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<div class="section" id="modules">
<span id="tut-modules"></span><h1>6. Modules<a class="headerlink" href="#modules" title="Permalink to this headline">¶</a></h1>
<p>If you quit from the Python interpreter and enter it again, the definitions you
have made (functions and variables) are lost. Therefore, if you want to write a
somewhat longer program, you are better off using a text editor to prepare the
input for the interpreter and running it with that file as input instead. This
is known as creating a <em>script</em>. As your program gets longer, you may want to
split it into several files for easier maintenance. You may also want to use a
handy function that you’ve written in several programs without copying its
definition into each program.</p>
<p>To support this, Python has a way to put definitions in a file and use them in a
script or in an interactive instance of the interpreter. Such a file is called a
<em>module</em>; definitions from a module can be <em>imported</em> into other modules or into
the <em>main</em> module (the collection of variables that you have access to in a
script executed at the top level and in calculator mode).</p>
<p>A module is a file containing Python definitions and statements. The file name
is the module name with the suffix <tt class="docutils literal"><span class="pre">.py</span></tt> appended. Within a module, the
module’s name (as a string) is available as the value of the global variable
<tt class="docutils literal"><span class="pre">__name__</span></tt>. For instance, use your favorite text editor to create a file
called <tt class="docutils literal"><span class="pre">fibo.py</span></tt> in the current directory with the following contents:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Fibonacci numbers module</span>
<span class="k">def</span> <span class="nf">fib</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="c"># write Fibonacci series up to n</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span>
<span class="k">while</span> <span class="n">b</span> <span class="o"><</span> <span class="n">n</span><span class="p">:</span>
<span class="k">print</span> <span class="n">b</span><span class="p">,</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span>
<span class="k">def</span> <span class="nf">fib2</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="c"># return Fibonacci series up to n</span>
<span class="n">result</span> <span class="o">=</span> <span class="p">[]</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span>
<span class="k">while</span> <span class="n">b</span> <span class="o"><</span> <span class="n">n</span><span class="p">:</span>
<span class="n">result</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span>
<span class="k">return</span> <span class="n">result</span>
</pre></div>
</div>
<p>Now enter the Python interpreter and import this module with the following
command:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fibo</span>
</pre></div>
</div>
<p>This does not enter the names of the functions defined in <tt class="docutils literal"><span class="pre">fibo</span></tt> directly in
the current symbol table; it only enters the module name <tt class="docutils literal"><span class="pre">fibo</span></tt> there. Using
the module name you can access the functions:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">fibo</span><span class="o">.</span><span class="n">fib</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span>
<span class="go">1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987</span>
<span class="gp">>>> </span><span class="n">fibo</span><span class="o">.</span><span class="n">fib2</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span>
<span class="go">[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]</span>
<span class="gp">>>> </span><span class="n">fibo</span><span class="o">.</span><span class="n">__name__</span>
<span class="go">'fibo'</span>
</pre></div>
</div>
<p>If you intend to use a function often you can assign it to a local name:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">fib</span> <span class="o">=</span> <span class="n">fibo</span><span class="o">.</span><span class="n">fib</span>
<span class="gp">>>> </span><span class="n">fib</span><span class="p">(</span><span class="mi">500</span><span class="p">)</span>
<span class="go">1 1 2 3 5 8 13 21 34 55 89 144 233 377</span>
</pre></div>
</div>
<div class="section" id="more-on-modules">
<span id="tut-moremodules"></span><h2>6.1. More on Modules<a class="headerlink" href="#more-on-modules" title="Permalink to this headline">¶</a></h2>
<p>A module can contain executable statements as well as function definitions.
These statements are intended to initialize the module. They are executed only
the <em>first</em> time the module is imported somewhere. <a class="footnote-reference" href="#id2" id="id1">[1]</a></p>
<p>Each module has its own private symbol table, which is used as the global symbol
table by all functions defined in the module. Thus, the author of a module can
use global variables in the module without worrying about accidental clashes
with a user’s global variables. On the other hand, if you know what you are
doing you can touch a module’s global variables with the same notation used to
refer to its functions, <tt class="docutils literal"><span class="pre">modname.itemname</span></tt>.</p>
<p>Modules can import other modules. It is customary but not required to place all
<a class="reference external" href="../reference/simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> statements at the beginning of a module (or script, for that
matter). The imported module names are placed in the importing module’s global
symbol table.</p>
<p>There is a variant of the <a class="reference external" href="../reference/simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> statement that imports names from a
module directly into the importing module’s symbol table. For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">fibo</span> <span class="kn">import</span> <span class="n">fib</span><span class="p">,</span> <span class="n">fib2</span>
<span class="gp">>>> </span><span class="n">fib</span><span class="p">(</span><span class="mi">500</span><span class="p">)</span>
<span class="go">1 1 2 3 5 8 13 21 34 55 89 144 233 377</span>
</pre></div>
</div>
<p>This does not introduce the module name from which the imports are taken in the
local symbol table (so in the example, <tt class="docutils literal"><span class="pre">fibo</span></tt> is not defined).</p>
<p>There is even a variant to import all names that a module defines:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">fibo</span> <span class="kn">import</span> <span class="o">*</span>
<span class="gp">>>> </span><span class="n">fib</span><span class="p">(</span><span class="mi">500</span><span class="p">)</span>
<span class="go">1 1 2 3 5 8 13 21 34 55 89 144 233 377</span>
</pre></div>
</div>
<p>This imports all names except those beginning with an underscore (<tt class="docutils literal"><span class="pre">_</span></tt>).</p>
<p>Note that in general the practice of importing <tt class="docutils literal"><span class="pre">*</span></tt> 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.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">For efficiency reasons, each module is only imported once per interpreter
session. Therefore, if you change your modules, you must restart the
interpreter – or, if it’s just one module you want to test interactively,
use <a title="reload" class="reference external" href="../library/functions.html#reload"><tt class="xref docutils literal"><span class="pre">reload()</span></tt></a>, e.g. <tt class="docutils literal"><span class="pre">reload(modulename)</span></tt>.</p>
</div>
<div class="section" id="executing-modules-as-scripts">
<span id="tut-modulesasscripts"></span><h3>6.1.1. Executing modules as scripts<a class="headerlink" href="#executing-modules-as-scripts" title="Permalink to this headline">¶</a></h3>
<p>When you run a Python module with</p>
<div class="highlight-python"><pre>python fibo.py <arguments></pre>
</div>
<p>the code in the module will be executed, just as if you imported it, but with
the <tt class="docutils literal"><span class="pre">__name__</span></tt> set to <tt class="docutils literal"><span class="pre">"__main__"</span></tt>. That means that by adding this code at
the end of your module:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s">"__main__"</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="n">fib</span><span class="p">(</span><span class="nb">int</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]))</span>
</pre></div>
</div>
<p>you can make the file usable as a script as well as an importable module,
because the code that parses the command line only runs if the module is
executed as the “main” file:</p>
<div class="highlight-python"><pre>$ python fibo.py 50
1 1 2 3 5 8 13 21 34</pre>
</div>
<p>If the module is imported, the code is not run:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fibo</span>
<span class="go">>>></span>
</pre></div>
</div>
<p>This is often used either to provide a convenient user interface to a module, or
for testing purposes (running the module as a script executes a test suite).</p>
</div>
<div class="section" id="the-module-search-path">
<span id="tut-searchpath"></span><h3>6.1.2. The Module Search Path<a class="headerlink" href="#the-module-search-path" title="Permalink to this headline">¶</a></h3>
<p id="index-1097">When a module named <tt class="xref docutils literal"><span class="pre">spam</span></tt> is imported, the interpreter searches for a file
named <tt class="docutils literal"><span class="pre">spam.py</span></tt> in the current directory, and then in the list of
directories specified by the environment variable <span class="target" id="index-1098"></span><a class="reference external" href="../using/cmdline.html#envvar-PYTHONPATH"><strong class="xref">PYTHONPATH</strong></a>. This
has the same syntax as the shell variable <span class="target" id="index-1099"></span><strong class="xref">PATH</strong>, that is, a list of
directory names. When <span class="target" id="index-1100"></span><a class="reference external" href="../using/cmdline.html#envvar-PYTHONPATH"><strong class="xref">PYTHONPATH</strong></a> is not set, or when the file is not
found there, the search continues in an installation-dependent default path; on
Unix, this is usually <tt class="docutils literal"><span class="pre">.:/usr/local/lib/python</span></tt>.</p>
<p>Actually, modules are searched in the list of directories given by the variable
<tt class="docutils literal"><span class="pre">sys.path</span></tt> which is initialized from the directory containing the input script
(or the current directory), <span class="target" id="index-1101"></span><a class="reference external" href="../using/cmdline.html#envvar-PYTHONPATH"><strong class="xref">PYTHONPATH</strong></a> and the installation- dependent
default. This allows Python programs that know what they’re doing to modify or
replace the module search path. Note that because the directory containing the
script being run is on the search path, it is important that the script not have
the same name as a standard module, or Python will attempt to load the script as
a module when that module is imported. This will generally be an error. See
section <a class="reference internal" href="#tut-standardmodules"><em>Standard Modules</em></a> for more information.</p>
</div>
<div class="section" id="compiled-python-files">
<h3>6.1.3. “Compiled” Python files<a class="headerlink" href="#compiled-python-files" title="Permalink to this headline">¶</a></h3>
<p>As an important speed-up of the start-up time for short programs that use a lot
of standard modules, if a file called <tt class="docutils literal"><span class="pre">spam.pyc</span></tt> exists in the directory
where <tt class="docutils literal"><span class="pre">spam.py</span></tt> is found, this is assumed to contain an
already-“byte-compiled” version of the module <tt class="xref docutils literal"><span class="pre">spam</span></tt>. The modification time
of the version of <tt class="docutils literal"><span class="pre">spam.py</span></tt> used to create <tt class="docutils literal"><span class="pre">spam.pyc</span></tt> is recorded in
<tt class="docutils literal"><span class="pre">spam.pyc</span></tt>, and the <tt class="docutils literal"><span class="pre">.pyc</span></tt> file is ignored if these don’t match.</p>
<p>Normally, you don’t need to do anything to create the <tt class="docutils literal"><span class="pre">spam.pyc</span></tt> file.
Whenever <tt class="docutils literal"><span class="pre">spam.py</span></tt> is successfully compiled, an attempt is made to write
the compiled version to <tt class="docutils literal"><span class="pre">spam.pyc</span></tt>. It is not an error if this attempt
fails; if for any reason the file is not written completely, the resulting
<tt class="docutils literal"><span class="pre">spam.pyc</span></tt> file will be recognized as invalid and thus ignored later. The
contents of the <tt class="docutils literal"><span class="pre">spam.pyc</span></tt> file are platform independent, so a Python
module directory can be shared by machines of different architectures.</p>
<p>Some tips for experts:</p>
<ul>
<li><p class="first">When the Python interpreter is invoked with the <a class="reference external" href="../using/cmdline.html#cmdoption-O"><em class="xref">-O</em></a> flag, optimized
code is generated and stored in <tt class="docutils literal"><span class="pre">.pyo</span></tt> files. The optimizer currently
doesn’t help much; it only removes <a class="reference external" href="../reference/simple_stmts.html#assert"><tt class="xref docutils literal"><span class="pre">assert</span></tt></a> statements. When
<a class="reference external" href="../using/cmdline.html#cmdoption-O"><em class="xref">-O</em></a> is used, <em>all</em> <a class="reference external" href="../glossary.html#term-bytecode"><em class="xref">bytecode</em></a> is optimized; <tt class="docutils literal"><span class="pre">.pyc</span></tt> files are
ignored and <tt class="docutils literal"><span class="pre">.py</span></tt> files are compiled to optimized bytecode.</p>
</li>
<li><p class="first">Passing two <a class="reference external" href="../using/cmdline.html#cmdoption-O"><em class="xref">-O</em></a> flags to the Python interpreter (<a class="reference external" href="../using/cmdline.html#cmdoption-OO"><em class="xref">-OO</em></a>) will
cause the bytecode compiler to perform optimizations that could in some rare
cases result in malfunctioning programs. Currently only <tt class="docutils literal"><span class="pre">__doc__</span></tt> strings are
removed from the bytecode, resulting in more compact <tt class="docutils literal"><span class="pre">.pyo</span></tt> files. Since
some programs may rely on having these available, you should only use this
option if you know what you’re doing.</p>
</li>
<li><p class="first">A program doesn’t run any faster when it is read from a <tt class="docutils literal"><span class="pre">.pyc</span></tt> or
<tt class="docutils literal"><span class="pre">.pyo</span></tt> file than when it is read from a <tt class="docutils literal"><span class="pre">.py</span></tt> file; the only thing
that’s faster about <tt class="docutils literal"><span class="pre">.pyc</span></tt> or <tt class="docutils literal"><span class="pre">.pyo</span></tt> files is the speed with which
they are loaded.</p>
</li>
<li><p class="first">When a script is run by giving its name on the command line, the bytecode for
the script is never written to a <tt class="docutils literal"><span class="pre">.pyc</span></tt> or <tt class="docutils literal"><span class="pre">.pyo</span></tt> file. Thus, the
startup time of a script may be reduced by moving most of its code to a module
and having a small bootstrap script that imports that module. It is also
possible to name a <tt class="docutils literal"><span class="pre">.pyc</span></tt> or <tt class="docutils literal"><span class="pre">.pyo</span></tt> file directly on the command
line.</p>
</li>
<li><p class="first">It is possible to have a file called <tt class="docutils literal"><span class="pre">spam.pyc</span></tt> (or <tt class="docutils literal"><span class="pre">spam.pyo</span></tt>
when <a class="reference external" href="../using/cmdline.html#cmdoption-O"><em class="xref">-O</em></a> is used) without a file <tt class="docutils literal"><span class="pre">spam.py</span></tt> for the same module.
This can be used to distribute a library of Python code in a form that is
moderately hard to reverse engineer.</p>
</li>
<li id="index-1102"><p class="first">The module <a title="Tools for byte-compiling all Python source files in a directory tree." class="reference external" href="../library/compileall.html#module-compileall"><tt class="xref docutils literal"><span class="pre">compileall</span></tt></a> can create <tt class="docutils literal"><span class="pre">.pyc</span></tt> files (or <tt class="docutils literal"><span class="pre">.pyo</span></tt>
files when <a class="reference external" href="../using/cmdline.html#cmdoption-O"><em class="xref">-O</em></a> is used) for all modules in a directory.</p>
</li>
</ul>
</div>
</div>
<div class="section" id="standard-modules">
<span id="tut-standardmodules"></span><h2>6.2. Standard Modules<a class="headerlink" href="#standard-modules" title="Permalink to this headline">¶</a></h2>
<p id="index-1103">Python comes with a library of standard modules, described in a separate
document, the Python Library Reference (“Library Reference” hereafter). Some
modules are built into the interpreter; these provide access to operations that
are not part of the core of the language but are nevertheless built in, either
for efficiency or to provide access to operating system primitives such as
system calls. The set of such modules is a configuration option which also
depends on the underlying platform For example, the <tt class="xref docutils literal"><span class="pre">winreg</span></tt> module is only
provided on Windows systems. One particular module deserves some attention:
<a title="Access system-specific parameters and functions." class="reference external" href="../library/sys.html#module-sys"><tt class="xref docutils literal"><span class="pre">sys</span></tt></a>, which is built into every Python interpreter. The variables
<tt class="docutils literal"><span class="pre">sys.ps1</span></tt> and <tt class="docutils literal"><span class="pre">sys.ps2</span></tt> define the strings used as primary and secondary
prompts:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sys</span>
<span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">ps1</span>
<span class="go">'>>> '</span>
<span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">ps2</span>
<span class="go">'... '</span>
<span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">ps1</span> <span class="o">=</span> <span class="s">'C> '</span>
<span class="go">C> print 'Yuck!'</span>
<span class="go">Yuck!</span>
<span class="go">C></span>
</pre></div>
</div>
<p>These two variables are only defined if the interpreter is in interactive mode.</p>
<p>The variable <tt class="docutils literal"><span class="pre">sys.path</span></tt> is a list of strings that determines the interpreter’s
search path for modules. It is initialized to a default path taken from the
environment variable <span class="target" id="index-1104"></span><a class="reference external" href="../using/cmdline.html#envvar-PYTHONPATH"><strong class="xref">PYTHONPATH</strong></a>, or from a built-in default if
<span class="target" id="index-1105"></span><a class="reference external" href="../using/cmdline.html#envvar-PYTHONPATH"><strong class="xref">PYTHONPATH</strong></a> is not set. You can modify it using standard list
operations:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sys</span>
<span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s">'/ufs/guido/lib/python'</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="the-dir-function">
<span id="tut-dir"></span><h2>6.3. The <a title="dir" class="reference external" href="../library/functions.html#dir"><tt class="xref docutils literal"><span class="pre">dir()</span></tt></a> Function<a class="headerlink" href="#the-dir-function" title="Permalink to this headline">¶</a></h2>
<p>The built-in function <a title="dir" class="reference external" href="../library/functions.html#dir"><tt class="xref docutils literal"><span class="pre">dir()</span></tt></a> is used to find out which names a module
defines. It returns a sorted list of strings:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fibo</span><span class="o">,</span> <span class="nn">sys</span>
<span class="gp">>>> </span><span class="nb">dir</span><span class="p">(</span><span class="n">fibo</span><span class="p">)</span>
<span class="go">['__name__', 'fib', 'fib2']</span>
<span class="gp">>>> </span><span class="nb">dir</span><span class="p">(</span><span class="n">sys</span><span class="p">)</span>
<span class="go">['__displayhook__', '__doc__', '__excepthook__', '__name__', '__stderr__',</span>
<span class="go"> '__stdin__', '__stdout__', '_getframe', 'api_version', 'argv',</span>
<span class="go"> 'builtin_module_names', 'byteorder', 'callstats', 'copyright',</span>
<span class="go"> 'displayhook', 'exc_clear', 'exc_info', 'exc_type', 'excepthook',</span>
<span class="go"> 'exec_prefix', 'executable', 'exit', 'getdefaultencoding', 'getdlopenflags',</span>
<span class="go"> 'getrecursionlimit', 'getrefcount', 'hexversion', 'maxint', 'maxunicode',</span>
<span class="go"> 'meta_path', 'modules', 'path', 'path_hooks', 'path_importer_cache',</span>
<span class="go"> 'platform', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setdlopenflags',</span>
<span class="go"> 'setprofile', 'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout',</span>
<span class="go"> 'version', 'version_info', 'warnoptions']</span>
</pre></div>
</div>
<p>Without arguments, <a title="dir" class="reference external" href="../library/functions.html#dir"><tt class="xref docutils literal"><span class="pre">dir()</span></tt></a> lists the names you have defined currently:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fibo</span>
<span class="gp">>>> </span><span class="n">fib</span> <span class="o">=</span> <span class="n">fibo</span><span class="o">.</span><span class="n">fib</span>
<span class="gp">>>> </span><span class="nb">dir</span><span class="p">()</span>
<span class="go">['__builtins__', '__doc__', '__file__', '__name__', 'a', 'fib', 'fibo', 'sys']</span>
</pre></div>
</div>
<p>Note that it lists all types of names: variables, modules, functions, etc.</p>
<p id="index-1106"><a title="dir" class="reference external" href="../library/functions.html#dir"><tt class="xref docutils literal"><span class="pre">dir()</span></tt></a> does not list the names of built-in functions and variables. If you
want a list of those, they are defined in the standard module
<a title="The module that provides the built-in namespace." class="reference external" href="../library/__builtin__.html#module-__builtin__"><tt class="xref docutils literal"><span class="pre">__builtin__</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">__builtin__</span>
<span class="gp">>>> </span><span class="nb">dir</span><span class="p">(</span><span class="n">__builtin__</span><span class="p">)</span>
<span class="go">['ArithmeticError', 'AssertionError', 'AttributeError', 'DeprecationWarning',</span>
<span class="go"> 'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False',</span>
<span class="go"> 'FloatingPointError', 'FutureWarning', 'IOError', 'ImportError',</span>
<span class="go"> 'IndentationError', 'IndexError', 'KeyError', 'KeyboardInterrupt',</span>
<span class="go"> 'LookupError', 'MemoryError', 'NameError', 'None', 'NotImplemented',</span>
<span class="go"> 'NotImplementedError', 'OSError', 'OverflowError',</span>
<span class="go"> 'PendingDeprecationWarning', 'ReferenceError', 'RuntimeError',</span>
<span class="go"> 'RuntimeWarning', 'StandardError', 'StopIteration', 'SyntaxError',</span>
<span class="go"> 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'True',</span>
<span class="go"> 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError',</span>
<span class="go"> 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError',</span>
<span class="go"> 'UserWarning', 'ValueError', 'Warning', 'WindowsError',</span>
<span class="go"> 'ZeroDivisionError', '_', '__debug__', '__doc__', '__import__',</span>
<span class="go"> '__name__', 'abs', 'apply', 'basestring', 'bool', 'buffer',</span>
<span class="go"> 'callable', 'chr', 'classmethod', 'cmp', 'coerce', 'compile',</span>
<span class="go"> 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'divmod',</span>
<span class="go"> 'enumerate', 'eval', 'execfile', 'exit', 'file', 'filter', 'float',</span>
<span class="go"> 'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex',</span>
<span class="go"> 'id', 'input', 'int', 'intern', 'isinstance', 'issubclass', 'iter',</span>
<span class="go"> 'len', 'license', 'list', 'locals', 'long', 'map', 'max', 'min',</span>
<span class="go"> 'object', 'oct', 'open', 'ord', 'pow', 'property', 'quit', 'range',</span>
<span class="go"> 'raw_input', 'reduce', 'reload', 'repr', 'reversed', 'round', 'set',</span>
<span class="go"> 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super',</span>
<span class="go"> 'tuple', 'type', 'unichr', 'unicode', 'vars', 'xrange', 'zip']</span>
</pre></div>
</div>
</div>
<div class="section" id="packages">
<span id="tut-packages"></span><h2>6.4. Packages<a class="headerlink" href="#packages" title="Permalink to this headline">¶</a></h2>
<p>Packages are a way of structuring Python’s module namespace by using “dotted
module names”. For example, the module name <tt class="xref docutils literal"><span class="pre">A.B</span></tt> designates a submodule
named <tt class="docutils literal"><span class="pre">B</span></tt> in a package named <tt class="docutils literal"><span class="pre">A</span></tt>. 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.</p>
<p>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: <tt class="docutils literal"><span class="pre">.wav</span></tt>,
<tt class="docutils literal"><span class="pre">.aiff</span></tt>, <tt class="docutils literal"><span class="pre">.au</span></tt>), 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):</p>
<div class="highlight-python"><pre>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
...</pre>
</div>
<p>When importing the package, Python searches through the directories on
<tt class="docutils literal"><span class="pre">sys.path</span></tt> looking for the package subdirectory.</p>
<p>The <tt class="docutils literal"><span class="pre">__init__.py</span></tt> files are required to make Python treat the directories
as containing packages; this is done to prevent directories with a common name,
such as <tt class="docutils literal"><span class="pre">string</span></tt>, from unintentionally hiding valid modules that occur later
on the module search path. In the simplest case, <tt class="docutils literal"><span class="pre">__init__.py</span></tt> can just be
an empty file, but it can also execute initialization code for the package or
set the <tt class="docutils literal"><span class="pre">__all__</span></tt> variable, described later.</p>
<p>Users of the package can import individual modules from the package, for
example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sound.effects.echo</span>
</pre></div>
</div>
<p>This loads the submodule <tt class="xref docutils literal"><span class="pre">sound.effects.echo</span></tt>. It must be referenced with
its full name.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">sound</span><span class="o">.</span><span class="n">effects</span><span class="o">.</span><span class="n">echo</span><span class="o">.</span><span class="n">echofilter</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">output</span><span class="p">,</span> <span class="n">delay</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">atten</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
</pre></div>
</div>
<p>An alternative way of importing the submodule is:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sound.effects</span> <span class="kn">import</span> <span class="n">echo</span>
</pre></div>
</div>
<p>This also loads the submodule <tt class="xref docutils literal"><span class="pre">echo</span></tt>, and makes it available without its
package prefix, so it can be used as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">echo</span><span class="o">.</span><span class="n">echofilter</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">output</span><span class="p">,</span> <span class="n">delay</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">atten</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
</pre></div>
</div>
<p>Yet another variation is to import the desired function or variable directly:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sound.effects.echo</span> <span class="kn">import</span> <span class="n">echofilter</span>
</pre></div>
</div>
<p>Again, this loads the submodule <tt class="xref docutils literal"><span class="pre">echo</span></tt>, but this makes its function
<tt class="xref docutils literal"><span class="pre">echofilter()</span></tt> directly available:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">echofilter</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">output</span><span class="p">,</span> <span class="n">delay</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">atten</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that when using <tt class="docutils literal"><span class="pre">from</span> <span class="pre">package</span> <span class="pre">import</span> <span class="pre">item</span></tt>, 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 <tt class="docutils literal"><span class="pre">import</span></tt> 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 <a title="exceptions.ImportError" class="reference external" href="../library/exceptions.html#exceptions.ImportError"><tt class="xref docutils literal"><span class="pre">ImportError</span></tt></a>
exception is raised.</p>
<p>Contrarily, when using syntax like <tt class="docutils literal"><span class="pre">import</span> <span class="pre">item.subitem.subsubitem</span></tt>, 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.</p>
<div class="section" id="importing-from-a-package">
<span id="tut-pkg-import-star"></span><h3>6.4.1. Importing * From a Package<a class="headerlink" href="#importing-from-a-package" title="Permalink to this headline">¶</a></h3>
<p id="index-1107">Now what happens when the user writes <tt class="docutils literal"><span class="pre">from</span> <span class="pre">sound.effects</span> <span class="pre">import</span> <span class="pre">*</span></tt>? 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.</p>
<p>The only solution is for the package author to provide an explicit index of the
package. The <a class="reference external" href="../reference/simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> statement uses the following convention: if a package’s
<tt class="docutils literal"><span class="pre">__init__.py</span></tt> code defines a list named <tt class="docutils literal"><span class="pre">__all__</span></tt>, it is taken to be the
list of module names that should be imported when <tt class="docutils literal"><span class="pre">from</span> <span class="pre">package</span> <span class="pre">import</span> <span class="pre">*</span></tt> 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 <tt class="docutils literal"><span class="pre">sounds/effects/__init__.py</span></tt> could contain the following
code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">__all__</span> <span class="o">=</span> <span class="p">[</span><span class="s">"echo"</span><span class="p">,</span> <span class="s">"surround"</span><span class="p">,</span> <span class="s">"reverse"</span><span class="p">]</span>
</pre></div>
</div>
<p>This would mean that <tt class="docutils literal"><span class="pre">from</span> <span class="pre">sound.effects</span> <span class="pre">import</span> <span class="pre">*</span></tt> would import the three
named submodules of the <tt class="xref docutils literal"><span class="pre">sound</span></tt> package.</p>
<p>If <tt class="docutils literal"><span class="pre">__all__</span></tt> is not defined, the statement <tt class="docutils literal"><span class="pre">from</span> <span class="pre">sound.effects</span> <span class="pre">import</span> <span class="pre">*</span></tt>
does <em>not</em> import all submodules from the package <tt class="xref docutils literal"><span class="pre">sound.effects</span></tt> into the
current namespace; it only ensures that the package <tt class="xref docutils literal"><span class="pre">sound.effects</span></tt> has
been imported (possibly running any initialization code in <tt class="docutils literal"><span class="pre">__init__.py</span></tt>)
and then imports whatever names are defined in the package. This includes any
names defined (and submodules explicitly loaded) by <tt class="docutils literal"><span class="pre">__init__.py</span></tt>. It
also includes any submodules of the package that were explicitly loaded by
previous <a class="reference external" href="../reference/simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> statements. Consider this code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sound.effects.echo</span>
<span class="kn">import</span> <span class="nn">sound.effects.surround</span>
<span class="kn">from</span> <span class="nn">sound.effects</span> <span class="kn">import</span> <span class="o">*</span>
</pre></div>
</div>
<p>In this example, the <tt class="xref docutils literal"><span class="pre">echo</span></tt> and <tt class="xref docutils literal"><span class="pre">surround</span></tt> modules are imported in the
current namespace because they are defined in the <tt class="xref docutils literal"><span class="pre">sound.effects</span></tt> package
when the <tt class="docutils literal"><span class="pre">from...import</span></tt> statement is executed. (This also works when
<tt class="docutils literal"><span class="pre">__all__</span></tt> is defined.)</p>
<p>Although certain modules are designed to export only names that follow certain
patterns when you use <tt class="docutils literal"><span class="pre">import</span> <span class="pre">*</span></tt>, it is still considered bad practise in
production code.</p>
<p>Remember, there is nothing wrong with using <tt class="docutils literal"><span class="pre">from</span> <span class="pre">Package</span> <span class="pre">import</span>
<span class="pre">specific_submodule</span></tt>! In fact, this is the recommended notation unless the
importing module needs to use submodules with the same name from different
packages.</p>
</div>
<div class="section" id="intra-package-references">
<h3>6.4.2. Intra-package References<a class="headerlink" href="#intra-package-references" title="Permalink to this headline">¶</a></h3>
<p>The submodules often need to refer to each other. For example, the
<tt class="xref docutils literal"><span class="pre">surround</span></tt> module might use the <tt class="xref docutils literal"><span class="pre">echo</span></tt> module. In fact, such
references are so common that the <a class="reference external" href="../reference/simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> statement first looks in the
containing package before looking in the standard module search path. Thus, the
<tt class="xref docutils literal"><span class="pre">surround</span></tt> module can simply use <tt class="docutils literal"><span class="pre">import</span> <span class="pre">echo</span></tt> or <tt class="docutils literal"><span class="pre">from</span> <span class="pre">echo</span> <span class="pre">import</span>
<span class="pre">echofilter</span></tt>. If the imported module is not found in the current package (the
package of which the current module is a submodule), the <a class="reference external" href="../reference/simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a>
statement looks for a top-level module with the given name.</p>
<p>When packages are structured into subpackages (as with the <tt class="xref docutils literal"><span class="pre">sound</span></tt> package
in the example), you can use absolute imports to refer to submodules of siblings
packages. For example, if the module <tt class="xref docutils literal"><span class="pre">sound.filters.vocoder</span></tt> needs to use
the <tt class="xref docutils literal"><span class="pre">echo</span></tt> module in the <tt class="xref docutils literal"><span class="pre">sound.effects</span></tt> package, it can use <tt class="docutils literal"><span class="pre">from</span>
<span class="pre">sound.effects</span> <span class="pre">import</span> <span class="pre">echo</span></tt>.</p>
<p>Starting with Python 2.5, in addition to the implicit relative imports described
above, you can write explicit relative imports with the <tt class="docutils literal"><span class="pre">from</span> <span class="pre">module</span> <span class="pre">import</span>
<span class="pre">name</span></tt> form of import statement. These explicit relative imports use leading
dots to indicate the current and parent packages involved in the relative
import. From the <tt class="xref docutils literal"><span class="pre">surround</span></tt> module for example, you might use:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">.</span> <span class="kn">import</span> <span class="n">echo</span>
<span class="kn">from</span> <span class="nn">..</span> <span class="kn">import</span> <span class="n">formats</span>
<span class="kn">from</span> <span class="nn">..filters</span> <span class="kn">import</span> <span class="n">equalizer</span>
</pre></div>
</div>
<p>Note that both explicit and implicit relative imports are based on the name of
the current module. Since the name of the main module is always <tt class="docutils literal"><span class="pre">"__main__"</span></tt>,
modules intended for use as the main module of a Python application should
always use absolute imports.</p>
</div>
<div class="section" id="packages-in-multiple-directories">
<h3>6.4.3. Packages in Multiple Directories<a class="headerlink" href="#packages-in-multiple-directories" title="Permalink to this headline">¶</a></h3>
<p>Packages support one more special attribute, <tt class="xref docutils literal"><span class="pre">__path__</span></tt>. This is
initialized to be a list containing the name of the directory holding the
package’s <tt class="docutils literal"><span class="pre">__init__.py</span></tt> 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.</p>
<p>While this feature is not often needed, it can be used to extend the set of
modules found in a package.</p>
<p class="rubric">Footnotes</p>
<table class="docutils footnote" frame="void" id="id2" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>In fact function definitions are also ‘statements’ that are ‘executed’; the
execution of a module-level function enters the function name in the module’s
global symbol table.</td></tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar">
<div class="sphinxsidebarwrapper">
<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference external" href="#">6. Modules</a><ul>
<li><a class="reference external" href="#more-on-modules">6.1. More on Modules</a><ul>
<li><a class="reference external" href="#executing-modules-as-scripts">6.1.1. Executing modules as scripts</a></li>
<li><a class="reference external" href="#the-module-search-path">6.1.2. The Module Search Path</a></li>
<li><a class="reference external" href="#compiled-python-files">6.1.3. “Compiled” Python files</a></li>
</ul>
</li>
<li><a class="reference external" href="#standard-modules">6.2. Standard Modules</a></li>
<li><a class="reference external" href="#the-dir-function">6.3. The <tt class="docutils literal"><span class="pre">dir()</span></tt> Function</a></li>
<li><a class="reference external" href="#packages">6.4. Packages</a><ul>
<li><a class="reference external" href="#importing-from-a-package">6.4.1. Importing * From a Package</a></li>
<li><a class="reference external" href="#intra-package-references">6.4.2. Intra-package References</a></li>
<li><a class="reference external" href="#packages-in-multiple-directories">6.4.3. Packages in Multiple Directories</a></li>
</ul>
</li>
</ul>
</li>
</ul>
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<h4>Next topic</h4>
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title="next chapter">7. Input and Output</a></p>
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