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An Introduction to Python
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
ISBN 0954161769
RRP £12.95 ($19.95)

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6.4.1 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. Unfortunately, this operation does not work very well on Mac and Windows platforms, where the filesystem does not always have accurate information about the case of a filename! On these platforms, there is no guaranteed way to know whether a file ‘ECHO.PY’ should be imported as a module ‘echo’, ‘Echo’ or ‘ECHO’. (For example, Windows 95 has the annoying practice of showing all file names with a capitalized first letter.) The DOS 8+3 filename restriction adds another interesting problem for long module names.

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. For example, the file ‘Sounds/Effects/__init__.py’ could contain the following code:

    __all__ = ["echo", "surround", "reverse"]

This would mean that from Sound.Effects import * would import the three named submodules of the ‘Sound’ package.

If __all__ is not defined, the statement from Sound.Effects import * does not import all submodules from the package ‘Sound.Effects’ into the current namespace; it only ensures that the package ‘Sound.Effects’ has been imported (possibly running any initialization code in ‘__init__.py’) and then imports whatever names are defined in the package. This includes any names defined (and submodules explicitly loaded) by ‘__init__.py’. It also includes any submodules of the package that were explicitly loaded by previous import statements. Consider this code:

    import Sound.Effects.echo
    import Sound.Effects.surround
    from Sound.Effects import *

In this example, the echo and surround modules are imported in the current namespace because they are defined in the ‘Sound.Effects’ package when the from...import statement is executed. (This also works when __all__ is defined.)

Note that in general the practice of importing * from a module or package is frowned upon, since it often causes poorly readable code. However, it is okay to use it to save typing in interactive sessions, and certain modules are designed to export only names that follow certain patterns.

Remember, there is nothing wrong with using from Package import specific_submodule! In fact, this is the recommended notation unless the importing module needs to use submodules with the same name from different packages.

ISBN 0954161769An Introduction to PythonSee the print edition