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Code Editor : parser.html

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 <section id="module-parser">
<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">parser</code></a> — Access Python parse trees<a class="headerlink" href="#module-parser" title="Permalink to this headline">¶</a></h1>
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">parser</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.
<div class="admonition note">
Note
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">ast</code></a>
module.
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">parser</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 ast, not st.
The “ast” functions have been removed in Python 3.
</div>
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">parser</code></a> module are
presented.
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">The Python Language Reference</a>. The parser
itself is created from a grammar specification defined in the file
<code class="file docutils literal notranslate">Grammar/Grammar</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">expr()</code></a> or <a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate">suite()</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">sequence2st()</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.
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">st2list()</code></a> or <a class="reference internal" href="#parser.st2tuple" title="parser.st2tuple"><code class="xref py py-func docutils literal notranslate">st2tuple()</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">Include/graminit.h</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">symbol</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">if</code></a> in an <code class="xref py py-const docutils literal notranslate">if_stmt</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">if</code></a> keyword
is represented by the tuple <code class="docutils literal notranslate">(1, 'if')</code>, where <code class="docutils literal notranslate">1</code> is the numeric value
associated with all <code class="xref py py-const docutils literal notranslate">NAME</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">(1, 'if', 12)</code>, where
the <code class="docutils literal notranslate">12</code> represents the line number at which the terminal symbol was found.
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">if</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">Include/token.h</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">token</code></a>.
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.
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">parser</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.
<div class="admonition seealso">
See also
<dl class="simple">
<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">symbol</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">token</code></a></dt><dd>Useful constants representing leaf nodes of the parse tree and functions for
testing node values.
</dd>
</dl>
</div>
<section id="creating-st-objects">
<h2>32.1.1. Creating ST Objects<a class="headerlink" href="#creating-st-objects" title="Permalink to this headline">¶</a></h2>
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">'eval'</code>
and <code class="docutils literal notranslate">'exec'</code> forms.
<dl class="py function">
<dt class="sig sig-object py" id="parser.expr">
parser.expr(source)<a class="headerlink" href="#parser.expr" title="Permalink to this definition">¶</a></dt>
<dd>The <a class="reference internal" href="#parser.expr" title="parser.expr"><code class="xref py py-func docutils literal notranslate">expr()</code></a> function parses the parameter source as if it were an input
to <code class="docutils literal notranslate">compile(source, 'file.py', 'eval')</code>. If the parse succeeds, an ST object
is created to hold the internal parse tree representation, otherwise an
appropriate exception is raised.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="parser.suite">
parser.suite(source)<a class="headerlink" href="#parser.suite" title="Permalink to this definition">¶</a></dt>
<dd>The <a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate">suite()</code></a> function parses the parameter source as if it were an input
to <code class="docutils literal notranslate">compile(source, 'file.py', 'exec')</code>. If the parse succeeds, an ST object
is created to hold the internal parse tree representation, otherwise an
appropriate exception is raised.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="parser.sequence2st">
parser.sequence2st(sequence)<a class="headerlink" href="#parser.sequence2st" title="Permalink to this definition">¶</a></dt>
<dd>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">ParserError</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">compilest()</code></a>. This may indicate problems not related to syntax
(such as a <code class="xref py py-exc docutils literal notranslate">MemoryError</code> exception), but may also be due to constructs such
as the result of parsing <code class="docutils literal notranslate">del f(0)</code>, which escapes the Python parser but is
checked by the bytecode compiler.
Sequences representing terminal tokens may be represented as either two-element
lists of the form <code class="docutils literal notranslate">(1, 'name')</code> or as three-element lists of the form <code class="docutils literal notranslate">(1,
'name', 56)</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.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="parser.tuple2st">
parser.tuple2st(sequence)<a class="headerlink" href="#parser.tuple2st" title="Permalink to this definition">¶</a></dt>
<dd>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">sequence2st()</code></a>. This entry point is
maintained for backward compatibility.
</dd></dl>

</section>
<section id="converting-st-objects">
<h2>32.1.2. Converting ST Objects<a class="headerlink" href="#converting-st-objects" title="Permalink to this headline">¶</a></h2>
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.
<dl class="py function">
<dt class="sig sig-object py" id="parser.st2list">
parser.st2list(ast[, line_info])<a class="headerlink" href="#parser.st2list" title="Permalink to this definition">¶</a></dt>
<dd>This function accepts an ST object from the caller in ast 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">st2tuple()</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.
If line_info 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 ends.
This information is omitted if the flag is false or omitted.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="parser.st2tuple">
parser.st2tuple(ast[, line_info])<a class="headerlink" href="#parser.st2tuple" title="Permalink to this definition">¶</a></dt>
<dd>This function accepts an ST object from the caller in ast 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">st2list()</code></a>.
If line_info 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.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="parser.compilest">
parser.compilest(ast, filename='&lt;syntax-tree&gt;')<a class="headerlink" href="#parser.compilest" title="Permalink to this definition">¶</a></dt>
<dd>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">exec</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">eval()</code></a> function. This function provides the interface to the
compiler, passing the internal parse tree from ast to the parser, using the
source file name specified by the filename parameter. The default value
supplied for filename indicates that the source was an ST object.
Compiling an ST object may result in exceptions related to compilation; an
example would be a <code class="xref py py-exc docutils literal notranslate">SyntaxError</code> caused by the parse tree for <code class="docutils literal notranslate">del f(0)</code>:
this statement is considered legal within the formal grammar for Python but is
not a legal language construct. The <code class="xref py py-exc docutils literal notranslate">SyntaxError</code> 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">parser</code></a> module. Most causes of
compilation failure can be diagnosed programmatically by inspection of the parse
tree.
</dd></dl>

</section>
<section id="queries-on-st-objects">
<h2>32.1.3. Queries on ST Objects<a class="headerlink" href="#queries-on-st-objects" title="Permalink to this headline">¶</a></h2>
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">expr()</code></a> or
<a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate">suite()</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">sequence2st()</code></a>.
<dl class="py function">
<dt class="sig sig-object py" id="parser.isexpr">
parser.isexpr(ast)<a class="headerlink" href="#parser.isexpr" title="Permalink to this definition">¶</a></dt>
<dd>When ast represents an <code class="docutils literal notranslate">'eval'</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">compilest()</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">compile()</code></a> function.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="parser.issuite">
parser.issuite(ast)<a class="headerlink" href="#parser.issuite" title="Permalink to this definition">¶</a></dt>
<dd>This function mirrors <a class="reference internal" href="#parser.isexpr" title="parser.isexpr"><code class="xref py py-func docutils literal notranslate">isexpr()</code></a> in that it reports whether an ST object
represents an <code class="docutils literal notranslate">'exec'</code> form, commonly known as a “suite.” It is not safe to
assume that this function is equivalent to <code class="docutils literal notranslate">not isexpr(ast)</code>, as additional
syntactic fragments may be supported in the future.
</dd></dl>

</section>
<section id="exceptions-and-error-handling">
<h2>32.1.4. Exceptions and Error Handling<a class="headerlink" href="#exceptions-and-error-handling" title="Permalink to this headline">¶</a></h2>
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.
<dl class="py exception">
<dt class="sig sig-object py" id="parser.ParserError">
exception parser.ParserError<a class="headerlink" href="#parser.ParserError" title="Permalink to this definition">¶</a></dt>
<dd>Exception raised when a failure occurs within the parser module. This is
generally produced for validation failures rather than the built-in
<code class="xref py py-exc docutils literal notranslate">SyntaxError</code> 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">sequence2st()</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">sequence2st()</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.
</dd></dl>

Note that the functions <a class="reference internal" href="#parser.compilest" title="parser.compilest"><code class="xref py py-func docutils literal notranslate">compilest()</code></a>, <a class="reference internal" href="#parser.expr" title="parser.expr"><code class="xref py py-func docutils literal notranslate">expr()</code></a>, and <a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate">suite()</code></a> may
raise exceptions which are normally raised by the parsing and compilation
process. These include the built in exceptions <code class="xref py py-exc docutils literal notranslate">MemoryError</code>,
<code class="xref py py-exc docutils literal notranslate">OverflowError</code>, <code class="xref py py-exc docutils literal notranslate">SyntaxError</code>, and <code class="xref py py-exc docutils literal notranslate">SystemError</code>. In these
cases, these exceptions carry all the meaning normally associated with them.
Refer to the descriptions of each function for detailed information.
</section>
<section id="st-objects">
<h2>32.1.5. ST Objects<a class="headerlink" href="#st-objects" title="Permalink to this headline">¶</a></h2>
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">pickle</code></a> module) is also supported.
<dl class="py data">
<dt class="sig sig-object py" id="parser.STType">
parser.STType<a class="headerlink" href="#parser.STType" title="Permalink to this definition">¶</a></dt>
<dd>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">expr()</code></a>, <a class="reference internal" href="#parser.suite" title="parser.suite"><code class="xref py py-func docutils literal notranslate">suite()</code></a> and
<a class="reference internal" href="#parser.sequence2st" title="parser.sequence2st"><code class="xref py py-func docutils literal notranslate">sequence2st()</code></a>.
</dd></dl>

ST objects have the following methods:
<dl class="py method">
<dt class="sig sig-object py" id="parser.ST.compile">
ST.compile([filename])<a class="headerlink" href="#parser.ST.compile" title="Permalink to this definition">¶</a></dt>
<dd>Same as <code class="docutils literal notranslate">compilest(st, filename)</code>.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="parser.ST.isexpr">
ST.isexpr()<a class="headerlink" href="#parser.ST.isexpr" title="Permalink to this definition">¶</a></dt>
<dd>Same as <code class="docutils literal notranslate">isexpr(st)</code>.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="parser.ST.issuite">
ST.issuite()<a class="headerlink" href="#parser.ST.issuite" title="Permalink to this definition">¶</a></dt>
<dd>Same as <code class="docutils literal notranslate">issuite(st)</code>.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="parser.ST.tolist">
ST.tolist([line_info])<a class="headerlink" href="#parser.ST.tolist" title="Permalink to this definition">¶</a></dt>
<dd>Same as <code class="docutils literal notranslate">st2list(st, line_info)</code>.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="parser.ST.totuple">
ST.totuple([line_info])<a class="headerlink" href="#parser.ST.totuple" title="Permalink to this definition">¶</a></dt>
<dd>Same as <code class="docutils literal notranslate">st2tuple(st, line_info)</code>.
</dd></dl>

</section>
<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">compile()</code></a><a class="headerlink" href="#example-emulation-of-compile" title="Permalink to this headline">¶</a></h2>
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">parser</code></a> module to produce an intermediate data structure is equivalent
to the code
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; code = compile(&#39;a + 5&#39;, &#39;file.py&#39;, &#39;eval&#39;)
&gt;&gt;&gt; a = 5
&gt;&gt;&gt; eval(code)
10
</pre></div>
</div>
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">parser</code></a> module is somewhat longer, and
allows the intermediate internal parse tree to be retained as an ST object:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import parser
&gt;&gt;&gt; st = parser.expr(&#39;a + 5&#39;)
&gt;&gt;&gt; code = st.compile(&#39;file.py&#39;)
&gt;&gt;&gt; a = 5
&gt;&gt;&gt; eval(code)
10
</pre></div>
</div>
An application which needs both ST and code objects can package this code into
readily available functions:
<div class="highlight-default notranslate"><div class="highlight"><pre>import parser

def load_suite(source_string):
 st = parser.suite(source_string)
 return st, st.compile()

def load_expression(source_string):
 st = parser.expr(source_string)
 return st, st.compile()
</pre></div>
</div>
</section>
</section>

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 <ul>
<li><a class="reference internal" href="#">32.1. <code class="xref py py-mod docutils literal notranslate">parser</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="xref py py-func docutils literal notranslate">compile()</code></a></li>
</ul>
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