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<H1><A NAME="SECTION0081200000000000000000"></A><A NAME="import"></A>
<BR>
6.12 The <tt class="keyword">import</tt> statement
</H1>
<a id='l2h-529' xml:id='l2h-529'></a><a id='l2h-550' xml:id='l2h-550'></a>
<a id='l2h-530' xml:id='l2h-530'></a><a id='l2h-531' xml:id='l2h-531'></a>
<P>
<dl><dd class="grammar">
<div class="productions">
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<tr>
    <td><a id='tok-import_stmt' xml:id='tok-import_stmt'>import_stmt</a></td>
    <td>::=</td>
    <td>"import" <a class='grammartoken' href="import.html#tok-module">module</a> ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>]
                ( "," <a class='grammartoken' href="import.html#tok-module">module</a> ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>] )*</td></tr>
  <tr>
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    <td></td>
    <td><code>| "from" <a class='grammartoken' href="import.html#tok-module">module</a> "import" <a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a>
                    ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>]</code></td></tr>
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    <td></td>
    <td></td>
    <td><code>&nbsp;&nbsp;( "," <a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a> ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>] )*</code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>| "from" <a class='grammartoken' href="import.html#tok-module">module</a> "import" "(" <a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a>
                    ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>]</code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>&nbsp;&nbsp;( "," <a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a> ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>] )* [","] ")"</code></td></tr>
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    <td><a id='tok-module' xml:id='tok-module'>module</a></td>
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</table>
</div>
<a class="grammar-footer"
  href="grammar.txt" type="text/plain"
  >Download entire grammar as text.</a>
</dd></dl>

<P>
Import statements are executed in two steps: (1) find a module, and
initialize it if necessary; (2) define a name or names in the local
namespace (of the scope where the <tt class="keyword">import</tt> statement occurs).
The first form (without <tt class="keyword">from</tt>) repeats these steps for each
identifier in the list.  The form with <tt class="keyword">from</tt> performs step
(1) once, and then performs step (2) repeatedly.

<P>
In this context, to ``initialize'' a built-in or extension module means to
call an initialization function that the module must provide for the purpose
(in the reference implementation, the function's name is obtained by
prepending string ``init'' to the module's name); to ``initialize'' a
Python-coded module means to execute the module's body.

<P>
The system maintains a table of modules that have been or are being
initialized,
indexed by module name.  This table is
accessible as <code>sys.modules</code>.  When a module name is found in
this table, step (1) is finished.  If not, a search for a module
definition is started.  When a module is found, it is loaded.  Details
of the module searching and loading process are implementation and
platform specific.  It generally involves searching for a ``built-in''
module with the given name and then searching a list of locations
given as <code>sys.path</code>.
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<a id='l2h-534' xml:id='l2h-534'></a><a id='l2h-535' xml:id='l2h-535'></a><a id='l2h-536' xml:id='l2h-536'></a><a id='l2h-552' xml:id='l2h-552'></a>
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<P>
If a built-in module is found,<a id='l2h-539' xml:id='l2h-539'></a> its
built-in initialization code is executed and step (1) is finished.  If
no matching file is found,
<tt class="exception">ImportError</tt><a id='l2h-540' xml:id='l2h-540'></a> is raised.
<a id='l2h-553' xml:id='l2h-553'></a>If a file is found, it is parsed,
yielding an executable code block.  If a syntax error occurs,
<tt class="exception">SyntaxError</tt><a id='l2h-541' xml:id='l2h-541'></a> is raised.  Otherwise, an
empty module of the given name is created and inserted in the module
table, and then the code block is executed in the context of this
module.  Exceptions during this execution terminate step (1).

<P>
When step (1) finishes without raising an exception, step (2) can
begin.

<P>
The first form of <tt class="keyword">import</tt> statement binds the module name in
the local namespace to the module object, and then goes on to import
the next identifier, if any.  If the module name is followed by
<tt class="keyword">as</tt>, the name following <tt class="keyword">as</tt> is used as the local
name for the module.

<P>
The <tt class="keyword">from</tt> form does not bind the module name: it goes through the
list of identifiers, looks each one of them up in the module found in step
(1), and binds the name in the local namespace to the object thus found.
As with the first form of <tt class="keyword">import</tt>, an alternate local name can be
supplied by specifying "<tt class="keyword">as</tt> localname".  If a name is not found,
<tt class="exception">ImportError</tt> is raised.  If the list of identifiers is replaced
by a star ("<tt class="character">*</tt>"), all public names defined in the module are
bound in the local namespace of the <tt class="keyword">import</tt> statement..
<a id='l2h-542' xml:id='l2h-542'></a><a id='l2h-543' xml:id='l2h-543'></a>
<P>
The <em>public names</em> defined by a module are determined by checking
the module's namespace for a variable named <code>__all__</code>; if
defined, it must be a sequence of strings which are names defined or
imported by that module.  The names given in <code>__all__</code> are all
considered public and are required to exist.  If <code>__all__</code> is not
defined, the set of public names includes all names found in the
module's namespace which do not begin with an underscore character
("<tt class="character">_</tt>").  <code>__all__</code> should contain the entire public API.
It is intended to avoid accidentally exporting items that are not part
of the API (such as library modules which were imported and used within
the module).
<a id='l2h-545' xml:id='l2h-545'></a>
<P>
The <tt class="keyword">from</tt> form with "<tt class="samp">*</tt>" may only occur in a module
scope.  If the wild card form of import -- "<tt class="samp">import *</tt>" -- is
used in a function and the function contains or is a nested block with
free variables, the compiler will raise a <tt class="exception">SyntaxError</tt>.

<P>
<a id='l2h-546' xml:id='l2h-546'></a><a id='l2h-547' xml:id='l2h-547'></a>
<P>
<strong>Hierarchical module names:</strong><a id='l2h-548' xml:id='l2h-548'></a>when the module names contains one or more dots, the module search
path is carried out differently.  The sequence of identifiers up to
the last dot is used to find a ``package''<a id='l2h-554' xml:id='l2h-554'></a>; the final
identifier is then searched inside the package.  A package is
generally a subdirectory of a directory on <code>sys.path</code> that has a
file <span class="file">__init__.py</span>.<a id='l2h-555' xml:id='l2h-555'></a>
[XXX Can't be bothered to spell this out right now; see the URL
<a class="url" href="http://www.python.org/doc/essays/packages.html">http://www.python.org/doc/essays/packages.html</a> for more details, also
about how the module search works from inside a package.]

<P>
The built-in function <tt class="function">__import__()</tt> is provided to support
applications that determine which modules need to be loaded
dynamically; refer to <a class="ulink" href="../lib/built-in-funcs.html"
  >Built-in
Functions</a> in the
<em class="citetitle"><a
 href="../lib/lib.html"
 title="Python Library Reference"
 >Python Library Reference</a></em> for additional
information.
<a id='l2h-549' xml:id='l2h-549'></a>
<P>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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