[Tutor] about reload

Luke Paireepinart rabidpoobear at gmail.com
Sat Dec 30 11:19:02 CET 2006

> 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
print a

The output of this is going to be
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 
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.

a = 'apples'

 >>> import config
 >>> print config.a

#------- we go and edit config.py while the interactive interpreter is 
still running, and we change it to this:
a = 'aardvark'

 >>> print config.a
 >>> reload(config)
 >>> print config.a

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

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