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From: vsoler on 16 May 2010 17:42
Taken from www.python.org, FAQ 2.3 How do I share global variables
across modules?

config.py:

x = 0 # Default value of the 'x' configuration setting


mod.py:

import config
config.x = 1


main.py:

import config # try removing it
import mod
print config.x

The example, such as shown in the website, works perfectly well.
However, I don't fully understand why I have to import config in
main.py, since it has already been imported by mod.py.

As the website explains, there is only one module namespace for each
module, and mod.py has aleady created the config namespace by
importing it. Why should I import it again in main.py if that
namespace already exists?

If I remove -> import config # try removing it in main.py,
the application does not run

What am I missing?
From: Patrick Maupin on 16 May 2010 18:05
On May 16, 4:42 pm, vsoler <vicente.so...(a)gmail.com> wrote:
> Taken fromwww.python.org, FAQ 2.3 How do I share global variables
> across modules?
>
> config.py:
>
> x = 0   # Default value of the 'x' configuration setting
>
> mod.py:
>
> import config
> config.x = 1
>
> main.py:
>
> import config       # try removing it
> import mod
> print config.x
>
> The example, such as shown in the website, works perfectly well.
> However, I don't fully understand why I have to import config in
> main.py, since it has already been imported by mod.py.
>
> As the website explains, there is only one module namespace for each
> module, and mod.py has aleady created the config namespace by
> importing it. Why should I import it again in main.py if that
> namespace already exists?
>
> If I remove ->   import config       # try removing it     in main.py,
> the application does not run
>
> What am I missing?

What you are missing is that the interpreter has to look *inside* a
namespace in order to actually find the object associated with a
name. As you found out, there is a namespace per module. So
main.py's namespace is where the code in main.py will search for
variables. If 'mod' imports config, then the 'mod' module's namespace
is updated with 'config' -> the config module. But the act of 'mod'
importing 'config' will not alter the namespace of 'main' at all. So
if you want to access variable 'x' inside 'config' from main you can
either import config directly into main and access it as config.x, or
you can import config into mod and import mod into main and access it
as mod.config.x.

Regards,
Pat
From: Rhodri James on 16 May 2010 18:23
On Sun, 16 May 2010 22:42:40 +0100, vsoler <vicente.soler(a)gmail.com> wrote:

> Taken from www.python.org, FAQ 2.3 How do I share global variables
> across modules?
>
> config.py:
>
> x = 0 # Default value of the 'x' configuration setting
>
>
> mod.py:
>
> import config
> config.x = 1
>
>
> main.py:
>
> import config # try removing it
> import mod
> print config.x
>
> The example, such as shown in the website, works perfectly well.
> However, I don't fully understand why I have to import config in
> main.py, since it has already been imported by mod.py.

Globals are only global to a module. As you can see above, they
don't magically appear in the namespace of the module you import
them into (not unless you do something careless like "from config
import *", at which point you deserve all the confusion you're
going to get). In both mod.py and main.py you get at the global
x through its module: config.x

Now as you say, main.py imports mod. In exactly the same way as
mod imports config. *Exactly*. We get at the contents of mod
(the things in its namespace) in the same way: mod.thing. When
mod imports config it imports it into its own namespace only;
there is no magic that makes it appear in the namespace of anything
importing mod any more than happens for anything else.

If you want to access config in main.py without importing it
directly because you know that (in this case) mod has already
imported it, you have to access it through the module that did
the importing. Instead of "config.x", in main it would be
"mod.config.x".

--
Rhodri James *-* Wildebeeste Herder to the Masses
From: vsoler on 16 May 2010 18:26
On 17 mayo, 00:05, Patrick Maupin <pmau...(a)gmail.com> wrote:
> On May 16, 4:42 pm, vsoler <vicente.so...(a)gmail.com> wrote:
>
>
>
> > Taken fromwww.python.org, FAQ 2.3 How do I share global variables
> > across modules?
>
> > config.py:
>
> > x = 0   # Default value of the 'x' configuration setting
>
> > mod.py:
>
> > import config
> > config.x = 1
>
> > main.py:
>
> > import config       # try removing it
> > import mod
> > print config.x
>
> > The example, such as shown in the website, works perfectly well.
> > However, I don't fully understand why I have to import config in
> > main.py, since it has already been imported by mod.py.
>
> > As the website explains, there is only one module namespace for each
> > module, and mod.py has aleady created the config namespace by
> > importing it. Why should I import it again in main.py if that
> > namespace already exists?
>
> > If I remove ->   import config       # try removing it     in main.py,
> > the application does not run
>
> > What am I missing?
>
> What you are missing is that the interpreter has to look *inside* a
> namespace in order to actually find the object associated with a
> name.  As you found out, there is a namespace per module.  So
> main.py's namespace is where the code in main.py will search for
> variables.  If 'mod' imports config, then the 'mod' module's namespace
> is updated with 'config' -> the config module.  But the act of 'mod'
> importing 'config' will not alter the namespace of 'main' at all.  So
> if you want to access variable 'x' inside 'config' from main you can
> either import config directly into main and access it as config.x, or
> you can import config into mod and import mod into main and access it
> as mod.config.x.
>
> Regards,
> Pat

Thank you Pat, it's very clear.

However, can I be 100% sure that,no matter how I access variable
'x' (with config.x or mod.config.x) it is always the same 'x'. I mean
that either reference of 'x' points to the same id(memory position)?

Thank you
From: James Mills on 16 May 2010 18:38
On Mon, May 17, 2010 at 8:26 AM, vsoler <vicente.soler(a)gmail.com> wrote:
> However, can I be 100% sure that,no matter how I access variable
> 'x' (with config.x or mod.config.x) it is always the same 'x'. I mean
> that either reference of 'x' points to the same id(memory position)?

Yes it does unless you re-assign it.

--James
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