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<div class="section" id="top-level-components">
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<span id="top-level"></span><h1>9. Top-level components<a class="headerlink" href="#top-level-components" title="Permalink to this headline">¶</a></h1>
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<p id="index-0">The Python interpreter can get its input from a number of sources: from a script
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passed to it as standard input or as program argument, typed in interactively,
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from a module source file, etc. This chapter gives the syntax used in these
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cases.</p>
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<div class="section" id="complete-python-programs">
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<span id="programs"></span><h2>9.1. Complete Python programs<a class="headerlink" href="#complete-python-programs" title="Permalink to this headline">¶</a></h2>
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<span class="target" id="index-1"></span><p id="index-2">While a language specification need not prescribe how the language interpreter
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is invoked, it is useful to have a notion of a complete Python program. A
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complete Python program is executed in a minimally initialized environment: all
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built-in and standard modules are available, but none have been initialized,
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except for <a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">sys</span></code></a> (various system services), <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a> (built-in
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functions, exceptions and <code class="docutils literal notranslate"><span class="pre">None</span></code>) and <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a>. The latter is used to
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provide the local and global namespace for execution of the complete program.</p>
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<p>The syntax for a complete Python program is that for file input, described in
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the next section.</p>
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<p id="index-3">The interpreter may also be invoked in interactive mode; in this case, it does
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not read and execute a complete program but reads and executes one statement
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(possibly compound) at a time. The initial environment is identical to that of
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a complete program; each statement is executed in the namespace of
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<a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a>.</p>
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<p id="index-4">A complete program can be passed to the interpreter
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in three forms: with the <a class="reference internal" href="../using/cmdline.html#cmdoption-c"><code class="xref std std-option docutils literal notranslate"><span class="pre">-c</span></code></a> <em>string</em> command line option, as a file
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passed as the first command line argument, or as standard input. If the file
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or standard input is a tty device, the interpreter enters interactive mode;
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otherwise, it executes the file as a complete program.</p>
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</div>
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<div class="section" id="file-input">
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<span id="id1"></span><h2>9.2. File input<a class="headerlink" href="#file-input" title="Permalink to this headline">¶</a></h2>
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<p>All input read from non-interactive files has the same form:</p>
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<pre>
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<strong id="grammar-token-file-input">file_input</strong> ::= (NEWLINE | <a class="reference internal" href="compound_stmts.html#grammar-token-statement"><code class="xref docutils literal notranslate"><span class="pre">statement</span></code></a>)*
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</pre>
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<p>This syntax is used in the following situations:</p>
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<ul class="simple">
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<li><p>when parsing a complete Python program (from a file or from a string);</p></li>
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<li><p>when parsing a module;</p></li>
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<li><p>when parsing a string passed to the <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> function;</p></li>
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</ul>
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</div>
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<div class="section" id="interactive-input">
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<span id="interactive"></span><h2>9.3. Interactive input<a class="headerlink" href="#interactive-input" title="Permalink to this headline">¶</a></h2>
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<p>Input in interactive mode is parsed using the following grammar:</p>
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<pre>
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<strong id="grammar-token-interactive-input">interactive_input</strong> ::= [<a class="reference internal" href="compound_stmts.html#grammar-token-stmt-list"><code class="xref docutils literal notranslate"><span class="pre">stmt_list</span></code></a>] NEWLINE | <a class="reference internal" href="compound_stmts.html#grammar-token-compound-stmt"><code class="xref docutils literal notranslate"><span class="pre">compound_stmt</span></code></a> NEWLINE
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</pre>
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<p>Note that a (top-level) compound statement must be followed by a blank line in
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interactive mode; this is needed to help the parser detect the end of the input.</p>
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</div>
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<div class="section" id="expression-input">
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<span id="id2"></span><h2>9.4. Expression input<a class="headerlink" href="#expression-input" title="Permalink to this headline">¶</a></h2>
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<span class="target" id="index-5"></span><p id="index-6"><a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> is used for expression input. It ignores leading whitespace. The
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string argument to <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> must have the following form:</p>
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<pre>
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<strong id="grammar-token-eval-input">eval_input</strong> ::= <a class="reference internal" href="expressions.html#grammar-token-expression-list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a> NEWLINE*
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<h3><a href="../contents.html">Table of Contents</a></h3>
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<ul>
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<li><a class="reference internal" href="#">9. Top-level components</a><ul>
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<li><a class="reference internal" href="#complete-python-programs">9.1. Complete Python programs</a></li>
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<li><a class="reference internal" href="#file-input">9.2. File input</a></li>
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<li><a class="reference internal" href="#interactive-input">9.3. Interactive input</a></li>
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<li><a class="reference internal" href="#expression-input">9.4. Expression input</a></li>
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