python FSM Fysom callbacks on the other way

Question

I am using Fysom to create FSM. I want to use callbacks in an other way:

TABLE = {
'initial': 'OFF',
'events': [{'name': 'load', 'src': 'OFF', 'dst': 'LOADED'},],
'callbacks': {'onload': myfunction,}}

fsm = Fysom(TABLE)

Here if I launch fsm.onload() it will execute myfunction. Instead I want If I launch myfunction() it will lunch fsm.onload().

I took a look on the script and related part is here:

def _enter_state(self, e):
    '''
        Executes the callback for onenter_state_ or on_state_.
    '''
    for fnname in ['onenter' + e.dst, 'on' + e.dst]:
        if hasattr(self, fnname):
            return getattr(self, fnname)(e)

I don't see how to change this peace of code for my purpose.

score 4 · Accepted Answer · edited May 18 '14 at 13:17

4

You cannot implement callbacks "in the other direction" without touching myfunction. A callback is actually an inverted call (hollywood principle), so the inverse of a callback is a simple call.

What this means is that myfunction should call the state machine transition directly. As a consequence, the state machine object must be in the scope of myfunction.

At this point you have some possibilities for implementation :

Have myfunction be a closure :

def outer():
    fsm = Fysom(TABLE)
    def myfunction():
        print("I call the state machine transition when called")
        fsm.onload()
    return fsm

Use a global state machine or scope it in a class :

class Foo(object):

    def __init__(self):
        self.fsm = Fysom(TABLE)

    def my_method(self):
        self.fsm.onload()

Create a decorator that calls the state transition method before/after executing the function. Then decorate away with
```
fsm = Fysom(TABLE)

@transition(fsm, "onload")
def myfunction():
    pass
```
Note that this also requires fsm to be defined in this scope.

edited May 18 '14 at 13:17

Ethan Furman

63,992
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answered May 13 '14 at 16:56

user3633465

106
1

First thanks' for help. I am using the 2nd point but actually I don't want to call it in my_method (that's the reason of my question). What I want is to define all function/method which will launch `self.fsm.onload()` (automatically) when called. And I want to define it in `TABLE`. As you said I cannot implement it without changing `myfunction`. Maybe I can if I change something in constructor (__init__) of class Foo and link between Foo and Fysom ? 3rd point: To my mind the decorator is exactly the same thing as calling it into the function. – Katsu May 14 '14 at 07:41
I don't see a way to do that right now. The fysom could probably rewrite the code of `myfunction` so that it calls `onload`, but that would be really messy. Maybe you can put a list of method names in `TABLE`, and then use `getattr`/`setattr` in the class constructor to replace the methods? That would look a lot like the decorated version, as you mentioned. – user3633465 May 14 '14 at 08:06
Ok so I finally choose the decorated version. It's easier because I don't really see how to implement with getattr/setattr version. Thank you for help. – Katsu May 15 '14 at 07:00

python FSM Fysom callbacks on the other way

1 Answers1