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<h3>Modules</h3>
<p>You can turn any valid source file into a module by loading it with the <tT CLAss="monofont">import</tt> statement. For example, consider the following code:</P>
<PRE>
# file : spam.py
a = 37 # A variable
def foo: # A function
print "I'm foo"
class bar: # A class
def grok(self):
print "I'm bar.grok"
b = bar() # Create an instance </pre>
<p>To load this code as a module, you use the statement <TT CLass="monofont">import spam</tt>. The first time <tt class="monofont">import</tt> is used to load a module, it does three things:</p>
<ol typE="1">
<li>
<P> It creates a new namespace that serves as a namespace to all the objects defined in the corresponding source file. This is the namespace accessed when functions and methods defined within the module use the <tt cLass="monofont">global</tT> statement.</p>
</li>
<LI>
<P> It executes the code contained in the module within the newly created namespace.</P>
</li>
<li>
<P> It creates a name within the caller that refers to the module namespace. This name matches the name of the module and is used as follows:</P>
<PRe>
import spam # Loads and executes the module 'spam'
print spam.a # Accesses a member of module 'spam'
spam.foo()
c = spam.bar()
... </pre>
</LI>
</OL>
<p>To import multiple modules, supply <tt cLASS="monofont">import</tt> with a comma-separated list of module names, like this:</p>
<pre>
import socket, os, regex </pre>
<p>Modules can be imported using alternative names by using the <tt class="monofont">as</tt> qualifier. For example:</p>
<Pre>
import os as system
import socket as net, thread as threads
system.chdir("..")
net.gethostname() </Pre>
<p>Use the <Tt claSs="monofont">from</tt> statement to load specific definitions within a module into the current namespace. The <TT CLass="monofont">from</tT> statement is identical to <TT Class="monofont">import</TT> except that instead of creating a name referring to the newly created module namespace, references to one or more of the objects defined in the module are placed into the current namespace:</P>
<Pre>
from socket import gethostname
# Put gethostname in current namespace
print gethostname() # Use without module name
socket.gethostname() # NameError: socket </prE>
<P>The <TT class="monofont">from</tt> statement also accepts a comma-separated list of object names. The asterisk (<tt class="monofont">*</tt>) wildcard character can also be used to load all the definitions in a module except those that start with an underscore. For example:</p>
<pre>
from socket import gethostname, socket
from socket import * # Load all definitions into current namespace </Pre>
<P>Modules can more precisely control the set of names that are imported by <tt cLass="monofont">from </tT><i><tt CLASs="monofont">module</tt></i>
<TT CLass="monofont"> import *</tT> by defining a list <TT Class="monofont">_ _all_ _</TT>. For example:</P>
<Pre>
# module: foo.py
_ _all_ _ = [ 'bar', 'spam' ] # Names to be imported by * </pre>
<p>In addition, the <tt class="monofont">as</tt> qualifier can be used to rename specific objects imported with <tt claSs="monofont">from</tT>. For example:</p>
<prE>
from socket import gethostname as hostname
h = hostname() </pre>
<p>The <Tt clASS="monofont">import</Tt> statement can appear at any point in a program. However, the code in each module is loaded and executed only once, regardless of how often you use the <tt cLASS="monofont">import</tt> statement. Subsequent <tt CLASs="monofont">import</tt> statements simply create a reference to the module namespace created on a previous import. You can find a dictionary containing all currently loaded modules in the variable <tT CLAss="monofont">sys.modules</tt>, which is a dictionary that maps module names to module objects. The contents of this dictionary are used to determine whether <tt class="monofont">import</tt> loads a fresh copy of a module.</p>
<p>The <tt claSs="monofont">from </tT><i><tt Class="monofont">module</Tt></i>
<tT CLAss="monofont"> import *</tt> statement can only be used at the top level of a module. In particular, it抯 illegal to use this form of <TT CLass="monofont">import</tT> inside function bodies, due to the way in which it interacts with function scoping rules.</P>
<P>Each module defines a variable <Tt claSS="monofont">_ _name_ _</TT> that contains the module name. Programs can examine this variable to determine the module in which they抮e executing. The top-level module of the interpreter is named <tt class="monofont">_ _main_ _</tt>. Programs specified on the command line or entered interactively run inside the <tt class="monofont">_ _main_ _</tt> module. Sometimes, a program may alter its behavior, depending on whether it has been imported as a module or is running in <tt ClaSs="monofont">_ _main_ _</tt>. This can be done as follows:</P>
<pre>
# Check if running as a program
if _ _name_ _ == '_ _main_ _':
# Yes
<i>statements</I>
else:
# No, I must have been imported as a module
<i>statements</i> </pRE>
</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, © 2002 New Riders Publishing</font></TD></TR></TABLE></P>
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