print("goodbye, old state!")
...
>>> lump = Matter()
>>>
>>> states = [
...     State(name='solid', on_exit=['say_goodbye']),
...     'liquid',
...     { 'name': 'gas' }
...     ]
>>>
>>> machine = Machine(lump, states=states)
>>> machine.add_transition('sublimate', 'solid', 'gas')
>>>
>>> lump.on_exit_solid('say_goodbye')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Matter' object has no attribute 'on_exit_solid'
>>>
>>> machine.set_state('solid')
>>> lump.sublimate()

[aleneum]

Hi,

they are created in core.py in __getattr__:

def __getattr__(self, name):
        if name.startswith('__'):
            if name in self.__dict__:
                return self.__dict__[name]
            else:
                raise AttributeError("{} does not exist".format(name))
        terms = name.split('_')
        if terms[0] in ['before', 'after']:
            name = '_'.join(terms[1:])
            if name not in self.events:
                raise MachineError('Event "%s" is not registered.' % name)
            return partial(self.events[name].add_callback, terms[0])

        # that's where the magic happens
        # on_enter / on_exit queries are concatenated with the current state 
        # and predefined with partial
        elif name.startswith('on_enter') or name.startswith('on_exit'):
            state = self.get_state('_'.join(terms[2:]))
            return partial(state.add_callback, terms[1])
        else:
            if name in self.__dict__:
                return self.__dict__[name]
            else:
                raise AttributeError("{} does not exist".format(name))

on_exit_solid has to be called from the machine, not from the model.

machine.on_exit_solid('say_goodbye')

should work.

[aleneum]

Okay,

after some more careful reading what you said and what the documentation states, I guess you are right. Afaik, it does not work the way its stated in the documentation.

[tyarkoni]

Good catch. I'll fix this right now. Thanks @TheMysteriousX!

[aleneum]

in core.Machine.add_states there is a check if an on_enter_State method exists but it is not created otherwise.

#  Add enter/exit callbacks if there are existing bound methods
    enter_callback = 'on_enter_' + state_name
    if hasattr(self.model, enter_callback) and \
               inspect.ismethod(getattr(self.model, enter_callback)):
        state.add_callback('enter', enter_callback)
    exit_callback = 'on_exit_' + state_name
    if hasattr(self.model, exit_callback) and \
               inspect.ismethod(getattr(self.model, exit_callback)):
        state.add_callback('exit', exit_callback)

[tyarkoni]

I assume we don't want to allow on_enter_* to work on the model itself, right? I mean, we definitely don't want to start patching an arbitrary model class's getattr method. So I'm just going to update the README to be clear about this.

[aleneum]

👍 agree