Advanced FSM Tidbits

request vs. demand

As stated previously, you normally request an FSM to change its state by calling either fsm.request('NewState', arg1, arg2, ...), or fsm.request('inputString', arg1, arg2, ...), where arg1, arg2, … represent optional arguments to the destination state’s enter function (or to the filter function). The call to request() will either succeed or fail, according to what the filter function for the current state does. If it succeeds, it will return the tuple ('NewState', arg1, arg2), indicating the new state it has transitioned to. If it fails, it will simply return None (unless the filter function was written to throw an exception on failure).

If you request an FSM to make a transition, and the request fails, you might consider this an error condition, and you might prefer to have your code to stop right away rather than continuing. In this case, you should call fsm.demand() instead. The syntax is the same as that for request(), but instead of returning None on failure, it will always raise an exception if the state transition is denied. There is no return value from demand(); if it returns, the transition was accepted.


An FSM is always in exactly one state, except while it is in the process of transitioning between states (that is, while it is calling the exitStateName method for the previous state, followed by the enterStateName method for the new state). During this time, the FSM is not considered in either state, and if you query fsm.state it will contain None.

During this transition time, it is not legal to call fsm.request() to request a new state. If you try to do this, the FSM will raise the exception FSM.AlreadyInTransition. This is a particularly common error if some cleanup code that is called from the exitStateName method has a side-effect that triggers a transition to a new state.

However, there’s a simple solution to this problem: call fsm.demand() instead. Unlike request(), demand() can be called while the FSM is currently in transition. When this happens, the FSM will queue up the demand, and will carry it out as soon as it has fully transitioned into its new state.


There is also a method fsm.forceTransition(). This is similar to demand() in that it never fails and does not have a return value, but it’s different in that it completely bypasses the filter function. You should therefore only pass an uppercase state name (along with any optional arguments) to forceTransition, never a lowercase input string. The FSM will always transition to the named state, even if it wouldn’t otherwise be allowed. Thus, forceTransition() can be useful in special cases to skip to another state that’s not necessarily connected to the current state (for instance, to handle emergency cleanup when an exception occurs). Be careful that you don’t overuse forceTransition(), though; consider whether demand() would be a better choice. If you find yourself making lots of calls to forceTransition(), it may be that your filter functions (or your defaultTransitions) are poorly written and are disallowing what should be legitimate state transitions.

Filtering the optional arguments

The filterStateName method receives two parameters: the string request, and a tuple, which contains the additional arguments passed to the request (or demand) call. It then normally returns the state name the FSM should transition to, or it returns None to indicate the transition is denied.

However, the filter function can also return a tuple. If it returns a tuple, it should be of the form ('StateName', arg1, arg2, ...), where arg1, arg2, … represent the optional arguments that should be passed to the enterStateName method. Usually, these are the same arguments that were passed to the filterStateName method (in this case, you can generate the return value tuple with the python syntax ('StateName',) + args).

The returned arguments are not necessarily the same as the ones passed in, however. The filter function is free to check, modify, or rearrange any of them; or it might even make up a completely new set of arguments. In this way, the filter function can filter not only the state transitions themselves, but also the set of data passed along with the request.