http://docs.python.org/tutorial/modules.html#importing-from-a-package
Now what happens when the user writes
from sound.effects import *? Ideally,
one would hope that this somehow goes
out to the filesystem, finds which
submodules are present in the package,
and imports them all. This could take
a long time and importing sub-modules
might have unwanted side-effects that
should only happen when the sub-module
is explicitly imported.
The only solution is for the package
author to provide an explicit index of
the package. The import statement uses
the following convention: if a
package’s __init__.py code defines a
list named __all__, it is taken to be
the list of module names that should
be imported when from package import *
is encountered. It is up to the
package author to keep this list
up-to-date when a new version of the
package is released. Package authors
may also decide not to support it, if
they don’t see a use for importing *
from their package.
