[Tutor] about reload

[linda.s]

On 12/30/06, Luke Paireepinart <rabidpoobear at gmail.com> wrote:
>
> > I got confused now:-> if IDLE keeps the imports in between runs of the
> > program, do I still need import and reload? or click F5 in IDLE is OK?
> >
> The purpose of import is so that python knows what packages you intend
> to use,
> because if it imported every package every time, then many bad things
> would occur
> - one broken package would cause every python script on your computer to
> stop running until you deleted or fixed the problem package
> - it would clutter the global namespace
> - it would slow down startup of your scripts.
> - various other things that I can't think of.
>
> So this explains why we need import.
>
> Now in IDLE, because it is itself written in Python,
> there are strange side effects (when it can't open a subprocess).
> If you write a script in IDLE that goes into an infinite loop, the IDLE
> windows will stop responding.
> As we mentioned before, if you run one script through an IDLE shell that
> imports a module,
> on all the subsequent runs, even of different scripts, through the same
> IDLE shell, the imports will still be in the global namespace.
>
> That does not mean that you don't need to have an import at the top of
> each of your scripts.
> You want the program to run outside of IDLE, right?  How about in a
> fresh copy of IDLE?
> then you better put the imports in.
> Python won't know what you mean when you say something like 'print
> math.pi' unless you have an 'import math' at the beginning of your script.
>
> As far as reload goes:
>
> reload does NOTHING if you're running your script from the command line
> (as far as you're concerned, at this stage in learning Python.).
> Take this for example:
>
> #----- config.py
> a = 'apples'
> #----- end
>
> #----- script.py
>
> import config
> print a
> reload(config)
> print a
>
> #------
> The output of this is going to be
> apples
> apples
> unless you change the 'config.py' file in between the first 'print a'
> and the 'reload'.
>
> In other words, reload updates changes that you make to your
> modules/packages.
> If you aren't planning on editing modules while your script is running,
> you don't have to worry about what 'reload' does.
> The imports will always be reloaded automatically when you run the
> Python interpreter again.
>
> There are 2 situations, as a beginner, that you would want reload.
> Take this for example.
>
> #-----config.py
> a = 'apples'
> #-----
>
>  >>> import config
>  >>> print config.a
> apples
>
> #------- we go and edit config.py while the interactive interpreter is
> still running, and we change it to this:
> a = 'aardvark'
> #-----
>
>  >>> print config.a
> apples
>  >>> reload(config)
>  >>> print config.a
> aardvark
>
> See, the 'config' module's attributes didn't change until we reloaded
> it.  So case 1: when using an interactive interpreter and editing modules,
> reload updates our changes.
>
> The second case is, if IDLE is running, remember how we said that it
> keeps the imported modules imported?
>
> Well, say your program does this:
> #----
> import config
> print config.a
> #-----
> Now, the first time, and all subsequent times we run this program, it
> will do the same thing:
> it will print whatever 'a' was when config was imported in the FIRST run
> of the program.
> So if we went and edited config.py, none of the changes would be
> reflected in the print statement of this script.
>
> Basically, if you're designing a module in IDLE and you're making lots
> of changes to it, you have 2 choices:
> First, you could add a reload() to the top of the script that's using
> the module,
> and take it out when you're done.
>
> Or, a better solution, start IDLE from the start menu, instead of
> right-clicking and choosing "Edit..." and it will open a subprocess,
> and none of these 'imported stuff hanging around' problems will occur.
>
> HTH - tell me if anything's unclear
> -Luke
>
>
Thanks a lot!