Find the location of a loaded module

[rdmurray at bitdance.com]

"Gabriel Genellina" <gagsl-py2 at yahoo.com.ar> wrote:
> En Fri, 20 Feb 2009 20:44:21 -0200, Aaron Scott
> <aaron.hildebrandt at gmail.com> escribi=F3:
> 
> > So, the problem lies with how Python cached the modules in memory.
> > Yes, the modules were in two different locations and yes, the one that
> > I specified using its direct path should be the one loaded. The
> > problem is, the module isn't always loaded -- if it's already in
> > memory, it'll use that instead. And since the modules had the same
> > name, Python wouldn't distinguish between them, even though they
> > weren't exactly the same.
> 
> Yes, that's how import works. It's barely documented, and you finally
> learned it the hard way...

I'd argue a little bit with "barely documented".  In the reference, the
discussion of the import statement starts off saying:

    Import statements are executed in two steps: (1) find a module,
    and initialize it if necessary; (2) define a name or names in the
    local namespace (of the scope where the import statement occurs).

The third paragraph then says:

    The system maintains a table of modules that have been or are being
    initialized, indexed by module name. This table is accessible as
    sys.modules. When a module name is found in this table, step (1)
    is finished.

That is pretty up front and unambiguous documentation.

However, the consequences of that statement won't be immediately clear
on first reading.  I think it would have cleared up Aaron's confusion
if he'd happened to think to read it.  But since he knew the syntax
of the import statement already, I'm not surprised he did not read it.

The Tutorial, in the section on modules and import, says:

    A module can contain executable statements as well as function
    definitions. These statements are intended to initialize the
    module. They are executed only the first time the module is imported
    somewhere.

This is considerably less precise, if more superficially understandable.
I wonder if it would be worth expanding on that statement to mention
that the module is not even looked for on disk if a module by that
name has already been imported.

--RDM