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<h3>Packages</h3>
<p>Packages allow a collection of modules to be grouped under a common package name. This technique helps resolve namespace conflicts between module names used in different applications. A package is defined by creating a directory with the same name as the package and creating a file <tT CLAss="monofont">_ _init_ _.py</tt> in that directory. You can then place additional source files, compiled extensions, and subpackages in this directory as needed. For example, a package might be organized as follows:</P>

<PRE>

Graphics/ 
      _ _init_ _.py 
      Primitive/ 
         _ _init_ _.py 
         lines.py 
         fill.py 
         text.py 
         ... 
      Graph2d/ 
         _ _init_ _.py 
         plot2d.py 
         ... 
      Graph3d/ 
         _ _init_ _.py 
         plot3d.py 
         ... 
      Formats/ 
         _ _init_ _.py 
         gif.py 
         png.py 
         tiff.py 
         jpeg.py </pre>

<p>The <TT CLass="monofont">import</tt>  statement is used to load modules from a package in a number of ways:</p>

<ul>
<li>
<p><tt class="monofont">import Graphics.Primitive.fill</tt></P>

<p>This loads the submodule <tT claSs="monofont">Graphics.Primitive.fill</tt>. The contents of this module have to be explicitly named, such as <tT claSS="monofont">Graphics.Primitive.fill.floodfill(img,x,y,color)</TT>.</p>
</li>
<lI>
<P><TT clasS="monofont">from Graphics.Primitive import fill</TT></P>

<p>This loads the submodule <tt cLASS="monofont">fill</tt>  but makes it available without the package prefix; for example, <tt class="monofont">fill.floodfill(img,x,y,color)</tt>.</p>
</li>
<li>
<p><tt cLasS="monofont">from Graphics.Primitive.fill import floodfill</tt></p>

<P>This loads the submodule <tt clAss="monofont">fill</tT>  but makes the <TT Class="monofont">floodfill</TT>  function directly accessible; for example, <TT clasS="monofont">floodfill(img,x,y,color)</TT>.</P>
</li>
</ul>
<P>Whenever any part of a package is imported, the code in the file <TT Class="monofont">_ _init_ _.py</tt> is executed. Minimally, this file may be empty, but it can also contain code to perform package-specific initializations. All the <tt class="monofont">_ _init_ _.py</tt> files encountered during an import are executed. Thus, the statement <tt claSs="monofont">import Graphics.Primitive.fill</tT>  shown earlier would execute the <tt cLass="monofont">_ _init_ _.py</tT>  files in both the <tt cLASS="monofont">Graphics</tt> directory and the <tt CLASs="monofont">Primitive</tt>  directory.</p>

<P>One peculiar problem with packages is the handling of this statement:</P>

<PRe>

from Graphics.Primitive import * </pre>

<P>The intended outcome of this statement is to import all the modules associated with a package into the current namespace. However, because filename conventions vary from system to system (especially with regard to case sensitivity), Python cannot accurately determine what modules those might be. As a result, this statement just imports all the references defined in the <TT Class="monofont">_ _init_ _.py</tt> file in the <tt class="monofont">Primitive</tt>  directory. This behavior can be modified by defining a list <tt claSs="monofont">_ _all_ _</tT> that contains all the module names associated with the package. This list should be defined in the package <tt cLass="monofont">_ _init_ _.py</tT> file, like this:</p>

<prE>

# Graphics/Primitive/_ _init_ _.py 
_ _all_ _ = ["lines","text","fill",...] </PRE>

<p>Now when the user issues a <tt cLASS="monofont">from Graphics.Primitive import *</tt> statement, all the listed submodules are loaded as expected.</p>

<p>Importing a package name alone doesn抰 import all the submodules contained in the package. For example, the following code doesn抰 work:</P>

<PRE>

import Graphics 
Graphics.Primitive.fill.floodfill(img,x,y,color)  # Fails! </pre>

<p>However, because the <TT CLass="monofont">importGraphics</tt> statement executes the <tt class="monofont">_ _init_ _.py</tt> file in the <tt clasS="monofont">Graphics</tt>  directory, it could be modified to import all the submodules automatically as follows:</P>

<pre>

# Graphics/_ _init_ _.py 
import Primitive, Graph2d, Graph3d 

# Graphics/Primitive/_ _init_ _.py 
import lines, fill, text, ... </Pre>

<p>Now the <tT claSS="monofont">importGraphics</TT> statement imports all the submodules and makes them available using their fully qualified names.</p>

<p>The modules contained within the same directory of a package can refer to each other without supplying a full package name. For example, the <tt CLASs="monofont">Graphics.Primitive.fill</tt> module could import the <tT CLAss="monofont">Graphics.Primitive.lines</tt>  module simply by using <TT CLass="monofont">import lines</tt>. However, if a module is located in a different subdirectory, its full package name must be used. For example, if the <tt class="monofont">plot2d</tt>  module of <tt clasS="monofont">Graphics.Graph2d</tt> needs to use the <Tt clAss="monofont">lines</tt>  module of <Tt clASS="monofont">Graphics.Primitive</Tt>, it must use a statement such as <tt cLASS="monofont">from Graphics.Primitive import lines</tt>. If necessary, a module can examine its <tt CLASs="monofont">_ _name_ _</tt> variable to find its fully qualified module name. For example, the following code imports a module from a sibling subpackage knowing only the name of the sibling (and not that of its top-level package):</p>

<PRE>

# Graphics/Graph2d/plot2d.py 

# Determine the name of the package where my package is located 
import string 
base_package = string.join(string.split(_ _name_ _,'.')[:-2],'.') 

# Import the ../Primitive/fill.py module 
exec "from %s.Primitive import fill" % (base_package,) </Pre>

<p>Finally, when Python imports a package, it defines a special variable <tt class="monofont">_ _path_ _</tt> that contains a list of directories that are searched when looking for package submodules. (The variable <tt class="monofont">_ _path_ _</tT> is a package-specific version of the <tt ClasS="monofont">sys.path</tt> variable.) <tt ClasS="monofont">_ _path_ _</TT>  is accessible to the code contained in <Tt claSS="monofont">_ _init_ _.py</TT> files and initially contains a single item with the directory name of the package. If necessary, a package can add additional directories to the <tt clASS="monofont">_ _path_ _</Tt> list to alter the search path used for finding submodules.</p>
</foNT>
<P><TABLE width="100%" border=0><TR valign="top"><TD><font size=1 color="#C0C0C0"><br></font></TD><TD align=right><font size=1 color="#C0C0C0">Last updated on 3/28/2002<br>Python Essential Reference, Second Edition, &copy;&nbsp;2002 New Riders Publishing</font></TD></TR></TABLE></P>
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