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<div class="section" id="module-parser">
<span id="parser-access-python-parse-trees"></span><h1>30.1. <a class="reference internal" href="#module-parser" title="parser: Access parse trees for Python source code."><tt class="xref py py-mod docutils literal"><span class="pre">parser</span></tt></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."><tt class="xref py py-mod docutils literal"><span class="pre">parser</span></tt></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 class="last">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."><tt class="xref py py-mod docutils literal"><span class="pre">ast</span></tt></a>
module.</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."><tt class="xref py py-mod docutils literal"><span class="pre">parser</span></tt></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"><em>The Python Language Reference</em></a>. The parser
itself is created from a grammar specification defined in the file
<tt class="file docutils literal"><span class="pre">Grammar/Grammar</span></tt> 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"><tt class="xref py py-func docutils literal"><span class="pre">expr()</span></tt></a> or <a class="reference internal" href="#parser.suite" title="parser.suite"><tt class="xref py py-func docutils literal"><span class="pre">suite()</span></tt></a> functions,
described below. The ST objects created by <a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><tt class="xref py py-func docutils literal"><span class="pre">sequence2st()</span></tt></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"><tt class="xref py py-func docutils literal"><span class="pre">st2list()</span></tt></a> or <a class="reference internal" href="#parser.st2tuple" title="parser.st2tuple"><tt class="xref py py-func docutils literal"><span class="pre">st2tuple()</span></tt></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 <tt class="file docutils literal"><span class="pre">Include/graminit.h</span></tt> and the Python module
<a class="reference internal" href="symbol.html#module-symbol" title="symbol: Constants representing internal nodes of the parse tree."><tt class="xref py py-mod docutils literal"><span class="pre">symbol</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">if</span></tt></a> in an <tt class="xref py py-const docutils literal"><span class="pre">if_stmt</span></tt>, are included in the
node tree without any special treatment. For example, the <a class="reference internal" href="../reference/compound_stmts.html#if"><tt class="xref std std-keyword docutils literal"><span class="pre">if</span></tt></a> keyword
is represented by the tuple <tt class="docutils literal"><span class="pre">(1,</span> <span class="pre">'if')</span></tt>, where <tt class="docutils literal"><span class="pre">1</span></tt> is the numeric value
associated with all <tt class="xref py py-const docutils literal"><span class="pre">NAME</span></tt> 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 <tt class="docutils literal"><span class="pre">(1,</span> <span class="pre">'if',</span> <span class="pre">12)</span></tt>, where
the <tt class="docutils literal"><span class="pre">12</span></tt> 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"><tt class="xref std std-keyword docutils literal"><span class="pre">if</span></tt></a> keyword above is representative. The various types of
terminal symbols are defined in the C header file <tt class="file docutils literal"><span class="pre">Include/token.h</span></tt> and
the Python module <a class="reference internal" href="token.html#module-token" title="token: Constants representing terminal nodes of the parse tree."><tt class="xref py py-mod docutils literal"><span class="pre">token</span></tt></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."><tt class="xref py py-mod docutils literal"><span class="pre">parser</span></tt></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-see-also 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."><tt class="xref py py-mod docutils literal"><span class="pre">symbol</span></tt></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."><tt class="xref py py-mod docutils literal"><span class="pre">token</span></tt></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>30.1.1. 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 <tt class="docutils literal"><span class="pre">'eval'</span></tt>
and <tt class="docutils literal"><span class="pre">'exec'</span></tt> forms.</p>
<dl class="function">
<dt id="parser.expr">
<tt class="descclassname">parser.</tt><tt class="descname">expr</tt><big>(</big><em>source</em><big>)</big><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"><tt class="xref py py-func docutils literal"><span class="pre">expr()</span></tt></a> function parses the parameter <em>source</em> as if it were an input
to <tt class="docutils literal"><span class="pre">compile(source,</span> <span class="pre">'file.py',</span> <span class="pre">'eval')</span></tt>. 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">
<tt class="descclassname">parser.</tt><tt class="descname">suite</tt><big>(</big><em>source</em><big>)</big><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"><tt class="xref py py-func docutils literal"><span class="pre">suite()</span></tt></a> function parses the parameter <em>source</em> as if it were an input
to <tt class="docutils literal"><span class="pre">compile(source,</span> <span class="pre">'file.py',</span> <span class="pre">'exec')</span></tt>. 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">
<tt class="descclassname">parser.</tt><tt class="descname">sequence2st</tt><big>(</big><em>sequence</em><big>)</big><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"><tt class="xref py py-exc docutils literal"><span class="pre">ParserError</span></tt></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"><tt class="xref py py-func docutils literal"><span class="pre">compilest()</span></tt></a>. This may indicate problems not related to syntax
(such as a <a class="reference internal" href="exceptions.html#MemoryError" title="MemoryError"><tt class="xref py py-exc docutils literal"><span class="pre">MemoryError</span></tt></a> exception), but may also be due to constructs such
as the result of parsing <tt class="docutils literal"><span class="pre">del</span> <span class="pre">f(0)</span></tt>, 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 <tt class="docutils literal"><span class="pre">(1,</span> <span class="pre">'name')</span></tt> or as three-element lists of the form <tt class="docutils literal"><span class="pre">(1,</span>
<span class="pre">'name',</span> <span class="pre">56)</span></tt>. 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">
<tt class="descclassname">parser.</tt><tt class="descname">tuple2st</tt><big>(</big><em>sequence</em><big>)</big><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"><tt class="xref py py-func docutils literal"><span class="pre">sequence2st()</span></tt></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>30.1.2. 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">
<tt class="descclassname">parser.</tt><tt class="descname">st2list</tt><big>(</big><em>st</em>, <em>line_info=False</em>, <em>col_info=False</em><big>)</big><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>st</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"><tt class="xref py py-func docutils literal"><span class="pre">st2tuple()</span></tt></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">
<tt class="descclassname">parser.</tt><tt class="descname">st2tuple</tt><big>(</big><em>st</em>, <em>line_info=False</em>, <em>col_info=False</em><big>)</big><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>st</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"><tt class="xref py py-func docutils literal"><span class="pre">st2list()</span></tt></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">
<tt class="descclassname">parser.</tt><tt class="descname">compilest</tt><big>(</big><em>st</em>, <em>filename='<syntax-tree>'</em><big>)</big><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 a call to the built-in <a class="reference internal" href="functions.html#exec" title="exec"><tt class="xref py py-func docutils literal"><span class="pre">exec()</span></tt></a> or <a class="reference internal" href="functions.html#eval" title="eval"><tt class="xref py py-func docutils literal"><span class="pre">eval()</span></tt></a>
functions. This function provides the interface to the compiler, passing the
internal parse tree from <em>st</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#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></a> caused by the parse tree for <tt class="docutils literal"><span class="pre">del</span> <span class="pre">f(0)</span></tt>:
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#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></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."><tt class="xref py py-mod docutils literal"><span class="pre">parser</span></tt></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>30.1.3. 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"><tt class="xref py py-func docutils literal"><span class="pre">expr()</span></tt></a> or
<a class="reference internal" href="#parser.suite" title="parser.suite"><tt class="xref py py-func docutils literal"><span class="pre">suite()</span></tt></a> or from a parse tree via <a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><tt class="xref py py-func docutils literal"><span class="pre">sequence2st()</span></tt></a>.</p>
<dl class="function">
<dt id="parser.isexpr">
<tt class="descclassname">parser.</tt><tt class="descname">isexpr</tt><big>(</big><em>st</em><big>)</big><a class="headerlink" href="#parser.isexpr" title="Permalink to this definition">¶</a></dt>
<dd><p id="index-2">When <em>st</em> represents an <tt class="docutils literal"><span class="pre">'eval'</span></tt> 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"><tt class="xref py py-func docutils literal"><span class="pre">compilest()</span></tt></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"><tt class="xref py py-func docutils literal"><span class="pre">compile()</span></tt></a> function.</p>
</dd></dl>
<dl class="function">
<dt id="parser.issuite">
<tt class="descclassname">parser.</tt><tt class="descname">issuite</tt><big>(</big><em>st</em><big>)</big><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"><tt class="xref py py-func docutils literal"><span class="pre">isexpr()</span></tt></a> in that it reports whether an ST object
represents an <tt class="docutils literal"><span class="pre">'exec'</span></tt> form, commonly known as a “suite.” It is not safe to
assume that this function is equivalent to <tt class="docutils literal"><span class="pre">not</span> <span class="pre">isexpr(st)</span></tt>, 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>30.1.4. 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><tt class="descclassname">parser.</tt><tt class="descname">ParserError</tt><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#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></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"><tt class="xref py py-func docutils literal"><span class="pre">sequence2st()</span></tt></a>
and an explanatory string. Calls to <a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><tt class="xref py py-func docutils literal"><span class="pre">sequence2st()</span></tt></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"><tt class="xref py py-func docutils literal"><span class="pre">compilest()</span></tt></a>, <a class="reference internal" href="#parser.expr" title="parser.expr"><tt class="xref py py-func docutils literal"><span class="pre">expr()</span></tt></a>, and <a class="reference internal" href="#parser.suite" title="parser.suite"><tt class="xref py py-func docutils literal"><span class="pre">suite()</span></tt></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#MemoryError" title="MemoryError"><tt class="xref py py-exc docutils literal"><span class="pre">MemoryError</span></tt></a>,
<a class="reference internal" href="exceptions.html#OverflowError" title="OverflowError"><tt class="xref py py-exc docutils literal"><span class="pre">OverflowError</span></tt></a>, <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></a>, and <a class="reference internal" href="exceptions.html#SystemError" title="SystemError"><tt class="xref py py-exc docutils literal"><span class="pre">SystemError</span></tt></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>30.1.5. 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."><tt class="xref py py-mod docutils literal"><span class="pre">pickle</span></tt></a> module) is also supported.</p>
<dl class="data">
<dt id="parser.STType">
<tt class="descclassname">parser.</tt><tt class="descname">STType</tt><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"><tt class="xref py py-func docutils literal"><span class="pre">expr()</span></tt></a>, <a class="reference internal" href="#parser.suite" title="parser.suite"><tt class="xref py py-func docutils literal"><span class="pre">suite()</span></tt></a> and
<a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><tt class="xref py py-func docutils literal"><span class="pre">sequence2st()</span></tt></a>.</p>
</dd></dl>
<p>ST objects have the following methods:</p>
<dl class="method">
<dt id="parser.ST.compile">
<tt class="descclassname">ST.</tt><tt class="descname">compile</tt><big>(</big><em>filename='<syntax-tree>'</em><big>)</big><a class="headerlink" href="#parser.ST.compile" title="Permalink to this definition">¶</a></dt>
<dd><p>Same as <tt class="docutils literal"><span class="pre">compilest(st,</span> <span class="pre">filename)</span></tt>.</p>
</dd></dl>
<dl class="method">
<dt id="parser.ST.isexpr">
<tt class="descclassname">ST.</tt><tt class="descname">isexpr</tt><big>(</big><big>)</big><a class="headerlink" href="#parser.ST.isexpr" title="Permalink to this definition">¶</a></dt>
<dd><p>Same as <tt class="docutils literal"><span class="pre">isexpr(st)</span></tt>.</p>
</dd></dl>
<dl class="method">
<dt id="parser.ST.issuite">
<tt class="descclassname">ST.</tt><tt class="descname">issuite</tt><big>(</big><big>)</big><a class="headerlink" href="#parser.ST.issuite" title="Permalink to this definition">¶</a></dt>
<dd><p>Same as <tt class="docutils literal"><span class="pre">issuite(st)</span></tt>.</p>
</dd></dl>
<dl class="method">
<dt id="parser.ST.tolist">
<tt class="descclassname">ST.</tt><tt class="descname">tolist</tt><big>(</big><em>line_info=False</em>, <em>col_info=False</em><big>)</big><a class="headerlink" href="#parser.ST.tolist" title="Permalink to this definition">¶</a></dt>
<dd><p>Same as <tt class="docutils literal"><span class="pre">st2list(st,</span> <span class="pre">line_info,</span> <span class="pre">col_info)</span></tt>.</p>
</dd></dl>
<dl class="method">
<dt id="parser.ST.totuple">
<tt class="descclassname">ST.</tt><tt class="descname">totuple</tt><big>(</big><em>line_info=False</em>, <em>col_info=False</em><big>)</big><a class="headerlink" href="#parser.ST.totuple" title="Permalink to this definition">¶</a></dt>
<dd><p>Same as <tt class="docutils literal"><span class="pre">st2tuple(st,</span> <span class="pre">line_info,</span> <span class="pre">col_info)</span></tt>.</p>
</dd></dl>
</div>
<div class="section" id="example-emulation-of-compile">
<h2>30.1.6. Example: Emulation of <a class="reference internal" href="functions.html#compile" title="compile"><tt class="xref py py-func docutils literal"><span class="pre">compile()</span></tt></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."><tt class="xref py py-mod docutils literal"><span class="pre">parser</span></tt></a> module to produce an intermediate data structure is equivalent
to the code</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">code</span> <span class="o">=</span> <span class="nb">compile</span><span class="p">(</span><span class="s">'a + 5'</span><span class="p">,</span> <span class="s">'file.py'</span><span class="p">,</span> <span class="s">'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."><tt class="xref py py-mod docutils literal"><span class="pre">parser</span></tt></a> module is somewhat longer, and
allows the intermediate internal parse tree to be retained as an ST object:</p>
<div class="highlight-python3"><div class="highlight"><pre><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="s">'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="s">'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-python3"><div class="highlight"><pre><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>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">30.1. <tt class="docutils literal"><span class="pre">parser</span></tt> — Access Python parse trees</a><ul>
<li><a class="reference internal" href="#creating-st-objects">30.1.1. Creating ST Objects</a></li>
<li><a class="reference internal" href="#converting-st-objects">30.1.2. Converting ST Objects</a></li>
<li><a class="reference internal" href="#queries-on-st-objects">30.1.3. Queries on ST Objects</a></li>
<li><a class="reference internal" href="#exceptions-and-error-handling">30.1.4. Exceptions and Error Handling</a></li>
<li><a class="reference internal" href="#st-objects">30.1.5. ST Objects</a></li>
<li><a class="reference internal" href="#example-emulation-of-compile">30.1.6. Example: Emulation of <tt class="docutils literal"><span class="pre">compile()</span></tt></a></li>
</ul>
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contrib-backports
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