Surprising imports
==================

If ``from a import b ; print b.c`` works, shouldn't 
``import a ; print a.b.c`` work too? Not necessarily, here is why.

To use an object in Python, the objects needs to be present in the
namespace. To create a new entry in a module namespace, it is possible
to either define the functions and classes *or import* them from a
package or a module. Packages are also namespaces, **the objects which
are included in the current namespace when the package is imported are
the objects which are defined and imported by the** :mod:`__init__.py`
module of the package. If :mod:`__init__.py` is empty for a
package, importing the package does not make the modules in the
package available.

Rule of thumb, if you don't want surprises always import modules
instead of packages, because with packages, what is imported depends
on the :mod:`__init__` package and changes.

Here is an example

.. sourcecode:: sh

   ~$ tree package
   package
   |-- blabi.py
   |-- __init__.py
   `-- shbonz
       |-- __init__.py
       `-- shekel.py
   

:mod:`package` and :mod:`shbonz` are packages (:mod:`shbonz` is a
*subpackage*), which means they are filesystem directories with a file
named *__init__.py* in it. Those can be empty files.

.. sourcecode:: sh
      
   ~$ cat package/__init__.py package/shbonz/__init__.py
   ~$
   ~$  # the __init__.py are just empty files

:mod:`blabi` et :mod:`shekel` are modules, they contain classes,
functions and constants. Fore example, :mod:`shekel` defines a
constant :obj:`pi`

.. sourcecode:: sh
   
   ~$ cat package/shbonz/shekel.py
   pi=3.14

As the *__init__.py* files are empty, they do not import :obj:`pi`, it
is not available when importing the packages:

.. sourcecode:: sh

   ~$ python -c 'import package;  print package.shbonz.shekel.pi'
   Traceback (most recent call last):
   AttributeError: 'module' object has no attribute 'shbonz'
   
   ~$ python -c 'from package import shbonz;  print shbonz.shekel.pi'
   Traceback (most recent call last):
   AttributeError: 'module' object has no attribute 'shekel'
   
The *module*, and not its parent packages must be imported, for the
constant to be available:

   ~$ python -c 'from package.shbonz import shekel; print shekel.pi'
   3.14

Now if the package's :mod:`__init__` module import the constant, it
will be available:

.. sourcecode:: sh
   
   ~$ echo "import shbonz" > package/__init__.py
   ~$ echo "import shekel" > package/shbonz/__init__.py
   ~$ python -c 'import package;print package.shbonz.shekel.pi'
   3.14
   
http://effbot.org/zone/import-confusion.htm

*5 May 2010*