12.7. Packages

A package is a hierarchical file directory structure that defines a single Python application environment that consists of modules and subpackages. Packages were added to Python 1.5 to aid with a variety of problems including:

• Adding hierarchical organization to flat namespace

• Allowing developers to group-related modules

• Allowing distributors to ship directories vs. bunch of files

• Helping resolve conflicting module names

Along with classes and modules, packages use the familiar attribute/dotted attribute notation to access their elements. Importing modules within packages use the standard import and from-import statements.

12.7.1. Directory Structure

The example package directory structure below is available for you to experiment with on the CD-ROM accompanying this book. Just browse the code samples and navigate to Chapter 12.

Phone/
    __init__.py
    Voicedta/
        __init__.py
        Pots.py
        Isdn.py
    Fax/
        __init__.py
        G3.py
    Mobile/
        __init__.py
        Analog.py
        Digital.py
    Pager/
        __init__.py
        Numeric.py

Phone is top-level package and Voicedta, etc., are subpackages. Import subpackages by using import like this:

							import Phone.Mobile.Analog

Phone.Mobile.Analog.dial()

Alternatively, you can use from-import in a variety of ways:

The first way is importing just the top-level subpackage and referencing down the subpackage tree using the attribute/dotted notation:

							from Phone import Mobile

Mobile.Analog.dial('4 555–1212')

Further more, we can do down one more subpackage for referencing:

							from Phone.Mobile import Analog

Analog.dial('555–1212')

In fact, you can go all the way down in the subpackage tree structure:

							from Phone.Mobile.Analog import dial

dial('555–1212')

In our above directory structure hierarchy, we observe a number of __init__.py files. These are initializer modules that are required when using from-import to import subpackages, but should otherwise exist though they can remain empty.

12.7.2. Using from-import with Packages

Packages also support the from-import all statement:

							from
							package.module
							import *
						

However, such a statement is too operating system filesystem-dependent for Python to make the determination which files to import. Thus the __all__ variable in __init__.py is required. This variable contains all the module names that should be imported when the above statement is invoked if there is such a thing. It consists of a list of module names as strings.