__init__.py in Python
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:
.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 format conversions __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.
__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:
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.
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. 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. For example, the file
sound/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
__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
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.)
Although certain modules are designed to export only names that follow certain patterns when you use
import *, it is still considered bad practise in production code.
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.
6.4.2. Intra-package References
When packages are structured into subpackages (as with the
sound package in the example), you can use absolute imports to refer to submodules of siblings packages. For example, if the module
sound.filters.vocoder needs to use the
echo module in the
sound.effects package, it can use
from sound.effects import echo.
You can also write relative imports, with the
from module import name form of import statement. These imports use leading dots to indicate the current and parent packages involved in the relative import. From the
surround module for example, you might use:
from . import echo from .. import formats from ..filters import equalizer
Note that relative imports are based on the name of the current module. Since the name of the main module is always
"__main__", modules intended for use as the main module of a Python application must always use absolute imports.
6.4.3. Packages in Multiple Directories
Packages support one more special attribute,
__path__. This is initialized to be a list containing the name of the directory holding the package’s
__init__.py before the code in that file is executed. This variable can be modified; doing so affects future searches for modules and subpackages contained in the package.
While this feature is not often needed, it can be used to extend the set of modules found in a package.