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<div class="section" id="top-level-components">
<span id="top-level"></span><h1><span class="section-number">9. </span>Top-level components<a class="headerlink" href="#top-level-components" title="Permalink to this headline">¶</a></h1>
<p id="index-0">The Python interpreter can get its input from a number of sources: from a script
passed to it as standard input or as program argument, typed in interactively,
from a module source file, etc. This chapter gives the syntax used in these
cases.</p>
<div class="section" id="complete-python-programs">
<span id="programs"></span><h2><span class="section-number">9.1. </span>Complete Python programs<a class="headerlink" href="#complete-python-programs" title="Permalink to this headline">¶</a></h2>
<span class="target" id="index-1"></span><p id="index-2">While a language specification need not prescribe how the language interpreter
is invoked, it is useful to have a notion of a complete Python program. A
complete Python program is executed in a minimally initialized environment: all
built-in and standard modules are available, but none have been initialized,
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
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
provide the local and global namespace for execution of the complete program.</p>
<p>The syntax for a complete Python program is that for file input, described in
the next section.</p>
<p id="index-3">The interpreter may also be invoked in interactive mode; in this case, it does
not read and execute a complete program but reads and executes one statement
(possibly compound) at a time. The initial environment is identical to that of
a complete program; each statement is executed in the namespace of
<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>
<p id="index-4">A complete program can be passed to the interpreter
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
passed as the first command line argument, or as standard input. If the file
or standard input is a tty device, the interpreter enters interactive mode;
otherwise, it executes the file as a complete program.</p>
</div>
<div class="section" id="file-input">
<span id="id1"></span><h2><span class="section-number">9.2. </span>File input<a class="headerlink" href="#file-input" title="Permalink to this headline">¶</a></h2>
<p>All input read from non-interactive files has the same form:</p>
<pre>
<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>)*
</pre>
<p>This syntax is used in the following situations:</p>
<ul class="simple">
<li><p>when parsing a complete Python program (from a file or from a string);</p></li>
<li><p>when parsing a module;</p></li>
<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>
</ul>
</div>
<div class="section" id="interactive-input">
<span id="interactive"></span><h2><span class="section-number">9.3. </span>Interactive input<a class="headerlink" href="#interactive-input" title="Permalink to this headline">¶</a></h2>
<p>Input in interactive mode is parsed using the following grammar:</p>
<pre>
<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
</pre>
<p>Note that a (top-level) compound statement must be followed by a blank line in
interactive mode; this is needed to help the parser detect the end of the input.</p>
</div>
<div class="section" id="expression-input">
<span id="id2"></span><h2><span class="section-number">9.4. </span>Expression input<a class="headerlink" href="#expression-input" title="Permalink to this headline">¶</a></h2>
<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
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>
<pre>
<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*
</pre>
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<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">9. Top-level components</a><ul>
<li><a class="reference internal" href="#complete-python-programs">9.1. Complete Python programs</a></li>
<li><a class="reference internal" href="#file-input">9.2. File input</a></li>
<li><a class="reference internal" href="#interactive-input">9.3. Interactive input</a></li>
<li><a class="reference internal" href="#expression-input">9.4. Expression input</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="compound_stmts.html"
title="previous chapter"><span class="section-number">8. </span>Compound statements</a></p>
<h4>Next topic</h4>
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