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 <div class="section" id="modules">
<h1>6. Modules<a class="headerlink" href="#modules" title="Permalink to this headline">¶</a></h1>
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 script. 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&#8217;ve written in several programs without copying its
definition into each program.
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
module; definitions from a module can be imported into other modules or into
the main module (the collection of variables that you have access to in a
script executed at the top level and in calculator mode).
A module is a file containing Python definitions and statements. The file name
is the module name with the suffix <tt class="file docutils literal">.py</tt> appended. Within a module, the
module&#8217;s name (as a string) is available as the value of the global variable
<tt class="docutils literal">__name__</tt>. For instance, use your favorite text editor to create a file
called <tt class="file docutils literal">fibo.py</tt> in the current directory with the following contents:
<div class="highlight-python"><div class="highlight"><pre># Fibonacci numbers module

def fib(n): # write Fibonacci series up to n
 a, b = 0, 1
 while b &lt; n:
 print b,
 a, b = b, a+b

def fib2(n): # return Fibonacci series up to n
 result = []
 a, b = 0, 1
 while b &lt; n:
 result.append(b)
 a, b = b, a+b
 return result
</pre></div>
</div>
Now enter the Python interpreter and import this module with the following
command:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import fibo
</pre></div>
</div>
This does not enter the names of the functions defined in <tt class="docutils literal">fibo</tt> directly in
the current symbol table; it only enters the module name <tt class="docutils literal">fibo</tt> there. Using
the module name you can access the functions:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; fibo.fib(1000)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
&gt;&gt;&gt; fibo.fib2(100)
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
&gt;&gt;&gt; fibo.__name__
&#39;fibo&#39;
</pre></div>
</div>
If you intend to use a function often you can assign it to a local name:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; fib = fibo.fib
&gt;&gt;&gt; fib(500)
1 1 2 3 5 8 13 21 34 55 89 144 233 377
</pre></div>
</div>
<div class="section" id="more-on-modules">
<h2>6.1. More on Modules<a class="headerlink" href="#more-on-modules" title="Permalink to this headline">¶</a></h2>
A module can contain executable statements as well as function definitions.
These statements are intended to initialize the module. They are executed only
the first time the module name is encountered in an import statement. <a class="footnote-reference" href="#id2" id="id1">[1]</a>
(They are also run if the file is executed as a script.)
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&#8217;s global variables. On the other hand, if you know what you are
doing you can touch a module&#8217;s global variables with the same notation used to
refer to its functions, <tt class="docutils literal">modname.itemname</tt>.
Modules can import other modules. It is customary but not required to place all
<a class="reference internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal">import</tt></a> statements at the beginning of a module (or script, for that
matter). The imported module names are placed in the importing module&#8217;s global
symbol table.
There is a variant of the <a class="reference internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal">import</tt></a> statement that imports names from a
module directly into the importing module&#8217;s symbol table. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; from fibo import fib, fib2
&gt;&gt;&gt; fib(500)
1 1 2 3 5 8 13 21 34 55 89 144 233 377
</pre></div>
</div>
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">fibo</tt> is not defined).
There is even a variant to import all names that a module defines:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; from fibo import *
&gt;&gt;&gt; fib(500)
1 1 2 3 5 8 13 21 34 55 89 144 233 377
</pre></div>
</div>
This imports all names except those beginning with an underscore (<tt class="docutils literal">_</tt>).
Note that in general the practice of importing <tt class="docutils literal">*</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.
<div class="admonition note">
Note
For efficiency reasons, each module is only imported once per interpreter
session. Therefore, if you change your modules, you must restart the
interpreter &#8211; or, if it&#8217;s just one module you want to test interactively,
use <a class="reference internal" href="../library/functions.html#reload" title="reload"><tt class="xref py py-func docutils literal">reload()</tt></a>, e.g. <tt class="docutils literal">reload(modulename)</tt>.
</div>
<div class="section" id="executing-modules-as-scripts">
<h3>6.1.1. Executing modules as scripts<a class="headerlink" href="#executing-modules-as-scripts" title="Permalink to this headline">¶</a></h3>
When you run a Python module with
<div class="highlight-python"><pre>python fibo.py &lt;arguments&gt;</pre>
</div>
the code in the module will be executed, just as if you imported it, but with
the <tt class="docutils literal">__name__</tt> set to <tt class="docutils literal">&quot;__main__&quot;</tt>. That means that by adding this code at
the end of your module:
<div class="highlight-python"><div class="highlight"><pre>if __name__ == &quot;__main__&quot;:
 import sys
 fib(int(sys.argv[1]))
</pre></div>
</div>
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 &#8220;main&#8221; file:
<div class="highlight-python"><pre>$ python fibo.py 50
1 1 2 3 5 8 13 21 34</pre>
</div>
If the module is imported, the code is not run:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import fibo
&gt;&gt;&gt;
</pre></div>
</div>
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).
</div>
<div class="section" id="the-module-search-path">
<h3>6.1.2. The Module Search Path<a class="headerlink" href="#the-module-search-path" title="Permalink to this headline">¶</a></h3>
When a module named <tt class="xref py py-mod docutils literal">spam</tt> is imported, the interpreter first searches for
a built-in module with that name. If not found, it then searches for a file
named <tt class="file docutils literal">spam.py</tt> in a list of directories given by the variable
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><tt class="xref py py-data docutils literal">sys.path</tt></a>. <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><tt class="xref py py-data docutils literal">sys.path</tt></a> is initialized from these locations:
<ul class="simple">
<li>the directory containing the input script (or the current directory).</li>
<li><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><tt class="xref std std-envvar docutils literal">PYTHONPATH</tt></a> (a list of directory names, with the same syntax as the
shell variable <tt class="xref std std-envvar docutils literal">PATH</tt>).</li>
<li>the installation-dependent default.</li>
</ul>
After initialization, Python programs can modify <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><tt class="xref py py-data docutils literal">sys.path</tt></a>. The
directory containing the script being run is placed at the beginning of the
search path, ahead of the standard library path. This means that scripts in that
directory will be loaded instead of modules of the same name in the library
directory. This is an error unless the replacement is intended. See section
<a class="reference internal" href="#tut-standardmodules">Standard Modules</a> for more information.
</div>
<div class="section" id="compiled-python-files">
<h3>6.1.3. &#8220;Compiled&#8221; Python files<a class="headerlink" href="#compiled-python-files" title="Permalink to this headline">¶</a></h3>
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="file docutils literal">spam.pyc</tt> exists in the directory
where <tt class="file docutils literal">spam.py</tt> is found, this is assumed to contain an
already-&#8220;byte-compiled&#8221; version of the module <tt class="xref py py-mod docutils literal">spam</tt>. The modification time
of the version of <tt class="file docutils literal">spam.py</tt> used to create <tt class="file docutils literal">spam.pyc</tt> is recorded in
<tt class="file docutils literal">spam.pyc</tt>, and the <tt class="file docutils literal">.pyc</tt> file is ignored if these don&#8217;t match.
Normally, you don&#8217;t need to do anything to create the <tt class="file docutils literal">spam.pyc</tt> file.
Whenever <tt class="file docutils literal">spam.py</tt> is successfully compiled, an attempt is made to write
the compiled version to <tt class="file docutils literal">spam.pyc</tt>. It is not an error if this attempt
fails; if for any reason the file is not written completely, the resulting
<tt class="file docutils literal">spam.pyc</tt> file will be recognized as invalid and thus ignored later. The
contents of the <tt class="file docutils literal">spam.pyc</tt> file are platform independent, so a Python
module directory can be shared by machines of different architectures.
Some tips for experts:
<ul>
<li>When the Python interpreter is invoked with the <a class="reference internal" href="../using/cmdline.html#cmdoption-O">-O</a> flag, optimized
code is generated and stored in <tt class="file docutils literal">.pyo</tt> files. The optimizer currently
doesn&#8217;t help much; it only removes <a class="reference internal" href="../reference/simple_stmts.html#assert"><tt class="xref std std-keyword docutils literal">assert</tt></a> statements. When
<a class="reference internal" href="../using/cmdline.html#cmdoption-O">-O</a> is used, all <a class="reference internal" href="../glossary.html#term-bytecode">bytecode</a> is optimized; <tt class="docutils literal">.pyc</tt> files are
ignored and <tt class="docutils literal">.py</tt> files are compiled to optimized bytecode.
</li>
<li>Passing two <a class="reference internal" href="../using/cmdline.html#cmdoption-O">-O</a> flags to the Python interpreter (<a class="reference internal" href="../using/cmdline.html#cmdoption-OO">-OO</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">__doc__</tt> strings are
removed from the bytecode, resulting in more compact <tt class="file docutils literal">.pyo</tt> files. Since
some programs may rely on having these available, you should only use this
option if you know what you&#8217;re doing.
</li>
<li>A program doesn&#8217;t run any faster when it is read from a <tt class="file docutils literal">.pyc</tt> or
<tt class="file docutils literal">.pyo</tt> file than when it is read from a <tt class="file docutils literal">.py</tt> file; the only thing
that&#8217;s faster about <tt class="file docutils literal">.pyc</tt> or <tt class="file docutils literal">.pyo</tt> files is the speed with which
they are loaded.
</li>
<li>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="file docutils literal">.pyc</tt> or <tt class="file docutils literal">.pyo</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="file docutils literal">.pyc</tt> or <tt class="file docutils literal">.pyo</tt> file directly on the command
line.
</li>
<li>It is possible to have a file called <tt class="file docutils literal">spam.pyc</tt> (or <tt class="file docutils literal">spam.pyo</tt>
when <a class="reference internal" href="../using/cmdline.html#cmdoption-O">-O</a> is used) without a file <tt class="file docutils literal">spam.py</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.
</li>
<li id="index-3">The module <a class="reference internal" href="../library/compileall.html#module-compileall" title="compileall: Tools for byte-compiling all Python source files in a directory tree."><tt class="xref py py-mod docutils literal">compileall</tt></a> can create <tt class="file docutils literal">.pyc</tt> files (or <tt class="file docutils literal">.pyo</tt>
files when <a class="reference internal" href="../using/cmdline.html#cmdoption-O">-O</a> is used) for all modules in a directory.
</li>
</ul>
</div>
</div>
<div class="section" id="standard-modules">
<h2>6.2. Standard Modules<a class="headerlink" href="#standard-modules" title="Permalink to this headline">¶</a></h2>
Python comes with a library of standard modules, described in a separate
document, the Python Library Reference (&#8220;Library Reference&#8221; 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 py py-mod docutils literal">winreg</tt> module is only
provided on Windows systems. One particular module deserves some attention:
<a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><tt class="xref py py-mod docutils literal">sys</tt></a>, which is built into every Python interpreter. The variables
<tt class="docutils literal">sys.ps1</tt> and <tt class="docutils literal">sys.ps2</tt> define the strings used as primary and secondary
prompts:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import sys
&gt;&gt;&gt; sys.ps1
&#39;&gt;&gt;&gt; &#39;
&gt;&gt;&gt; sys.ps2
&#39;... &#39;
&gt;&gt;&gt; sys.ps1 = &#39;C&gt; &#39;
C&gt; print &#39;Yuck!&#39;
Yuck!
C&gt;
</pre></div>
</div>
These two variables are only defined if the interpreter is in interactive mode.
The variable <tt class="docutils literal">sys.path</tt> is a list of strings that determines the interpreter&#8217;s
search path for modules. It is initialized to a default path taken from the
environment variable <a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><tt class="xref std std-envvar docutils literal">PYTHONPATH</tt></a>, or from a built-in default if
<a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><tt class="xref std std-envvar docutils literal">PYTHONPATH</tt></a> is not set. You can modify it using standard list
operations:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import sys
&gt;&gt;&gt; sys.path.append(&#39;/ufs/guido/lib/python&#39;)
</pre></div>
</div>
</div>
<div class="section" id="the-dir-function">
<h2>6.3. The <a class="reference internal" href="../library/functions.html#dir" title="dir"><tt class="xref py py-func docutils literal">dir()</tt></a> Function<a class="headerlink" href="#the-dir-function" title="Permalink to this headline">¶</a></h2>
The built-in function <a class="reference internal" href="../library/functions.html#dir" title="dir"><tt class="xref py py-func docutils literal">dir()</tt></a> is used to find out which names a module
defines. It returns a sorted list of strings:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import fibo, sys
&gt;&gt;&gt; dir(fibo)
[&#39;__name__&#39;, &#39;fib&#39;, &#39;fib2&#39;]
&gt;&gt;&gt; dir(sys) 
[&#39;__displayhook__&#39;, &#39;__doc__&#39;, &#39;__excepthook__&#39;, &#39;__name__&#39;, &#39;__package__&#39;,
 &#39;__stderr__&#39;, &#39;__stdin__&#39;, &#39;__stdout__&#39;, &#39;_clear_type_cache&#39;,
 &#39;_current_frames&#39;, &#39;_getframe&#39;, &#39;_mercurial&#39;, &#39;api_version&#39;, &#39;argv&#39;,
 &#39;builtin_module_names&#39;, &#39;byteorder&#39;, &#39;call_tracing&#39;, &#39;callstats&#39;,
 &#39;copyright&#39;, &#39;displayhook&#39;, &#39;dont_write_bytecode&#39;, &#39;exc_clear&#39;, &#39;exc_info&#39;,
 &#39;exc_traceback&#39;, &#39;exc_type&#39;, &#39;exc_value&#39;, &#39;excepthook&#39;, &#39;exec_prefix&#39;,
 &#39;executable&#39;, &#39;exit&#39;, &#39;flags&#39;, &#39;float_info&#39;, &#39;float_repr_style&#39;,
 &#39;getcheckinterval&#39;, &#39;getdefaultencoding&#39;, &#39;getdlopenflags&#39;,
 &#39;getfilesystemencoding&#39;, &#39;getobjects&#39;, &#39;getprofile&#39;, &#39;getrecursionlimit&#39;,
 &#39;getrefcount&#39;, &#39;getsizeof&#39;, &#39;gettotalrefcount&#39;, &#39;gettrace&#39;, &#39;hexversion&#39;,
 &#39;long_info&#39;, &#39;maxint&#39;, &#39;maxsize&#39;, &#39;maxunicode&#39;, &#39;meta_path&#39;, &#39;modules&#39;,
 &#39;path&#39;, &#39;path_hooks&#39;, &#39;path_importer_cache&#39;, &#39;platform&#39;, &#39;prefix&#39;, &#39;ps1&#39;,
 &#39;py3kwarning&#39;, &#39;setcheckinterval&#39;, &#39;setdlopenflags&#39;, &#39;setprofile&#39;,
 &#39;setrecursionlimit&#39;, &#39;settrace&#39;, &#39;stderr&#39;, &#39;stdin&#39;, &#39;stdout&#39;, &#39;subversion&#39;,
 &#39;version&#39;, &#39;version_info&#39;, &#39;warnoptions&#39;]
</pre></div>
</div>
Without arguments, <a class="reference internal" href="../library/functions.html#dir" title="dir"><tt class="xref py py-func docutils literal">dir()</tt></a> lists the names you have defined currently:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a = [1, 2, 3, 4, 5]
&gt;&gt;&gt; import fibo
&gt;&gt;&gt; fib = fibo.fib
&gt;&gt;&gt; dir()
[&#39;__builtins__&#39;, &#39;__name__&#39;, &#39;__package__&#39;, &#39;a&#39;, &#39;fib&#39;, &#39;fibo&#39;, &#39;sys&#39;]
</pre></div>
</div>
Note that it lists all types of names: variables, modules, functions, etc.
<a class="reference internal" href="../library/functions.html#dir" title="dir"><tt class="xref py py-func docutils literal">dir()</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 class="reference internal" href="../library/__builtin__.html#module-__builtin__" title="__builtin__: The module that provides the built-in namespace."><tt class="xref py py-mod docutils literal">__builtin__</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import __builtin__
&gt;&gt;&gt; dir(__builtin__) 
[&#39;ArithmeticError&#39;, &#39;AssertionError&#39;, &#39;AttributeError&#39;, &#39;BaseException&#39;,
 &#39;BufferError&#39;, &#39;BytesWarning&#39;, &#39;DeprecationWarning&#39;, &#39;EOFError&#39;,
 &#39;Ellipsis&#39;, &#39;EnvironmentError&#39;, &#39;Exception&#39;, &#39;False&#39;, &#39;FloatingPointError&#39;,
 &#39;FutureWarning&#39;, &#39;GeneratorExit&#39;, &#39;IOError&#39;, &#39;ImportError&#39;, &#39;ImportWarning&#39;,
 &#39;IndentationError&#39;, &#39;IndexError&#39;, &#39;KeyError&#39;, &#39;KeyboardInterrupt&#39;,
 &#39;LookupError&#39;, &#39;MemoryError&#39;, &#39;NameError&#39;, &#39;None&#39;, &#39;NotImplemented&#39;,
 &#39;NotImplementedError&#39;, &#39;OSError&#39;, &#39;OverflowError&#39;,
 &#39;PendingDeprecationWarning&#39;, &#39;ReferenceError&#39;, &#39;RuntimeError&#39;,
 &#39;RuntimeWarning&#39;, &#39;StandardError&#39;, &#39;StopIteration&#39;, &#39;SyntaxError&#39;,
 &#39;SyntaxWarning&#39;, &#39;SystemError&#39;, &#39;SystemExit&#39;, &#39;TabError&#39;, &#39;True&#39;,
 &#39;TypeError&#39;, &#39;UnboundLocalError&#39;, &#39;UnicodeDecodeError&#39;,
 &#39;UnicodeEncodeError&#39;, &#39;UnicodeError&#39;, &#39;UnicodeTranslateError&#39;,
 &#39;UnicodeWarning&#39;, &#39;UserWarning&#39;, &#39;ValueError&#39;, &#39;Warning&#39;,
 &#39;ZeroDivisionError&#39;, &#39;_&#39;, &#39;__debug__&#39;, &#39;__doc__&#39;, &#39;__import__&#39;,
 &#39;__name__&#39;, &#39;__package__&#39;, &#39;abs&#39;, &#39;all&#39;, &#39;any&#39;, &#39;apply&#39;, &#39;basestring&#39;,
 &#39;bin&#39;, &#39;bool&#39;, &#39;buffer&#39;, &#39;bytearray&#39;, &#39;bytes&#39;, &#39;callable&#39;, &#39;chr&#39;,
 &#39;classmethod&#39;, &#39;cmp&#39;, &#39;coerce&#39;, &#39;compile&#39;, &#39;complex&#39;, &#39;copyright&#39;,
 &#39;credits&#39;, &#39;delattr&#39;, &#39;dict&#39;, &#39;dir&#39;, &#39;divmod&#39;, &#39;enumerate&#39;, &#39;eval&#39;,
 &#39;execfile&#39;, &#39;exit&#39;, &#39;file&#39;, &#39;filter&#39;, &#39;float&#39;, &#39;format&#39;, &#39;frozenset&#39;,
 &#39;getattr&#39;, &#39;globals&#39;, &#39;hasattr&#39;, &#39;hash&#39;, &#39;help&#39;, &#39;hex&#39;, &#39;id&#39;, &#39;input&#39;,
 &#39;int&#39;, &#39;intern&#39;, &#39;isinstance&#39;, &#39;issubclass&#39;, &#39;iter&#39;, &#39;len&#39;, &#39;license&#39;,
 &#39;list&#39;, &#39;locals&#39;, &#39;long&#39;, &#39;map&#39;, &#39;max&#39;, &#39;memoryview&#39;, &#39;min&#39;, &#39;next&#39;,
 &#39;object&#39;, &#39;oct&#39;, &#39;open&#39;, &#39;ord&#39;, &#39;pow&#39;, &#39;print&#39;, &#39;property&#39;, &#39;quit&#39;,
 &#39;range&#39;, &#39;raw_input&#39;, &#39;reduce&#39;, &#39;reload&#39;, &#39;repr&#39;, &#39;reversed&#39;, &#39;round&#39;,
 &#39;set&#39;, &#39;setattr&#39;, &#39;slice&#39;, &#39;sorted&#39;, &#39;staticmethod&#39;, &#39;str&#39;, &#39;sum&#39;, &#39;super&#39;,
 &#39;tuple&#39;, &#39;type&#39;, &#39;unichr&#39;, &#39;unicode&#39;, &#39;vars&#39;, &#39;xrange&#39;, &#39;zip&#39;]
</pre></div>
</div>
</div>
<div class="section" id="packages">
<h2>6.4. Packages<a class="headerlink" href="#packages" title="Permalink to this headline">¶</a></h2>
Packages are a way of structuring Python&#8217;s module namespace by using &#8220;dotted
module names&#8221;. For example, the module name <tt class="xref py py-mod docutils literal">A.B</tt> designates a submodule
named <tt class="docutils literal">B</tt> in a package named <tt class="docutils literal">A</tt>. Just like the use of modules saves the
authors of different modules from having to worry about each other&#8217;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&#8217;s module names.
Suppose you want to design a collection of modules (a &#8220;package&#8221;) 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="file docutils literal">.wav</tt>,
<tt class="file docutils literal">.aiff</tt>, <tt class="file docutils literal">.au</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&#8217;s a possible structure for
your package (expressed in terms of a hierarchical filesystem):
<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>
When importing the package, Python searches through the directories on
<tt class="docutils literal">sys.path</tt> looking for the package subdirectory.
The <tt class="file docutils literal">__init__.py</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">string</tt>, from unintentionally hiding valid modules that occur later
on the module search path. In the simplest case, <tt class="file docutils literal">__init__.py</tt> can just be
an empty file, but it can also execute initialization code for the package or
set the <tt class="docutils literal">__all__</tt> variable, described later.
Users of the package can import individual modules from the package, for
example:
<div class="highlight-python"><div class="highlight"><pre>import sound.effects.echo
</pre></div>
</div>
This loads the submodule <tt class="xref py py-mod docutils literal">sound.effects.echo</tt>. It must be referenced with
its full name.
<div class="highlight-python"><div class="highlight"><pre>sound.effects.echo.echofilter(input, output, delay=0.7, atten=4)
</pre></div>
</div>
An alternative way of importing the submodule is:
<div class="highlight-python"><div class="highlight"><pre>from sound.effects import echo
</pre></div>
</div>
This also loads the submodule <tt class="xref py py-mod docutils literal">echo</tt>, and makes it available without its
package prefix, so it can be used as follows:
<div class="highlight-python"><div class="highlight"><pre>echo.echofilter(input, output, delay=0.7, atten=4)
</pre></div>
</div>
Yet another variation is to import the desired function or variable directly:
<div class="highlight-python"><div class="highlight"><pre>from sound.effects.echo import echofilter
</pre></div>
</div>
Again, this loads the submodule <tt class="xref py py-mod docutils literal">echo</tt>, but this makes its function
<tt class="xref py py-func docutils literal">echofilter()</tt> directly available:
<div class="highlight-python"><div class="highlight"><pre>echofilter(input, output, delay=0.7, atten=4)
</pre></div>
</div>
Note that when using <tt class="docutils literal">from package import item</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">import</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 class="reference internal" href="../library/exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><tt class="xref py py-exc docutils literal">ImportError</tt></a>
exception is raised.
Contrarily, when using syntax like <tt class="docutils literal">import item.subitem.subsubitem</tt>, each item
except for the last must be a package; the last item can be a module or a
package but can&#8217;t be a class or function or variable defined in the previous
item.
<div class="section" id="importing-from-a-package">
<h3>6.4.1. Importing * From a Package<a class="headerlink" href="#importing-from-a-package" title="Permalink to this headline">¶</a></h3>
Now what happens when the user writes <tt class="docutils literal">from sound.effects import *</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.
The only solution is for the package author to provide an explicit index of the
package. The <a class="reference internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal">import</tt></a> statement uses the following convention: if a package&#8217;s
<tt class="file docutils literal">__init__.py</tt> code defines a list named <tt class="docutils literal">__all__</tt>, it is taken to be the
list of module names that should be imported when <tt class="docutils literal">from package import *</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&#8217;t see a use for importing * from their package. For
example, the file <tt class="file docutils literal">sounds/effects/__init__.py</tt> could contain the following
code:
<div class="highlight-python"><div class="highlight"><pre>__all__ = [&quot;echo&quot;, &quot;surround&quot;, &quot;reverse&quot;]
</pre></div>
</div>
This would mean that <tt class="docutils literal">from sound.effects import *</tt> would import the three
named submodules of the <tt class="xref py py-mod docutils literal">sound</tt> package.
If <tt class="docutils literal">__all__</tt> is not defined, the statement <tt class="docutils literal">from sound.effects import *</tt>
does not import all submodules from the package <tt class="xref py py-mod docutils literal">sound.effects</tt> into the
current namespace; it only ensures that the package <tt class="xref py py-mod docutils literal">sound.effects</tt> has
been imported (possibly running any initialization code in <tt class="file docutils literal">__init__.py</tt>)
and then imports whatever names are defined in the package. This includes any
names defined (and submodules explicitly loaded) by <tt class="file docutils literal">__init__.py</tt>. It
also includes any submodules of the package that were explicitly loaded by
previous <a class="reference internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal">import</tt></a> statements. Consider this code:
<div class="highlight-python"><div class="highlight"><pre>import sound.effects.echo
import sound.effects.surround
from sound.effects import *
</pre></div>
</div>
In this example, the <tt class="xref py py-mod docutils literal">echo</tt> and <tt class="xref py py-mod docutils literal">surround</tt> modules are imported in the
current namespace because they are defined in the <tt class="xref py py-mod docutils literal">sound.effects</tt> package
when the <tt class="docutils literal">from...import</tt> statement is executed. (This also works when
<tt class="docutils literal">__all__</tt> is defined.)
Although certain modules are designed to export only names that follow certain
patterns when you use <tt class="docutils literal">import *</tt>, it is still considered bad practise in
production code.
Remember, there is nothing wrong with using <tt class="docutils literal">from Package import
specific_submodule</tt>! In fact, this is the recommended notation unless the
importing module needs to use submodules with the same name from different
packages.
</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>
The submodules often need to refer to each other. For example, the
<tt class="xref py py-mod docutils literal">surround</tt> module might use the <tt class="xref py py-mod docutils literal">echo</tt> module. In fact, such
references are so common that the <a class="reference internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal">import</tt></a> statement first looks in the
containing package before looking in the standard module search path. Thus, the
<tt class="xref py py-mod docutils literal">surround</tt> module can simply use <tt class="docutils literal">import echo</tt> or <tt class="docutils literal">from echo import
echofilter</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 internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal">import</tt></a>
statement looks for a top-level module with the given name.
When packages are structured into subpackages (as with the <tt class="xref py py-mod docutils literal">sound</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 py py-mod docutils literal">sound.filters.vocoder</tt> needs to use
the <tt class="xref py py-mod docutils literal">echo</tt> module in the <tt class="xref py py-mod docutils literal">sound.effects</tt> package, it can use <tt class="docutils literal">from
sound.effects import echo</tt>.
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">from module import
name</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 py py-mod docutils literal">surround</tt> module for example, you might use:
<div class="highlight-python"><div class="highlight"><pre>from . import echo
from .. import formats
from ..filters import equalizer
</pre></div>
</div>
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">&quot;__main__&quot;</tt>,
modules intended for use as the main module of a Python application should
always use absolute imports.
</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>
Packages support one more special attribute, <tt class="xref py py-attr docutils literal">__path__</tt>. This is
initialized to be a list containing the name of the directory holding the
package&#8217;s <tt class="file docutils literal">__init__.py</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.
While this feature is not often needed, it can be used to extend the set of
modules found in a package.
Footnotes
<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 &#8216;statements&#8217; that are &#8216;executed&#8217;; the
execution of a module-level function definition enters the function name in
the module&#8217;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 internal" href="#">6. Modules</a><ul>
<li><a class="reference internal" href="#more-on-modules">6.1. More on Modules</a><ul>
<li><a class="reference internal" href="#executing-modules-as-scripts">6.1.1. Executing modules as scripts</a></li>
<li><a class="reference internal" href="#the-module-search-path">6.1.2. The Module Search Path</a></li>
<li><a class="reference internal" href="#compiled-python-files">6.1.3. &#8220;Compiled&#8221; Python files</a></li>
</ul>
</li>
<li><a class="reference internal" href="#standard-modules">6.2. Standard Modules</a></li>
<li><a class="reference internal" href="#the-dir-function">6.3. The <tt class="docutils literal">dir()</tt> Function</a></li>
<li><a class="reference internal" href="#packages">6.4. Packages</a><ul>
<li><a class="reference internal" href="#importing-from-a-package">6.4.1. Importing * From a Package</a></li>
<li><a class="reference internal" href="#intra-package-references">6.4.2. Intra-package References</a></li>
<li><a class="reference internal" href="#packages-in-multiple-directories">6.4.3. Packages in Multiple Directories</a></li>
</ul>
</li>
</ul>
</li>
</ul>

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