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| 54 | |
| 55 | <H1><A NAME="SECTION0081200000000000000000"></A><A NAME="import"></A> |
| 56 | <BR> |
| 57 | 6.12 The <tt class="keyword">import</tt> statement |
| 58 | </H1> |
| 59 | <a id='l2h-529' xml:id='l2h-529'></a><a id='l2h-550' xml:id='l2h-550'></a> |
| 60 | <a id='l2h-530' xml:id='l2h-530'></a><a id='l2h-531' xml:id='l2h-531'></a> |
| 61 | <P> |
| 62 | <dl><dd class="grammar"> |
| 63 | <div class="productions"> |
| 64 | <table> |
| 65 | <tr> |
| 66 | <td><a id='tok-import_stmt' xml:id='tok-import_stmt'>import_stmt</a></td> |
| 67 | <td>::=</td> |
| 68 | <td>"import" <a class='grammartoken' href="import.html#tok-module">module</a> ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>] |
| 69 | ( "," <a class='grammartoken' href="import.html#tok-module">module</a> ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>] )*</td></tr> |
| 70 | <tr> |
| 71 | <td></td> |
| 72 | <td></td> |
| 73 | <td><code>| "from" <a class='grammartoken' href="import.html#tok-module">module</a> "import" <a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a> |
| 74 | ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>]</code></td></tr> |
| 75 | <tr> |
| 76 | <td></td> |
| 77 | <td></td> |
| 78 | <td><code> ( "," <a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a> ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>] )*</code></td></tr> |
| 79 | <tr> |
| 80 | <td></td> |
| 81 | <td></td> |
| 82 | <td><code>| "from" <a class='grammartoken' href="import.html#tok-module">module</a> "import" "(" <a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a> |
| 83 | ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>]</code></td></tr> |
| 84 | <tr> |
| 85 | <td></td> |
| 86 | <td></td> |
| 87 | <td><code> ( "," <a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a> ["as" <a class='grammartoken' href="notation.html#tok-name">name</a>] )* [","] ")"</code></td></tr> |
| 88 | <tr> |
| 89 | <td></td> |
| 90 | <td></td> |
| 91 | <td><code>| "from" <a class='grammartoken' href="import.html#tok-module">module</a> "import" "*"</code></td></tr> |
| 92 | <tr> |
| 93 | <td><a id='tok-module' xml:id='tok-module'>module</a></td> |
| 94 | <td>::=</td> |
| 95 | <td>(<a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a> ".")* <a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a></td></tr> |
| 96 | </table> |
| 97 | </div> |
| 98 | <a class="grammar-footer" |
| 99 | href="grammar.txt" type="text/plain" |
| 100 | >Download entire grammar as text.</a> |
| 101 | </dd></dl> |
| 102 | |
| 103 | <P> |
| 104 | Import statements are executed in two steps: (1) find a module, and |
| 105 | initialize it if necessary; (2) define a name or names in the local |
| 106 | namespace (of the scope where the <tt class="keyword">import</tt> statement occurs). |
| 107 | The first form (without <tt class="keyword">from</tt>) repeats these steps for each |
| 108 | identifier in the list. The form with <tt class="keyword">from</tt> performs step |
| 109 | (1) once, and then performs step (2) repeatedly. |
| 110 | |
| 111 | <P> |
| 112 | In this context, to ``initialize'' a built-in or extension module means to |
| 113 | call an initialization function that the module must provide for the purpose |
| 114 | (in the reference implementation, the function's name is obtained by |
| 115 | prepending string ``init'' to the module's name); to ``initialize'' a |
| 116 | Python-coded module means to execute the module's body. |
| 117 | |
| 118 | <P> |
| 119 | The system maintains a table of modules that have been or are being |
| 120 | initialized, |
| 121 | indexed by module name. This table is |
| 122 | accessible as <code>sys.modules</code>. When a module name is found in |
| 123 | this table, step (1) is finished. If not, a search for a module |
| 124 | definition is started. When a module is found, it is loaded. Details |
| 125 | of the module searching and loading process are implementation and |
| 126 | platform specific. It generally involves searching for a ``built-in'' |
| 127 | module with the given name and then searching a list of locations |
| 128 | given as <code>sys.path</code>. |
| 129 | <a id='l2h-533' xml:id='l2h-533'></a><a id='l2h-551' xml:id='l2h-551'></a> |
| 130 | <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> |
| 131 | <a id='l2h-537' xml:id='l2h-537'></a><a id='l2h-538' xml:id='l2h-538'></a> |
| 132 | <P> |
| 133 | If a built-in module is found,<a id='l2h-539' xml:id='l2h-539'></a> its |
| 134 | built-in initialization code is executed and step (1) is finished. If |
| 135 | no matching file is found, |
| 136 | <tt class="exception">ImportError</tt><a id='l2h-540' xml:id='l2h-540'></a> is raised. |
| 137 | <a id='l2h-553' xml:id='l2h-553'></a>If a file is found, it is parsed, |
| 138 | yielding an executable code block. If a syntax error occurs, |
| 139 | <tt class="exception">SyntaxError</tt><a id='l2h-541' xml:id='l2h-541'></a> is raised. Otherwise, an |
| 140 | empty module of the given name is created and inserted in the module |
| 141 | table, and then the code block is executed in the context of this |
| 142 | module. Exceptions during this execution terminate step (1). |
| 143 | |
| 144 | <P> |
| 145 | When step (1) finishes without raising an exception, step (2) can |
| 146 | begin. |
| 147 | |
| 148 | <P> |
| 149 | The first form of <tt class="keyword">import</tt> statement binds the module name in |
| 150 | the local namespace to the module object, and then goes on to import |
| 151 | the next identifier, if any. If the module name is followed by |
| 152 | <tt class="keyword">as</tt>, the name following <tt class="keyword">as</tt> is used as the local |
| 153 | name for the module. |
| 154 | |
| 155 | <P> |
| 156 | The <tt class="keyword">from</tt> form does not bind the module name: it goes through the |
| 157 | list of identifiers, looks each one of them up in the module found in step |
| 158 | (1), and binds the name in the local namespace to the object thus found. |
| 159 | As with the first form of <tt class="keyword">import</tt>, an alternate local name can be |
| 160 | supplied by specifying "<tt class="keyword">as</tt> localname". If a name is not found, |
| 161 | <tt class="exception">ImportError</tt> is raised. If the list of identifiers is replaced |
| 162 | by a star ("<tt class="character">*</tt>"), all public names defined in the module are |
| 163 | bound in the local namespace of the <tt class="keyword">import</tt> statement.. |
| 164 | <a id='l2h-542' xml:id='l2h-542'></a><a id='l2h-543' xml:id='l2h-543'></a> |
| 165 | <P> |
| 166 | The <em>public names</em> defined by a module are determined by checking |
| 167 | the module's namespace for a variable named <code>__all__</code>; if |
| 168 | defined, it must be a sequence of strings which are names defined or |
| 169 | imported by that module. The names given in <code>__all__</code> are all |
| 170 | considered public and are required to exist. If <code>__all__</code> is not |
| 171 | defined, the set of public names includes all names found in the |
| 172 | module's namespace which do not begin with an underscore character |
| 173 | ("<tt class="character">_</tt>"). <code>__all__</code> should contain the entire public API. |
| 174 | It is intended to avoid accidentally exporting items that are not part |
| 175 | of the API (such as library modules which were imported and used within |
| 176 | the module). |
| 177 | <a id='l2h-545' xml:id='l2h-545'></a> |
| 178 | <P> |
| 179 | The <tt class="keyword">from</tt> form with "<tt class="samp">*</tt>" may only occur in a module |
| 180 | scope. If the wild card form of import -- "<tt class="samp">import *</tt>" -- is |
| 181 | used in a function and the function contains or is a nested block with |
| 182 | free variables, the compiler will raise a <tt class="exception">SyntaxError</tt>. |
| 183 | |
| 184 | <P> |
| 185 | <a id='l2h-546' xml:id='l2h-546'></a><a id='l2h-547' xml:id='l2h-547'></a> |
| 186 | <P> |
| 187 | <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 |
| 188 | path is carried out differently. The sequence of identifiers up to |
| 189 | the last dot is used to find a ``package''<a id='l2h-554' xml:id='l2h-554'></a>; the final |
| 190 | identifier is then searched inside the package. A package is |
| 191 | generally a subdirectory of a directory on <code>sys.path</code> that has a |
| 192 | file <span class="file">__init__.py</span>.<a id='l2h-555' xml:id='l2h-555'></a> |
| 193 | [XXX Can't be bothered to spell this out right now; see the URL |
| 194 | <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 |
| 195 | about how the module search works from inside a package.] |
| 196 | |
| 197 | <P> |
| 198 | The built-in function <tt class="function">__import__()</tt> is provided to support |
| 199 | applications that determine which modules need to be loaded |
| 200 | dynamically; refer to <a class="ulink" href="../lib/built-in-funcs.html" |
| 201 | >Built-in |
| 202 | Functions</a> in the |
| 203 | <em class="citetitle"><a |
| 204 | href="../lib/lib.html" |
| 205 | title="Python Library Reference" |
| 206 | >Python Library Reference</a></em> for additional |
| 207 | information. |
| 208 | <a id='l2h-549' xml:id='l2h-549'></a> |
| 209 | <P> |
| 210 | |
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| 214 | <A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a> |
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