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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>32.1. parser — Access Python parse trees — Python 2.7.18 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.18 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="32.2. ast — Abstract Syntax Trees" href="ast.html" /> <link rel="prev" title="32. 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You should install the python3 and python3-doc packages and read the <a href="file:///usr/share/doc/python3-doc/html/library/parser.html"> Python documentation for the Python3 version packaged in this release</a>. </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="ast.html" title="32.2. ast — Abstract Syntax Trees" accesskey="N">next</a> |</li> <li class="right" > <a href="language.html" title="32. Python Language Services" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="language.html" accesskey="U">32. Python Language Services</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="module-parser"> <span id="parser-access-python-parse-trees"></span><h1>32.1. <a class="reference internal" href="#module-parser" title="parser: Access parse trees for Python source code."><code class="xref py py-mod docutils literal notranslate"><span class="pre">parser</span></code></a> — Access Python parse trees<a class="headerlink" href="#module-parser" title="Permalink to this headline">¶</a></h1> <p id="index-0">The <a class="reference internal" href="#module-parser" title="parser: Access parse trees for Python source code."><code class="xref py py-mod docutils literal notranslate"><span class="pre">parser</span></code></a> module provides an interface to Python’s internal parser and byte-code compiler. The primary purpose for this interface is to allow Python code to edit the parse tree of a Python expression and create executable code from this. This is better than trying to parse and modify an arbitrary Python code fragment as a string because parsing is performed in a manner identical to the code forming the application. It is also faster.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p>From Python 2.5 onward, it’s much more convenient to cut in at the Abstract Syntax Tree (AST) generation and compilation stage, using the <a class="reference internal" href="ast.html#module-ast" title="ast: Abstract Syntax Tree classes and manipulation."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ast</span></code></a> module.</p> <p class="last">The <a class="reference internal" href="#module-parser" title="parser: Access parse trees for Python source code."><code class="xref py py-mod docutils literal notranslate"><span class="pre">parser</span></code></a> module exports the names documented here also with “st” replaced by “ast”; this is a legacy from the time when there was no other AST and has nothing to do with the AST found in Python 2.5. This is also the reason for the functions’ keyword arguments being called <em>ast</em>, not <em>st</em>. The “ast” functions have been removed in Python 3.</p> </div> <p>There are a few things to note about this module which are important to making use of the data structures created. This is not a tutorial on editing the parse trees for Python code, but some examples of using the <a class="reference internal" href="#module-parser" title="parser: Access parse trees for Python source code."><code class="xref py py-mod docutils literal notranslate"><span class="pre">parser</span></code></a> module are presented.</p> <p>Most importantly, a good understanding of the Python grammar processed by the internal parser is required. For full information on the language syntax, refer to <a class="reference internal" href="../reference/index.html#reference-index"><span class="std std-ref">The Python Language Reference</span></a>. The parser itself is created from a grammar specification defined in the file <code class="file docutils literal notranslate"><span class="pre">Grammar/Grammar</span></code> in the standard Python distribution. The parse trees stored in the ST objects created by this module are the actual output from the internal parser when created by the <a class="reference internal" href="#parser.expr" title="parser.expr"><code class="xref py py-func docutils literal notranslate"><span class="pre">expr()</span></code></a> or <a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate"><span class="pre">suite()</span></code></a> functions, described below. The ST objects created by <a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><code class="xref py py-func docutils literal notranslate"><span class="pre">sequence2st()</span></code></a> faithfully simulate those structures. Be aware that the values of the sequences which are considered “correct” will vary from one version of Python to another as the formal grammar for the language is revised. However, transporting code from one Python version to another as source text will always allow correct parse trees to be created in the target version, with the only restriction being that migrating to an older version of the interpreter will not support more recent language constructs. The parse trees are not typically compatible from one version to another, whereas source code has always been forward-compatible.</p> <p>Each element of the sequences returned by <a class="reference internal" href="#parser.st2list" title="parser.st2list"><code class="xref py py-func docutils literal notranslate"><span class="pre">st2list()</span></code></a> or <a class="reference internal" href="#parser.st2tuple" title="parser.st2tuple"><code class="xref py py-func docutils literal notranslate"><span class="pre">st2tuple()</span></code></a> has a simple form. Sequences representing non-terminal elements in the grammar always have a length greater than one. The first element is an integer which identifies a production in the grammar. These integers are given symbolic names in the C header file <code class="file docutils literal notranslate"><span class="pre">Include/graminit.h</span></code> and the Python module <a class="reference internal" href="symbol.html#module-symbol" title="symbol: Constants representing internal nodes of the parse tree."><code class="xref py py-mod docutils literal notranslate"><span class="pre">symbol</span></code></a>. Each additional element of the sequence represents a component of the production as recognized in the input string: these are always sequences which have the same form as the parent. An important aspect of this structure which should be noted is that keywords used to identify the parent node type, such as the keyword <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> in an <code class="xref py py-const docutils literal notranslate"><span class="pre">if_stmt</span></code>, are included in the node tree without any special treatment. For example, the <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> keyword is represented by the tuple <code class="docutils literal notranslate"><span class="pre">(1,</span> <span class="pre">'if')</span></code>, where <code class="docutils literal notranslate"><span class="pre">1</span></code> is the numeric value associated with all <code class="xref py py-const docutils literal notranslate"><span class="pre">NAME</span></code> tokens, including variable and function names defined by the user. In an alternate form returned when line number information is requested, the same token might be represented as <code class="docutils literal notranslate"><span class="pre">(1,</span> <span class="pre">'if',</span> <span class="pre">12)</span></code>, where the <code class="docutils literal notranslate"><span class="pre">12</span></code> represents the line number at which the terminal symbol was found.</p> <p>Terminal elements are represented in much the same way, but without any child elements and the addition of the source text which was identified. The example of the <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> keyword above is representative. The various types of terminal symbols are defined in the C header file <code class="file docutils literal notranslate"><span class="pre">Include/token.h</span></code> and the Python module <a class="reference internal" href="token.html#module-token" title="token: Constants representing terminal nodes of the parse tree."><code class="xref py py-mod docutils literal notranslate"><span class="pre">token</span></code></a>.</p> <p>The ST objects are not required to support the functionality of this module, but are provided for three purposes: to allow an application to amortize the cost of processing complex parse trees, to provide a parse tree representation which conserves memory space when compared to the Python list or tuple representation, and to ease the creation of additional modules in C which manipulate parse trees. A simple “wrapper” class may be created in Python to hide the use of ST objects.</p> <p>The <a class="reference internal" href="#module-parser" title="parser: Access parse trees for Python source code."><code class="xref py py-mod docutils literal notranslate"><span class="pre">parser</span></code></a> module defines functions for a few distinct purposes. The most important purposes are to create ST objects and to convert ST objects to other representations such as parse trees and compiled code objects, but there are also functions which serve to query the type of parse tree represented by an ST object.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt>Module <a class="reference internal" href="symbol.html#module-symbol" title="symbol: Constants representing internal nodes of the parse tree."><code class="xref py py-mod docutils literal notranslate"><span class="pre">symbol</span></code></a></dt> <dd>Useful constants representing internal nodes of the parse tree.</dd> <dt>Module <a class="reference internal" href="token.html#module-token" title="token: Constants representing terminal nodes of the parse tree."><code class="xref py py-mod docutils literal notranslate"><span class="pre">token</span></code></a></dt> <dd>Useful constants representing leaf nodes of the parse tree and functions for testing node values.</dd> </dl> </div> <div class="section" id="creating-st-objects"> <span id="creating-sts"></span><h2>32.1.1. Creating ST Objects<a class="headerlink" href="#creating-st-objects" title="Permalink to this headline">¶</a></h2> <p>ST objects may be created from source code or from a parse tree. When creating an ST object from source, different functions are used to create the <code class="docutils literal notranslate"><span class="pre">'eval'</span></code> and <code class="docutils literal notranslate"><span class="pre">'exec'</span></code> forms.</p> <dl class="function"> <dt id="parser.expr"> <code class="descclassname">parser.</code><code class="descname">expr</code><span class="sig-paren">(</span><em>source</em><span class="sig-paren">)</span><a class="headerlink" href="#parser.expr" title="Permalink to this definition">¶</a></dt> <dd><p>The <a class="reference internal" href="#parser.expr" title="parser.expr"><code class="xref py py-func docutils literal notranslate"><span class="pre">expr()</span></code></a> function parses the parameter <em>source</em> as if it were an input to <code class="docutils literal notranslate"><span class="pre">compile(source,</span> <span class="pre">'file.py',</span> <span class="pre">'eval')</span></code>. If the parse succeeds, an ST object is created to hold the internal parse tree representation, otherwise an appropriate exception is raised.</p> </dd></dl> <dl class="function"> <dt id="parser.suite"> <code class="descclassname">parser.</code><code class="descname">suite</code><span class="sig-paren">(</span><em>source</em><span class="sig-paren">)</span><a class="headerlink" href="#parser.suite" title="Permalink to this definition">¶</a></dt> <dd><p>The <a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate"><span class="pre">suite()</span></code></a> function parses the parameter <em>source</em> as if it were an input to <code class="docutils literal notranslate"><span class="pre">compile(source,</span> <span class="pre">'file.py',</span> <span class="pre">'exec')</span></code>. If the parse succeeds, an ST object is created to hold the internal parse tree representation, otherwise an appropriate exception is raised.</p> </dd></dl> <dl class="function"> <dt id="parser.sequence2st"> <code class="descclassname">parser.</code><code class="descname">sequence2st</code><span class="sig-paren">(</span><em>sequence</em><span class="sig-paren">)</span><a class="headerlink" href="#parser.sequence2st" title="Permalink to this definition">¶</a></dt> <dd><p>This function accepts a parse tree represented as a sequence and builds an internal representation if possible. If it can validate that the tree conforms to the Python grammar and all nodes are valid node types in the host version of Python, an ST object is created from the internal representation and returned to the called. If there is a problem creating the internal representation, or if the tree cannot be validated, a <a class="reference internal" href="#parser.ParserError" title="parser.ParserError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ParserError</span></code></a> exception is raised. An ST object created this way should not be assumed to compile correctly; normal exceptions raised by compilation may still be initiated when the ST object is passed to <a class="reference internal" href="#parser.compilest" title="parser.compilest"><code class="xref py py-func docutils literal notranslate"><span class="pre">compilest()</span></code></a>. This may indicate problems not related to syntax (such as a <a class="reference internal" href="exceptions.html#exceptions.MemoryError" title="exceptions.MemoryError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">MemoryError</span></code></a> exception), but may also be due to constructs such as the result of parsing <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">f(0)</span></code>, which escapes the Python parser but is checked by the bytecode compiler.</p> <p>Sequences representing terminal tokens may be represented as either two-element lists of the form <code class="docutils literal notranslate"><span class="pre">(1,</span> <span class="pre">'name')</span></code> or as three-element lists of the form <code class="docutils literal notranslate"><span class="pre">(1,</span> <span class="pre">'name',</span> <span class="pre">56)</span></code>. If the third element is present, it is assumed to be a valid line number. The line number may be specified for any subset of the terminal symbols in the input tree.</p> </dd></dl> <dl class="function"> <dt id="parser.tuple2st"> <code class="descclassname">parser.</code><code class="descname">tuple2st</code><span class="sig-paren">(</span><em>sequence</em><span class="sig-paren">)</span><a class="headerlink" href="#parser.tuple2st" title="Permalink to this definition">¶</a></dt> <dd><p>This is the same function as <a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><code class="xref py py-func docutils literal notranslate"><span class="pre">sequence2st()</span></code></a>. This entry point is maintained for backward compatibility.</p> </dd></dl> </div> <div class="section" id="converting-st-objects"> <span id="converting-sts"></span><h2>32.1.2. Converting ST Objects<a class="headerlink" href="#converting-st-objects" title="Permalink to this headline">¶</a></h2> <p>ST objects, regardless of the input used to create them, may be converted to parse trees represented as list- or tuple- trees, or may be compiled into executable code objects. Parse trees may be extracted with or without line numbering information.</p> <dl class="function"> <dt id="parser.st2list"> <code class="descclassname">parser.</code><code class="descname">st2list</code><span class="sig-paren">(</span><em>ast</em><span class="optional">[</span>, <em>line_info</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#parser.st2list" title="Permalink to this definition">¶</a></dt> <dd><p>This function accepts an ST object from the caller in <em>ast</em> and returns a Python list representing the equivalent parse tree. The resulting list representation can be used for inspection or the creation of a new parse tree in list form. This function does not fail so long as memory is available to build the list representation. If the parse tree will only be used for inspection, <a class="reference internal" href="#parser.st2tuple" title="parser.st2tuple"><code class="xref py py-func docutils literal notranslate"><span class="pre">st2tuple()</span></code></a> should be used instead to reduce memory consumption and fragmentation. When the list representation is required, this function is significantly faster than retrieving a tuple representation and converting that to nested lists.</p> <p>If <em>line_info</em> is true, line number information will be included for all terminal tokens as a third element of the list representing the token. Note that the line number provided specifies the line on which the token <em>ends</em>. This information is omitted if the flag is false or omitted.</p> </dd></dl> <dl class="function"> <dt id="parser.st2tuple"> <code class="descclassname">parser.</code><code class="descname">st2tuple</code><span class="sig-paren">(</span><em>ast</em><span class="optional">[</span>, <em>line_info</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#parser.st2tuple" title="Permalink to this definition">¶</a></dt> <dd><p>This function accepts an ST object from the caller in <em>ast</em> and returns a Python tuple representing the equivalent parse tree. Other than returning a tuple instead of a list, this function is identical to <a class="reference internal" href="#parser.st2list" title="parser.st2list"><code class="xref py py-func docutils literal notranslate"><span class="pre">st2list()</span></code></a>.</p> <p>If <em>line_info</em> is true, line number information will be included for all terminal tokens as a third element of the list representing the token. This information is omitted if the flag is false or omitted.</p> </dd></dl> <dl class="function"> <dt id="parser.compilest"> <code class="descclassname">parser.</code><code class="descname">compilest</code><span class="sig-paren">(</span><em>ast</em>, <em>filename='<syntax-tree>'</em><span class="sig-paren">)</span><a class="headerlink" href="#parser.compilest" title="Permalink to this definition">¶</a></dt> <dd><p id="index-1">The Python byte compiler can be invoked on an ST object to produce code objects which can be used as part of an <a class="reference internal" href="../reference/simple_stmts.html#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a> statement or a call to the built-in <a class="reference internal" href="functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> function. This function provides the interface to the compiler, passing the internal parse tree from <em>ast</em> to the parser, using the source file name specified by the <em>filename</em> parameter. The default value supplied for <em>filename</em> indicates that the source was an ST object.</p> <p>Compiling an ST object may result in exceptions related to compilation; an example would be a <a class="reference internal" href="exceptions.html#exceptions.SyntaxError" title="exceptions.SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> caused by the parse tree for <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">f(0)</span></code>: this statement is considered legal within the formal grammar for Python but is not a legal language construct. The <a class="reference internal" href="exceptions.html#exceptions.SyntaxError" title="exceptions.SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> raised for this condition is actually generated by the Python byte-compiler normally, which is why it can be raised at this point by the <a class="reference internal" href="#module-parser" title="parser: Access parse trees for Python source code."><code class="xref py py-mod docutils literal notranslate"><span class="pre">parser</span></code></a> module. Most causes of compilation failure can be diagnosed programmatically by inspection of the parse tree.</p> </dd></dl> </div> <div class="section" id="queries-on-st-objects"> <span id="querying-sts"></span><h2>32.1.3. Queries on ST Objects<a class="headerlink" href="#queries-on-st-objects" title="Permalink to this headline">¶</a></h2> <p>Two functions are provided which allow an application to determine if an ST was created as an expression or a suite. Neither of these functions can be used to determine if an ST was created from source code via <a class="reference internal" href="#parser.expr" title="parser.expr"><code class="xref py py-func docutils literal notranslate"><span class="pre">expr()</span></code></a> or <a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate"><span class="pre">suite()</span></code></a> or from a parse tree via <a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><code class="xref py py-func docutils literal notranslate"><span class="pre">sequence2st()</span></code></a>.</p> <dl class="function"> <dt id="parser.isexpr"> <code class="descclassname">parser.</code><code class="descname">isexpr</code><span class="sig-paren">(</span><em>ast</em><span class="sig-paren">)</span><a class="headerlink" href="#parser.isexpr" title="Permalink to this definition">¶</a></dt> <dd><p id="index-2">When <em>ast</em> represents an <code class="docutils literal notranslate"><span class="pre">'eval'</span></code> form, this function returns true, otherwise it returns false. This is useful, since code objects normally cannot be queried for this information using existing built-in functions. Note that the code objects created by <a class="reference internal" href="#parser.compilest" title="parser.compilest"><code class="xref py py-func docutils literal notranslate"><span class="pre">compilest()</span></code></a> cannot be queried like this either, and are identical to those created by the built-in <a class="reference internal" href="functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a> function.</p> </dd></dl> <dl class="function"> <dt id="parser.issuite"> <code class="descclassname">parser.</code><code class="descname">issuite</code><span class="sig-paren">(</span><em>ast</em><span class="sig-paren">)</span><a class="headerlink" href="#parser.issuite" title="Permalink to this definition">¶</a></dt> <dd><p>This function mirrors <a class="reference internal" href="#parser.isexpr" title="parser.isexpr"><code class="xref py py-func docutils literal notranslate"><span class="pre">isexpr()</span></code></a> in that it reports whether an ST object represents an <code class="docutils literal notranslate"><span class="pre">'exec'</span></code> form, commonly known as a “suite.” It is not safe to assume that this function is equivalent to <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">isexpr(ast)</span></code>, as additional syntactic fragments may be supported in the future.</p> </dd></dl> </div> <div class="section" id="exceptions-and-error-handling"> <span id="st-errors"></span><h2>32.1.4. Exceptions and Error Handling<a class="headerlink" href="#exceptions-and-error-handling" title="Permalink to this headline">¶</a></h2> <p>The parser module defines a single exception, but may also pass other built-in exceptions from other portions of the Python runtime environment. See each function for information about the exceptions it can raise.</p> <dl class="exception"> <dt id="parser.ParserError"> <em class="property">exception </em><code class="descclassname">parser.</code><code class="descname">ParserError</code><a class="headerlink" href="#parser.ParserError" title="Permalink to this definition">¶</a></dt> <dd><p>Exception raised when a failure occurs within the parser module. This is generally produced for validation failures rather than the built-in <a class="reference internal" href="exceptions.html#exceptions.SyntaxError" title="exceptions.SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> raised during normal parsing. The exception argument is either a string describing the reason of the failure or a tuple containing a sequence causing the failure from a parse tree passed to <a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><code class="xref py py-func docutils literal notranslate"><span class="pre">sequence2st()</span></code></a> and an explanatory string. Calls to <a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><code class="xref py py-func docutils literal notranslate"><span class="pre">sequence2st()</span></code></a> need to be able to handle either type of exception, while calls to other functions in the module will only need to be aware of the simple string values.</p> </dd></dl> <p>Note that the functions <a class="reference internal" href="#parser.compilest" title="parser.compilest"><code class="xref py py-func docutils literal notranslate"><span class="pre">compilest()</span></code></a>, <a class="reference internal" href="#parser.expr" title="parser.expr"><code class="xref py py-func docutils literal notranslate"><span class="pre">expr()</span></code></a>, and <a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate"><span class="pre">suite()</span></code></a> may raise exceptions which are normally raised by the parsing and compilation process. These include the built in exceptions <a class="reference internal" href="exceptions.html#exceptions.MemoryError" title="exceptions.MemoryError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">MemoryError</span></code></a>, <a class="reference internal" href="exceptions.html#exceptions.OverflowError" title="exceptions.OverflowError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OverflowError</span></code></a>, <a class="reference internal" href="exceptions.html#exceptions.SyntaxError" title="exceptions.SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a>, and <a class="reference internal" href="exceptions.html#exceptions.SystemError" title="exceptions.SystemError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemError</span></code></a>. In these cases, these exceptions carry all the meaning normally associated with them. Refer to the descriptions of each function for detailed information.</p> </div> <div class="section" id="st-objects"> <span id="id1"></span><h2>32.1.5. ST Objects<a class="headerlink" href="#st-objects" title="Permalink to this headline">¶</a></h2> <p>Ordered and equality comparisons are supported between ST objects. Pickling of ST objects (using the <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> module) is also supported.</p> <dl class="data"> <dt id="parser.STType"> <code class="descclassname">parser.</code><code class="descname">STType</code><a class="headerlink" href="#parser.STType" title="Permalink to this definition">¶</a></dt> <dd><p>The type of the objects returned by <a class="reference internal" href="#parser.expr" title="parser.expr"><code class="xref py py-func docutils literal notranslate"><span class="pre">expr()</span></code></a>, <a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate"><span class="pre">suite()</span></code></a> and <a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><code class="xref py py-func docutils literal notranslate"><span class="pre">sequence2st()</span></code></a>.</p> </dd></dl> <p>ST objects have the following methods:</p> <dl class="method"> <dt id="parser.ST.compile"> <code class="descclassname">ST.</code><code class="descname">compile</code><span class="sig-paren">(</span><span class="optional">[</span><em>filename</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#parser.ST.compile" title="Permalink to this definition">¶</a></dt> <dd><p>Same as <code class="docutils literal notranslate"><span class="pre">compilest(st,</span> <span class="pre">filename)</span></code>.</p> </dd></dl> <dl class="method"> <dt id="parser.ST.isexpr"> <code class="descclassname">ST.</code><code class="descname">isexpr</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#parser.ST.isexpr" title="Permalink to this definition">¶</a></dt> <dd><p>Same as <code class="docutils literal notranslate"><span class="pre">isexpr(st)</span></code>.</p> </dd></dl> <dl class="method"> <dt id="parser.ST.issuite"> <code class="descclassname">ST.</code><code class="descname">issuite</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#parser.ST.issuite" title="Permalink to this definition">¶</a></dt> <dd><p>Same as <code class="docutils literal notranslate"><span class="pre">issuite(st)</span></code>.</p> </dd></dl> <dl class="method"> <dt id="parser.ST.tolist"> <code class="descclassname">ST.</code><code class="descname">tolist</code><span class="sig-paren">(</span><span class="optional">[</span><em>line_info</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#parser.ST.tolist" title="Permalink to this definition">¶</a></dt> <dd><p>Same as <code class="docutils literal notranslate"><span class="pre">st2list(st,</span> <span class="pre">line_info)</span></code>.</p> </dd></dl> <dl class="method"> <dt id="parser.ST.totuple"> <code class="descclassname">ST.</code><code class="descname">totuple</code><span class="sig-paren">(</span><span class="optional">[</span><em>line_info</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#parser.ST.totuple" title="Permalink to this definition">¶</a></dt> <dd><p>Same as <code class="docutils literal notranslate"><span class="pre">st2tuple(st,</span> <span class="pre">line_info)</span></code>.</p> </dd></dl> </div> <div class="section" id="example-emulation-of-compile"> <h2>32.1.6. Example: Emulation of <a class="reference internal" href="functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a><a class="headerlink" href="#example-emulation-of-compile" title="Permalink to this headline">¶</a></h2> <p>While many useful operations may take place between parsing and bytecode generation, the simplest operation is to do nothing. For this purpose, using the <a class="reference internal" href="#module-parser" title="parser: Access parse trees for Python source code."><code class="xref py py-mod docutils literal notranslate"><span class="pre">parser</span></code></a> module to produce an intermediate data structure is equivalent to the code</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">code</span> <span class="o">=</span> <span class="nb">compile</span><span class="p">(</span><span class="s1">'a + 5'</span><span class="p">,</span> <span class="s1">'file.py'</span><span class="p">,</span> <span class="s1">'eval'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="mi">5</span> <span class="gp">>>> </span><span class="nb">eval</span><span class="p">(</span><span class="n">code</span><span class="p">)</span> <span class="go">10</span> </pre></div> </div> <p>The equivalent operation using the <a class="reference internal" href="#module-parser" title="parser: Access parse trees for Python source code."><code class="xref py py-mod docutils literal notranslate"><span class="pre">parser</span></code></a> module is somewhat longer, and allows the intermediate internal parse tree to be retained as an ST object:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">parser</span> <span class="gp">>>> </span><span class="n">st</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">expr</span><span class="p">(</span><span class="s1">'a + 5'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">code</span> <span class="o">=</span> <span class="n">st</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s1">'file.py'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="mi">5</span> <span class="gp">>>> </span><span class="nb">eval</span><span class="p">(</span><span class="n">code</span><span class="p">)</span> <span class="go">10</span> </pre></div> </div> <p>An application which needs both ST and code objects can package this code into readily available functions:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">parser</span> <span class="k">def</span> <span class="nf">load_suite</span><span class="p">(</span><span class="n">source_string</span><span class="p">):</span> <span class="n">st</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">suite</span><span class="p">(</span><span class="n">source_string</span><span class="p">)</span> <span class="k">return</span> <span class="n">st</span><span class="p">,</span> <span class="n">st</span><span class="o">.</span><span class="n">compile</span><span class="p">()</span> <span class="k">def</span> <span class="nf">load_expression</span><span class="p">(</span><span class="n">source_string</span><span class="p">):</span> <span class="n">st</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">expr</span><span class="p">(</span><span class="n">source_string</span><span class="p">)</span> <span class="k">return</span> <span class="n">st</span><span class="p">,</span> <span class="n">st</span><span class="o">.</span><span class="n">compile</span><span class="p">()</span> </pre></div> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table of Contents</a></h3> <ul> <li><a class="reference internal" href="#">32.1. <code class="docutils literal notranslate"><span class="pre">parser</span></code> — Access Python parse trees</a><ul> <li><a class="reference internal" href="#creating-st-objects">32.1.1. Creating ST Objects</a></li> <li><a class="reference internal" href="#converting-st-objects">32.1.2. Converting ST Objects</a></li> <li><a class="reference internal" href="#queries-on-st-objects">32.1.3. Queries on ST Objects</a></li> <li><a class="reference internal" href="#exceptions-and-error-handling">32.1.4. Exceptions and Error Handling</a></li> <li><a class="reference internal" href="#st-objects">32.1.5. ST Objects</a></li> <li><a class="reference internal" href="#example-emulation-of-compile">32.1.6. Example: Emulation of <code class="docutils literal notranslate"><span class="pre">compile()</span></code></a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="language.html" title="previous chapter">32. 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