[Edu-sig] Using IDLE with students

John Zelle john.zelle at wartburg.edu
Fri Aug 17 05:25:12 CEST 2007

On Thursday 16 August 2007 8:40 pm, Richard Guenther wrote:

> you are still able to access variables that have just run in a program
> after the program is done running.  For instance, if I run a program that
> declares a="assigned in program", I can later print out that variable in
> shell mode.

Yes, this is true if your script manipulates global variables. It is 
equivalent to running the python interpreter on the script using the -i flag, 
which leaves you in interactive mode after executing the script. Again, you 
can get a "clean" slate by simply going to the shell menu and selecting 
restart. This happens automatically if you run another script, each time you 
hit <F5> you get a clean run, and then you can play around with the results 
afterwords. The important thing is that the next run is not "tainted" it 
happens in a clean namespace. 

As I mentioned before, you can also "protect" against this by simply writing 
your programs in functions to avoid global variables.

I personally put my scripts in a function called main, and I execute that. If 
I want some data to hang around so that I can play with it, then I poke it 
into a  global variable or two.

> So I've worked several Python books and tutorials, written both functional
> programs and some OOP, and yet I find myself not real clear on what exactly
> IDLE is ... :-(

It's not really too mysterious if you're used to read-eval-print loops. The 
main thing is you need a good understanding of things that produce 
side-effects. Globals variables are always tricky in that way. The fact that 
built-in functions are not reserved in Python is just a little extra twist 
(e.g. you can redefine things like raw_input).

> Ah well, at least my ubuntu menu launches IDLE in "normal" mode now. :-)

Yeah, now you run into the problem that you can only have 1 IDLE process 
running at a time. I don't understand why IDLE uses a fixed port.


> ----- Original Message ----
> From: John Zelle <john.zelle at wartburg.edu>
> To: edu-sig at python.org
> Cc: Richard Guenther <heistooheavy at yahoo.com>
> Sent: Wednesday, August 15, 2007 9:13:32 AM
> Subject: Re: [Edu-sig] Using IDLE with students
> On Wednesday 15 August 2007 9:04 am, Richard Guenther wrote:
> > Sorry if this is a bit simplistic:
> >
> > When teaching Python to beginners and using IDLE, it seems that one of
> > the dangers would be to have them assigning variables in the interactive
> > mode and then maybe using them in a script they're writing.  Then, when
> > they run the script, the variable is still in memory so the program
> > works--for now.
> >
> > Or, as happened recently "raw_input" gets accidentally assigned to a
> > string.  Then, any programs that end with "raw_input("Press Enter to exit
> > this program")" will cause an error, even though the program script
> > itself is fine.
> >
> >
> > Obviously quiting and reloading IDLE will take care of this, but I was
> > wondering what else may trip up students using IDLE.  Maybe it would be
> > nice if IDLE had an option called "Run fresh" that would clear any
> > variables first....just musing here.
> Provided you start IDLE in the "normal" mode, running scripts should
> execute in a separate subprocess, so the kinds of interactions you describe
> here are not really a problem. When running in this mode, you can also do a
> "restart" under the shell menu, and this will get you a fresh interactive
> environment.
> The problem is that the default IDLE setup in some environments starts up
> IDLE with the -n switch that causes it to run without separate subprocesses
> for scripts. For example, under Windows, if you right-click on a Python
> program and then select "edit with IDLE" it will open in the no-subprocess
> mode. I always have my students create a shortcut to IDLE in their working
> directories and make sure it starts IDLE without the -n switch, and I
> emphasize starting IDLE and then loading programs.
> By the way, another thing that will really help is getting students in the
> habit of writing scripts as functions and then just calling the function.
> That way variables are local to the function/script regardless of how IDLE
> is running (still doesn't solve problems like reassigning built-in
> functions though).
> --John

John M. Zelle, Ph.D.             Wartburg College
Professor of Computer Science    Waverly, IA     
john.zelle at wartburg.edu          (319) 352-8360  

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