A package import question

[Gabriel Genellina]

En Fri, 13 Jun 2008 20:01:56 -0300, Dan Yamins <dyamins at gmail.com>  
escribió:

> I'm having a problem importing a package in python, deleting some of  
> what's
> been imported, and then reimporting.  (I'm the sure the problem is  
> trivial,
> but I just don't understand it.)
>
> I have a directory of python modules called Operations.  It contains a
> python module called archive.py.    Here's a import of the archive module
> via package import:
>
>   Python 2.5.2 (r252:60911, Feb 22 2008, 07:57:53)
>   [GCC 4.0.1 (Apple Computer, Inc. build 5363)] on darwin
>   Type "help", "copyright", "credits" or "license" for more information.
>   >>> import Operations.archive
>   >>> Operations.archive
>   <module 'Operations.archive' from 'Operations/archive.pyc'>
>
> So  far, so good.

Note that if you execute dir() at this point, you'll see the Operations  
name, *not* Operations.archive.
The statement "import Operations.archive" first tries to locate and load a  
module named Operations - and *that* name is added to the current  
namespace, not Operations.archive (which is an invalid name by itself).

> But now, suppose I want to delete Operations.archive.  then, I can't
> reimport it.  instead, I
>
>    >>> del Operations.archive

You have removed the "archive" attribute from the object to which the  
"Operations" name is referring to.

>    >>> import Operations.archive

Python keeps a reference to all imported modules in sys.modules; if a  
module was already imported, any subsequent imports of the same module  
just return the existing reference.
If you want to force Python to re-read the module from file, use the  
reload function. But please read the warnings at  
http://docs.python.org/lib/built-in-funcs.html#l2h-61

>    >>> dir()
>      ['Operations', '__builtins__', '__doc__', '__name__']
>    >>>
>
> Instead of getting 'Operations.archive', I just seem to get 'Operations'.

You would never get a dotted name from dir(), unless you play tricks with  
locals()/globals()

> I can't seem to be able to import Operations.archive without quitting the
> python interpreter and starting again.
>
> What's going on here, and how do I fix it?

reload() may be what you need, but again, make sure you read the  
documentation before using it. reload is not a magic wand. Remember that  
names imported from the old module definition continue to be bound to the  
old objects, and all instances of classes defined in the old module  
continue to use the old class definitions, among other things.

-- 
Gabriel Genellina