Working around a lack of 'goto' in python

[Roy Smith]

Stephen Horne <steve at ninereeds.fsnet.co.uk> wrote:
>> goto allows the code for a state to be fallen into from the top.
> 
> How is this different to the more typical switch-within-loop method?

Not much.  The C switch statement isn't much better than a goto, for 
just that reason :-)

> returning a function pointer to a
> function that is immediately going to call that function pointer,
> you're basically just faking the effect of a goto anyway.

In some respects, you're right.  Certainly, both ways get you the same 
pseudo-random flow of control, but that's inherent in a state machine.  
What the function-per-state approach gets you is modularity.  Each 
function at least has its own namespace and set of local variables. 

To tie this in with the current thread on unit testing, it's also a lot 
easier to test a bunch of little functions than a collection of gotos.  
You said:

> In fact its basically a special case of the state variable method, 
> using the processors IP as the state variable.

That's a very good observation.  Imagine you implemented your state 
machine with gotos.  To test each transition, you need to get the 
machine into the right state before each test by feeding it a sequence 
of inputs which navigates your state graph in the right way.  That's 
complicated and error prone to design (the last thing you want is an 
error-prone test procedure).  If there were many states and edges, it 
would be very slow.  If the state machine is non-deterministic (timing 
dependencies, perhaps), it would be impossible.

Of course, this is all assuming that the logic implemented in each state 
is different enough to require writing individual functions.  Some state 
machines (like those that execute compiled regular expressions) are 
simple enough to be table driven.  With a tabular approach, you get the 
same testability you do with the functional approach; you can set the 
machine to any random state, apply a single input, and see what happens.  
You can't do that with gotos because the state is stored in a place 
that's not exposed at the language level (unless you're writing in 
assembler).