[Tutor] code structure terminology and advice

[Che M]

> To: tutor at python.org
> From: alan.gauld at btinternet.com
> Date: Wed, 22 Jul 2009 08:46:04 +0100
> Subject: Re: [Tutor] code structure terminology and advice
> 
> 
> "Che M" <pine508 at hotmail.com> wrote 
> 
> You pretty much answered your own question.
> 
> The ultimate answer depends on a few other contextual issues.
> 
> > Q1) Generally, what is the best structure for when there are 
> > a number of steps/actions that need to be taken?  
> 
> A sequence! :-)
> 
> > Q2) Is there a term/some jargon in programming that 
> > refers to the difference between examples 1-3 below?
> 
> Not that I'm aware of.
> 
> >  wish to perform four related sequential actions--steps, 
> > let's call them--in some part of the program.  
> 
> If the entire program consists of those 4 steps (as in a batch 
> data processing program, say) then you might structure things 
> differently than if the 4 steps are only part of a bigger 
> program - one menu option out of several, say.
> 
> In the former you would perform the steps in sequence and 
> may or may not put them in functions.depending on whether 
> they would ever be reused and how long the code was per step.
> In most cases the steps are likely to be reusable or of significant 
> length so you would put each one in its own function.
> 
> > 1. Do the steps all in one function:
> > 
> > def Function(self):
> >   do step1 action
> >   do step2 action
> >   do step3 action 
> >   do step4 action
> 
> This would be sensible in the menu selection context.
> 
> > 2. Have each step be a function, and have each function 
> > call the next function in the chain:
> 
> No, this is terrrible from every point of view. It removes any 
> opportunity of reuise of individual steps and builds up a deep 
> call stack which makes debugging harder and the coupling 
> between functions makes maintenance harder (for example 
> swapping two steps around in the sequence)
> 
> > def StepOne(self):
> >    do step1 action
> >    StepTwo()
> 
> > 3. Make each step a function, but call all of them in order in 
> > one master function:
> > 
> > def MasterFunction(self):
> >    self.StepOne()
> >    self.StepTwo()
> 
> This is just a variation on option 1 and the same applies.
> 
> The fourth option you do not consider but which could be the 
> best solution for the batch program I discussed is just to perform 
> each step from the top klevel of the program, ie no 
> enclosing function
> 
> StepOne()
> StepTwo()
> StepThree()
> StepFour()
> 
> 
> > It seems to me that example 3 is the most sensible.  
> 
> In general yes.
> 
> > 1 is bad because the steps should be functions, because then they 
> > can be easily reused, seen from an IDE explorer, etc. 
> > (I guess if they are really minor things it isn't necessary). 
> 
> This depends on the size of the steps and the nature of the overall 
> programme, but functions would usually be best.
> 
> > 2 is terrible 
> 
> Yes.
> 
> > Maybe there are other and better ways to think about organizing 
> > a sequence of steps, and if so I'd love to hear them.  
> 
> For longer and more complex processes you could use a table 
> or list of functions and step through them in a loop
> 
> process = [StepOne, StepTwo,...., stepN]
> 
> for step in process:
>      step()
> 
> Or for dynamically changing sequences you can create or hold 
> a sequence list and use that to index a list of steps:
> 
> steps = [StepOne, StepTwo,...., stepN]
> sequence = [1,2,3,4]
> 
> def do Process(sequence)
>     for step in sequence:
>          steps[step]()
> 
> This allows you to selectively peform some of the steps:
> 
> sequence = [1,3,4,N]
> 
> doProcess(sequence)
> 
> Finally for very complex scenarios you might implement a 
> state machine where the return value of each step controls 
> the next step in the sequence. That can be done by building 
> a set of state objects or using a data driven approach with 
> a table
> 
> You can read about state machines in several places, 
> I'd start with Wikipedia.
> 
> You might also find that reading about business process execution
> languages gives you some ideas about how to structure and 
> represent complex processes. BEPL would be a good 
> starting point, again try Wikipedia.
> 
> HTH,
> 
> 
> -- 
> Alan Gauld
> Author of the Learn to Program web site
> http://www.alan-g.me.uk/

Thank you, Alan.  It is sometimes helpful just to get corroboration, and I feel that sometimes the simplest things are the most important.  I also thought that maybe the mistake in example 2 had a name; I don't know if it is a common mistake or not for beginning programmers, but I have found myself doing it sometimes and I am making a point to undo it now in any old code.  The point about looping through a list of functions is also something that hadn't occurred to me.  So this is all very helpful.

Thanks,
Che

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