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An Introduction to Python
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
ISBN 0954161769
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6.4 Packages

Packages are a way of structuring Python's module namespace by using "dotted module names". For example, the module name ‘A.B’ designates a submodule named ‘B’ in a package named ‘A’. Just like the use of modules saves the authors of different modules from having to worry about each other's global variable names, the use of dotted module names saves the authors of multi-module packages like NumPy or the Python Imaging Library from having to worry about each other's module names.

Suppose you want to design a collection of modules (a "package") for the uniform handling of sound files and sound data. There are many different sound file formats (usually recognized by their extension, for example: ‘.wav’, ‘.aiff’, ‘.au’), so you may need to create and maintain a growing collection of modules for the conversion between the various file formats. There are also many different operations you might want to perform on sound data (such as mixing, adding echo, applying an equalizer function, creating an artificial stereo effect), so in addition you will be writing a never-ending stream of modules to perform these operations. Here's a possible structure for your package (expressed in terms of a hierarchical filesystem):

    Sound/                             Top-level package
          __init__.py       Initialize the sound package
          Formats/           Subpackage for file formats
                  __init__.py
                  wavread.py
                  wavwrite.py
                  aiffread.py
                  aiffwrite.py
                  auread.py
                  auwrite.py
                  ...
          Effects/          Subpackage for sound effects
                  __init__.py
                  echo.py
                  surround.py
                  reverse.py
                  ...
          Filters/                Subpackage for filters
                  __init__.py
                  equalizer.py
                  vocoder.py
                  karaoke.py
                  ...

When importing the package, Python searches through the directories on sys.path looking for the package subdirectory.

The ‘__init__.py’ files are required to make Python treat the directories as containing packages; this is done to prevent directories with a common name, such as ‘string’, from unintentionally hiding valid modules that occur later on the module search path. In the simplest case, ‘__init__.py’ can just be an empty file, but it can also execute initialization code for the package or set the __all__ variable, described later.

Users of the package can import individual modules from the package, for example:

    import Sound.Effects.echo

This loads the submodule ‘Sound.Effects.echo’. It must be referenced with its full name.

    Sound.Effects.echo.echofilter(input, output, delay=0.7,
                                  atten=4)

An alternative way of importing the submodule is:

    from Sound.Effects import echo

This also loads the submodule ‘echo’, and makes it available without its package prefix, so it can be used as follows:

    echo.echofilter(input, output, delay=0.7, atten=4)

Yet another variation is to import the desired function or variable directly:

    from Sound.Effects.echo import echofilter

Again, this loads the submodule ‘echo’, but this makes its function echofilter() directly available:

    echofilter(input, output, delay=0.7, atten=4)

Note that when using from package import item, the item can be either a submodule (or subpackage) of the package, or some other name defined in the package, like a function, class or variable. The import statement first tests whether the item is defined in the package; if not, it assumes it is a module and attempts to load it. If it fails to find it, an ImportError exception is raised.

Contrarily, when using syntax like import item.subitem.subsubitem, each item except for the last must be a package; the last item can be a module or a package but can't be a class or function or variable defined in the previous item.

ISBN 0954161769An Introduction to PythonSee the print edition