Exception when used with %autoreload

[xiao-fred]

• Python State Machine version: 0.8.0
• Python version: 3.7.6
• Operating System: macOS 10.15.2

Description

I am using statemachine in a project, with Jupyter notebook and the IPython %autoreload extension.

I have been using Jupyter and autoreload for years, and I haven't met any serious problem until now. But it leads to a systematic exception in statemachine. And unless it can be solved, I will have to give up statemachine, since there is no way I give up Jupyter and autoreload...

What I Did

Create two files:

File 1: my_state_machine.py

from statemachine import StateMachine, State


class MyStateMachine(StateMachine):
    state1 = State('State1', initial=True)
    state2 = State('State2')
    state3 = State('State3')

    transition12 = state1.to(state2)
    transition23 = state2.to(state3)

    def do_something(self):
        print('something')

File 2: Jupyter notebook
Create 2 cells:
Cell #1

%load_ext autoreload
%autoreload 2

from my_state_machine import MyStateMachine

Cell #2

state_machine = MyStateMachine()
state_machine.transition12()

Execute both cells (cell #1 then cell #2). It's working. Fine.
Execute cell #2 once again. It's working too. Fine.

Now modify my_state_machine.py. No need to modify the states or transitions. Just modify the do_something() method:

def do_something(self):
    print('something else')

Now run cell #2 once again. Autoreload automatically reloads the statemachine module. But obviously something goes wrong, and systematically leads to an exception:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-12-50d5b3013664> in <module>
      1 state_machine = MyStateMachine()
----> 2 state_machine.transition12()

~/opt/anaconda3/envs/tf20/lib/python3.7/site-packages/statemachine/statemachine.py in __call__(self, *args, **kwargs)
     61 
     62     def __call__(self, *args, **kwargs):
---> 63         return self.func(*args, **kwargs)
     64 
     65 

~/opt/anaconda3/envs/tf20/lib/python3.7/site-packages/statemachine/statemachine.py in transition_callback(*args, **kwargs)
     89         def transition_callback(*args, **kwargs):
     90             print('!@#$ Transition.__get__.transition_callback', 'machine', machine, 'self', id(self))
---> 91             return self._run(machine, *args, **kwargs)
     92 
     93         return CallableInstance(self, func=transition_callback)

~/opt/anaconda3/envs/tf20/lib/python3.7/site-packages/statemachine/statemachine.py in _run(self, machine, *args, **kwargs)
    116 
    117     def _run(self, machine, *args, **kwargs):
--> 118         self._verify_can_run(machine)
    119         self._validate(*args, **kwargs)
    120         return machine._activate(self, *args, **kwargs)

~/opt/anaconda3/envs/tf20/lib/python3.7/site-packages/statemachine/statemachine.py in _verify_can_run(self, machine)
    110 
    111     def _verify_can_run(self, machine):
--> 112         transition = self._can_run(machine)
    113         if not transition:
    114             raise TransitionNotAllowed(self, machine.current_state)

~/opt/anaconda3/envs/tf20/lib/python3.7/site-packages/statemachine/statemachine.py in _can_run(self, machine)
    106 
    107     def _can_run(self, machine):
--> 108         if machine.current_state == self.source:
    109             return self
    110 

AttributeError: 'NoneType' object has no attribute 'current_state'

Here's a (partial) callstack showing when autoreload is invoking Transition._ _ get _ _() with a None machine parameter, eventually leading to an exception, next time the transition is invoked. Unfortunately I am not familiar enough with autoreload or your own code to understand what is the expected workflow...

347, in update_generic
    update(a, b)
  File "/Users/fred/opt/anaconda3/envs/tf20/lib/python3.7/site-packages/IPython/extensions/autoreload.py", line 287, in update_class
    old_obj = getattr(old, key)
  File "/Users/fred/opt/anaconda3/envs/tf20/lib/python3.7/site-packages/statemachine/statemachine.py", line 88, in __get__
    traceback.print_stack()

[fgmacedo]

Hi @xiao-fred, thanks for reporting this. Can you please provide a notebook and your requirements with exact versions, so I can try for myself?

I'm not familiar with autoreload but can try to find a fix.

[brunolnetto]

May I join the issue? I installed the library with !pip install statemachine and shift-entered from statemachine import StateMachine, State. However, the following log appeared:

NameError: name 'StateMachine' is not defined

Weird.

[brunolnetto]

@xiao-fred I run the following steps (OS Linux distribution Ubuntu 20.04):

1. Create and Activate virtual environment: virtualenv .venv && source .venv/bin/activate;
2. Install ipykernel to allow jupyter notebook usage of above virtual environment: pip install ipykernel;
3. Make virtual kernel available: python -m ipykernel install --user --name .venv --display-name
4. Checkout available kernelspec routes: jupyter kernelspec list;
5. Open jupyter notebook session: jupyter notebook;
6. Click on file sm_notebook.ipynb;
7. Activate .venv kernel by:
   a. Click option Kernel on header-menu;
   b. Select option Change kernel > .venv.
8. Perform the described workflow on this issue first post.

[fgmacedo]

Closing as stale. If you still need help please open a new issue.