[ Avaa Bypassed ]

Upload:

Command:

hmhc3928@3.135.218.67: ~ $ 

<!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="Content-Type" content="text/html; charset=utf-8" />
    
    <title>31.2. ast — Abstract Syntax Trees &mdash; Python 2.7.5 documentation</title>
    
    <link rel="stylesheet" href="../_static/default.css" type="text/css" />
    <link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
    
    <script type="text/javascript">
      var DOCUMENTATION_OPTIONS = {
        URL_ROOT:    '../',
        VERSION:     '2.7.5',
        COLLAPSE_INDEX: false,
        FILE_SUFFIX: '.html',
        HAS_SOURCE:  true
      };
    </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/sidebar.js"></script>
    <link rel="search" type="application/opensearchdescription+xml"
          title="Search within Python 2.7.5 documentation"
          href="../_static/opensearch.xml"/>
    <link rel="author" title="About these documents" href="../about.html" />
    <link rel="copyright" title="Copyright" href="../copyright.html" />
    <link rel="top" title="Python 2.7.5 documentation" href="../index.html" />
    <link rel="up" title="31. Python Language Services" href="language.html" />
    <link rel="next" title="31.3. symtable — Access to the compiler’s symbol tables" href="symtable.html" />
    <link rel="prev" title="31.1. parser — Access Python parse trees" href="parser.html" />
    <link rel="shortcut icon" type="image/png" href="../_static/py.png" />
    <script type="text/javascript" src="../_static/copybutton.js"></script>
    
 

  </head>
  <body>
    <div class="related">
      <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="symtable.html" title="31.3. symtable — Access to the compiler’s symbol tables"
             accesskey="N">next</a> |</li>
        <li class="right" >
          <a href="parser.html" title="31.1. parser — Access Python parse trees"
             accesskey="P">previous</a> |</li>
        <li><img src="../_static/py.png" alt=""
                 style="vertical-align: middle; margin-top: -1px"/></li>
        <li><a href="http://www.python.org/">Python</a> &raquo;</li>
        <li>
          <a href="../index.html">Python 2.7.5 documentation</a> &raquo;
        </li>

          <li><a href="index.html" >The Python Standard Library</a> &raquo;</li>
          <li><a href="language.html" accesskey="U">31. Python Language Services</a> &raquo;</li> 
      </ul>
    </div>  

    <div class="document">
      <div class="documentwrapper">
        <div class="bodywrapper">
          <div class="body">
            
  <div class="section" id="module-ast">
<span id="ast-abstract-syntax-trees"></span><h1>31.2. <a class="reference internal" href="#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> &#8212; Abstract Syntax Trees<a class="headerlink" href="#module-ast" title="Permalink to this headline">¶</a></h1>
<p class="versionadded">
<span class="versionmodified">New in version 2.5: </span>The low-level <tt class="docutils literal"><span class="pre">_ast</span></tt> module containing only the node classes.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6: </span>The high-level <tt class="docutils literal"><span class="pre">ast</span></tt> module containing all helpers.</p>
<p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Lib/ast.py">Lib/ast.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#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 helps Python applications to process trees of the Python
abstract syntax grammar.  The abstract syntax itself might change with each
Python release; this module helps to find out programmatically what the current
grammar looks like.</p>
<p>An abstract syntax tree can be generated by passing <tt class="xref py py-data docutils literal"><span class="pre">ast.PyCF_ONLY_AST</span></tt> as
a flag to the <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> built-in function, or using the <a class="reference internal" href="#ast.parse" title="ast.parse"><tt class="xref py py-func docutils literal"><span class="pre">parse()</span></tt></a>
helper provided in this module.  The result will be a tree of objects whose
classes all inherit from <a class="reference internal" href="#ast.AST" title="ast.AST"><tt class="xref py py-class docutils literal"><span class="pre">ast.AST</span></tt></a>.  An abstract syntax tree can be
compiled into a Python code object using 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>
<div class="section" id="node-classes">
<h2>31.2.1. Node classes<a class="headerlink" href="#node-classes" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="ast.AST">
<em class="property">class </em><tt class="descclassname">ast.</tt><tt class="descname">AST</tt><a class="headerlink" href="#ast.AST" title="Permalink to this definition">¶</a></dt>
<dd><p>This is the base of all AST node classes.  The actual node classes are
derived from the <tt class="file docutils literal"><span class="pre">Parser/Python.asdl</span></tt> file, which is reproduced
<a class="reference internal" href="#abstract-grammar"><em>below</em></a>.  They are defined in the <tt class="xref py py-mod docutils literal"><span class="pre">_ast</span></tt> C
module and re-exported in <a class="reference internal" href="#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>.</p>
<p>There is one class defined for each left-hand side symbol in the abstract
grammar (for example, <tt class="xref py py-class docutils literal"><span class="pre">ast.stmt</span></tt> or <tt class="xref py py-class docutils literal"><span class="pre">ast.expr</span></tt>).  In addition,
there is one class defined for each constructor on the right-hand side; these
classes inherit from the classes for the left-hand side trees.  For example,
<tt class="xref py py-class docutils literal"><span class="pre">ast.BinOp</span></tt> inherits from <tt class="xref py py-class docutils literal"><span class="pre">ast.expr</span></tt>.  For production rules
with alternatives (aka &#8220;sums&#8221;), the left-hand side class is abstract: only
instances of specific constructor nodes are ever created.</p>
<dl class="attribute">
<dt id="ast.AST._fields">
<tt class="descname">_fields</tt><a class="headerlink" href="#ast.AST._fields" title="Permalink to this definition">¶</a></dt>
<dd><p>Each concrete class has an attribute <a class="reference internal" href="#ast.AST._fields" title="ast.AST._fields"><tt class="xref py py-attr docutils literal"><span class="pre">_fields</span></tt></a> which gives the names
of all child nodes.</p>
<p>Each instance of a concrete class has one attribute for each child node,
of the type as defined in the grammar.  For example, <tt class="xref py py-class docutils literal"><span class="pre">ast.BinOp</span></tt>
instances have an attribute <tt class="xref py py-attr docutils literal"><span class="pre">left</span></tt> of type <tt class="xref py py-class docutils literal"><span class="pre">ast.expr</span></tt>.</p>
<p>If these attributes are marked as optional in the grammar (using a
question mark), the value might be <tt class="docutils literal"><span class="pre">None</span></tt>.  If the attributes can have
zero-or-more values (marked with an asterisk), the values are represented
as Python lists.  All possible attributes must be present and have valid
values when compiling an AST with <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>.</p>
</dd></dl>

<dl class="attribute">
<dt id="ast.AST.lineno">
<tt class="descname">lineno</tt><a class="headerlink" href="#ast.AST.lineno" title="Permalink to this definition">¶</a></dt>
<dt id="ast.AST.col_offset">
<tt class="descname">col_offset</tt><a class="headerlink" href="#ast.AST.col_offset" title="Permalink to this definition">¶</a></dt>
<dd><p>Instances of <tt class="xref py py-class docutils literal"><span class="pre">ast.expr</span></tt> and <tt class="xref py py-class docutils literal"><span class="pre">ast.stmt</span></tt> subclasses have
<a class="reference internal" href="#ast.AST.lineno" title="ast.AST.lineno"><tt class="xref py py-attr docutils literal"><span class="pre">lineno</span></tt></a> and <a class="reference internal" href="#ast.AST.col_offset" title="ast.AST.col_offset"><tt class="xref py py-attr docutils literal"><span class="pre">col_offset</span></tt></a> attributes.  The <a class="reference internal" href="#ast.AST.lineno" title="ast.AST.lineno"><tt class="xref py py-attr docutils literal"><span class="pre">lineno</span></tt></a> is
the line number of source text (1-indexed so the first line is line 1) and
the <a class="reference internal" href="#ast.AST.col_offset" title="ast.AST.col_offset"><tt class="xref py py-attr docutils literal"><span class="pre">col_offset</span></tt></a> is the UTF-8 byte offset of the first token that
generated the node.  The UTF-8 offset is recorded because the parser uses
UTF-8 internally.</p>
</dd></dl>

<p>The constructor of a class <tt class="xref py py-class docutils literal"><span class="pre">ast.T</span></tt> parses its arguments as follows:</p>
<ul class="simple">
<li>If there are positional arguments, there must be as many as there are items
in <tt class="xref py py-attr docutils literal"><span class="pre">T._fields</span></tt>; they will be assigned as attributes of these names.</li>
<li>If there are keyword arguments, they will set the attributes of the same
names to the given values.</li>
</ul>
<p>For example, to create and populate an <tt class="xref py py-class docutils literal"><span class="pre">ast.UnaryOp</span></tt> node, you could
use</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">node</span> <span class="o">=</span> <span class="n">ast</span><span class="o">.</span><span class="n">UnaryOp</span><span class="p">()</span>
<span class="n">node</span><span class="o">.</span><span class="n">op</span> <span class="o">=</span> <span class="n">ast</span><span class="o">.</span><span class="n">USub</span><span class="p">()</span>
<span class="n">node</span><span class="o">.</span><span class="n">operand</span> <span class="o">=</span> <span class="n">ast</span><span class="o">.</span><span class="n">Num</span><span class="p">()</span>
<span class="n">node</span><span class="o">.</span><span class="n">operand</span><span class="o">.</span><span class="n">n</span> <span class="o">=</span> <span class="mi">5</span>
<span class="n">node</span><span class="o">.</span><span class="n">operand</span><span class="o">.</span><span class="n">lineno</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">node</span><span class="o">.</span><span class="n">operand</span><span class="o">.</span><span class="n">col_offset</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">node</span><span class="o">.</span><span class="n">lineno</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">node</span><span class="o">.</span><span class="n">col_offset</span> <span class="o">=</span> <span class="mi">0</span>
</pre></div>
</div>
<p>or the more compact</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">node</span> <span class="o">=</span> <span class="n">ast</span><span class="o">.</span><span class="n">UnaryOp</span><span class="p">(</span><span class="n">ast</span><span class="o">.</span><span class="n">USub</span><span class="p">(),</span> <span class="n">ast</span><span class="o">.</span><span class="n">Num</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="n">lineno</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">col_offset</span><span class="o">=</span><span class="mi">0</span><span class="p">),</span>
                   <span class="n">lineno</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">col_offset</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
<p class="versionadded">
<span class="versionmodified">New in version 2.6: </span>The constructor as explained above was added.  In Python 2.5 nodes had
to be created by calling the class constructor without arguments and
setting the attributes afterwards.</p>
</dd></dl>

</div>
<div class="section" id="abstract-grammar">
<span id="id1"></span><h2>31.2.2. Abstract Grammar<a class="headerlink" href="#abstract-grammar" title="Permalink to this headline">¶</a></h2>
<p>The module defines a string constant <tt class="docutils literal"><span class="pre">__version__</span></tt> which is the decimal
Subversion revision number of the file shown below.</p>
<p>The abstract grammar is currently defined as follows:</p>
<div class="highlight-python"><pre>-- ASDL's five builtin types are identifier, int, string, object, bool

module Python version "$Revision$"
{
	mod = Module(stmt* body)
	    | Interactive(stmt* body)
	    | Expression(expr body)

	    -- not really an actual node but useful in Jython's typesystem.
	    | Suite(stmt* body)

	stmt = FunctionDef(identifier name, arguments args, 
                            stmt* body, expr* decorator_list)
	      | ClassDef(identifier name, expr* bases, stmt* body, expr* decorator_list)
	      | Return(expr? value)

	      | Delete(expr* targets)
	      | Assign(expr* targets, expr value)
	      | AugAssign(expr target, operator op, expr value)

	      -- not sure if bool is allowed, can always use int
 	      | Print(expr? dest, expr* values, bool nl)

	      -- use 'orelse' because else is a keyword in target languages
	      | For(expr target, expr iter, stmt* body, stmt* orelse)
	      | While(expr test, stmt* body, stmt* orelse)
	      | If(expr test, stmt* body, stmt* orelse)
	      | With(expr context_expr, expr? optional_vars, stmt* body)

	      -- 'type' is a bad name
	      | Raise(expr? type, expr? inst, expr? tback)
	      | TryExcept(stmt* body, excepthandler* handlers, stmt* orelse)
	      | TryFinally(stmt* body, stmt* finalbody)
	      | Assert(expr test, expr? msg)

	      | Import(alias* names)
	      | ImportFrom(identifier? module, alias* names, int? level)

	      -- Doesn't capture requirement that locals must be
	      -- defined if globals is
	      -- still supports use as a function!
	      | Exec(expr body, expr? globals, expr? locals)

	      | Global(identifier* names)
	      | Expr(expr value)
	      | Pass | Break | Continue

	      -- XXX Jython will be different
	      -- col_offset is the byte offset in the utf8 string the parser uses
	      attributes (int lineno, int col_offset)

	      -- BoolOp() can use left &amp; right?
	expr = BoolOp(boolop op, expr* values)
	     | BinOp(expr left, operator op, expr right)
	     | UnaryOp(unaryop op, expr operand)
	     | Lambda(arguments args, expr body)
	     | IfExp(expr test, expr body, expr orelse)
	     | Dict(expr* keys, expr* values)
	     | Set(expr* elts)
	     | ListComp(expr elt, comprehension* generators)
	     | SetComp(expr elt, comprehension* generators)
	     | DictComp(expr key, expr value, comprehension* generators)
	     | GeneratorExp(expr elt, comprehension* generators)
	     -- the grammar constrains where yield expressions can occur
	     | Yield(expr? value)
	     -- need sequences for compare to distinguish between
	     -- x &lt; 4 &lt; 3 and (x &lt; 4) &lt; 3
	     | Compare(expr left, cmpop* ops, expr* comparators)
	     | Call(expr func, expr* args, keyword* keywords,
			 expr? starargs, expr? kwargs)
	     | Repr(expr value)
	     | Num(object n) -- a number as a PyObject.
	     | Str(string s) -- need to specify raw, unicode, etc?
	     -- other literals? bools?

	     -- the following expression can appear in assignment context
	     | Attribute(expr value, identifier attr, expr_context ctx)
	     | Subscript(expr value, slice slice, expr_context ctx)
	     | Name(identifier id, expr_context ctx)
	     | List(expr* elts, expr_context ctx) 
	     | Tuple(expr* elts, expr_context ctx)

	      -- col_offset is the byte offset in the utf8 string the parser uses
	      attributes (int lineno, int col_offset)

	expr_context = Load | Store | Del | AugLoad | AugStore | Param

	slice = Ellipsis | Slice(expr? lower, expr? upper, expr? step) 
	      | ExtSlice(slice* dims) 
	      | Index(expr value) 

	boolop = And | Or 

	operator = Add | Sub | Mult | Div | Mod | Pow | LShift 
                 | RShift | BitOr | BitXor | BitAnd | FloorDiv

	unaryop = Invert | Not | UAdd | USub

	cmpop = Eq | NotEq | Lt | LtE | Gt | GtE | Is | IsNot | In | NotIn

	comprehension = (expr target, expr iter, expr* ifs)

	-- not sure what to call the first argument for raise and except
	excepthandler = ExceptHandler(expr? type, expr? name, stmt* body)
	                attributes (int lineno, int col_offset)

	arguments = (expr* args, identifier? vararg, 
		     identifier? kwarg, expr* defaults)

        -- keyword arguments supplied to call
        keyword = (identifier arg, expr value)

        -- import name with optional 'as' alias.
        alias = (identifier name, identifier? asname)
}
</pre>
</div>
</div>
<div class="section" id="ast-helpers">
<h2>31.2.3. <a class="reference internal" href="#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> Helpers<a class="headerlink" href="#ast-helpers" title="Permalink to this headline">¶</a></h2>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
<p>Apart from the node classes, <a class="reference internal" href="#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 defines these utility functions
and classes for traversing abstract syntax trees:</p>
<dl class="function">
<dt id="ast.parse">
<tt class="descclassname">ast.</tt><tt class="descname">parse</tt><big>(</big><em>source</em>, <em>filename='&lt;unknown&gt;'</em>, <em>mode='exec'</em><big>)</big><a class="headerlink" href="#ast.parse" title="Permalink to this definition">¶</a></dt>
<dd><p>Parse the source into an AST node.  Equivalent to <tt class="docutils literal"><span class="pre">compile(source,</span>
<span class="pre">filename,</span> <span class="pre">mode,</span> <span class="pre">ast.PyCF_ONLY_AST)</span></tt>.</p>
</dd></dl>

<dl class="function">
<dt id="ast.literal_eval">
<tt class="descclassname">ast.</tt><tt class="descname">literal_eval</tt><big>(</big><em>node_or_string</em><big>)</big><a class="headerlink" href="#ast.literal_eval" title="Permalink to this definition">¶</a></dt>
<dd><p>Safely evaluate an expression node or a string containing a Python
expression.  The string or node provided may only consist of the following
Python literal structures: strings, numbers, tuples, lists, dicts, booleans,
and <tt class="docutils literal"><span class="pre">None</span></tt>.</p>
<p>This can be used for safely evaluating strings containing Python expressions
from untrusted sources without the need to parse the values oneself.</p>
</dd></dl>

<dl class="function">
<dt id="ast.get_docstring">
<tt class="descclassname">ast.</tt><tt class="descname">get_docstring</tt><big>(</big><em>node</em>, <em>clean=True</em><big>)</big><a class="headerlink" href="#ast.get_docstring" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the docstring of the given <em>node</em> (which must be a
<tt class="xref py py-class docutils literal"><span class="pre">FunctionDef</span></tt>, <tt class="xref py py-class docutils literal"><span class="pre">ClassDef</span></tt> or <tt class="xref py py-class docutils literal"><span class="pre">Module</span></tt> node), or <tt class="docutils literal"><span class="pre">None</span></tt>
if it has no docstring.  If <em>clean</em> is true, clean up the docstring&#8217;s
indentation with <a class="reference internal" href="inspect.html#inspect.cleandoc" title="inspect.cleandoc"><tt class="xref py py-func docutils literal"><span class="pre">inspect.cleandoc()</span></tt></a>.</p>
</dd></dl>

<dl class="function">
<dt id="ast.fix_missing_locations">
<tt class="descclassname">ast.</tt><tt class="descname">fix_missing_locations</tt><big>(</big><em>node</em><big>)</big><a class="headerlink" href="#ast.fix_missing_locations" title="Permalink to this definition">¶</a></dt>
<dd><p>When you compile a node tree with <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>, the compiler expects
<tt class="xref py py-attr docutils literal"><span class="pre">lineno</span></tt> and <tt class="xref py py-attr docutils literal"><span class="pre">col_offset</span></tt> attributes for every node that supports
them.  This is rather tedious to fill in for generated nodes, so this helper
adds these attributes recursively where not already set, by setting them to
the values of the parent node.  It works recursively starting at <em>node</em>.</p>
</dd></dl>

<dl class="function">
<dt id="ast.increment_lineno">
<tt class="descclassname">ast.</tt><tt class="descname">increment_lineno</tt><big>(</big><em>node</em>, <em>n=1</em><big>)</big><a class="headerlink" href="#ast.increment_lineno" title="Permalink to this definition">¶</a></dt>
<dd><p>Increment the line number of each node in the tree starting at <em>node</em> by <em>n</em>.
This is useful to &#8220;move code&#8221; to a different location in a file.</p>
</dd></dl>

<dl class="function">
<dt id="ast.copy_location">
<tt class="descclassname">ast.</tt><tt class="descname">copy_location</tt><big>(</big><em>new_node</em>, <em>old_node</em><big>)</big><a class="headerlink" href="#ast.copy_location" title="Permalink to this definition">¶</a></dt>
<dd><p>Copy source location (<tt class="xref py py-attr docutils literal"><span class="pre">lineno</span></tt> and <tt class="xref py py-attr docutils literal"><span class="pre">col_offset</span></tt>) from <em>old_node</em>
to <em>new_node</em> if possible, and return <em>new_node</em>.</p>
</dd></dl>

<dl class="function">
<dt id="ast.iter_fields">
<tt class="descclassname">ast.</tt><tt class="descname">iter_fields</tt><big>(</big><em>node</em><big>)</big><a class="headerlink" href="#ast.iter_fields" title="Permalink to this definition">¶</a></dt>
<dd><p>Yield a tuple of <tt class="docutils literal"><span class="pre">(fieldname,</span> <span class="pre">value)</span></tt> for each field in <tt class="docutils literal"><span class="pre">node._fields</span></tt>
that is present on <em>node</em>.</p>
</dd></dl>

<dl class="function">
<dt id="ast.iter_child_nodes">
<tt class="descclassname">ast.</tt><tt class="descname">iter_child_nodes</tt><big>(</big><em>node</em><big>)</big><a class="headerlink" href="#ast.iter_child_nodes" title="Permalink to this definition">¶</a></dt>
<dd><p>Yield all direct child nodes of <em>node</em>, that is, all fields that are nodes
and all items of fields that are lists of nodes.</p>
</dd></dl>

<dl class="function">
<dt id="ast.walk">
<tt class="descclassname">ast.</tt><tt class="descname">walk</tt><big>(</big><em>node</em><big>)</big><a class="headerlink" href="#ast.walk" title="Permalink to this definition">¶</a></dt>
<dd><p>Recursively yield all descendant nodes in the tree starting at <em>node</em>
(including <em>node</em> itself), in no specified order.  This is useful if you only
want to modify nodes in place and don&#8217;t care about the context.</p>
</dd></dl>

<dl class="class">
<dt id="ast.NodeVisitor">
<em class="property">class </em><tt class="descclassname">ast.</tt><tt class="descname">NodeVisitor</tt><a class="headerlink" href="#ast.NodeVisitor" title="Permalink to this definition">¶</a></dt>
<dd><p>A node visitor base class that walks the abstract syntax tree and calls a
visitor function for every node found.  This function may return a value
which is forwarded by the <a class="reference internal" href="#ast.NodeVisitor.visit" title="ast.NodeVisitor.visit"><tt class="xref py py-meth docutils literal"><span class="pre">visit()</span></tt></a> method.</p>
<p>This class is meant to be subclassed, with the subclass adding visitor
methods.</p>
<dl class="method">
<dt id="ast.NodeVisitor.visit">
<tt class="descname">visit</tt><big>(</big><em>node</em><big>)</big><a class="headerlink" href="#ast.NodeVisitor.visit" title="Permalink to this definition">¶</a></dt>
<dd><p>Visit a node.  The default implementation calls the method called
<tt class="samp docutils literal"><span class="pre">self.visit_</span><em><span class="pre">classname</span></em></tt> where <em>classname</em> is the name of the node
class, or <a class="reference internal" href="#ast.NodeVisitor.generic_visit" title="ast.NodeVisitor.generic_visit"><tt class="xref py py-meth docutils literal"><span class="pre">generic_visit()</span></tt></a> if that method doesn&#8217;t exist.</p>
</dd></dl>

<dl class="method">
<dt id="ast.NodeVisitor.generic_visit">
<tt class="descname">generic_visit</tt><big>(</big><em>node</em><big>)</big><a class="headerlink" href="#ast.NodeVisitor.generic_visit" title="Permalink to this definition">¶</a></dt>
<dd><p>This visitor calls <a class="reference internal" href="#ast.NodeVisitor.visit" title="ast.NodeVisitor.visit"><tt class="xref py py-meth docutils literal"><span class="pre">visit()</span></tt></a> on all children of the node.</p>
<p>Note that child nodes of nodes that have a custom visitor method won&#8217;t be
visited unless the visitor calls <a class="reference internal" href="#ast.NodeVisitor.generic_visit" title="ast.NodeVisitor.generic_visit"><tt class="xref py py-meth docutils literal"><span class="pre">generic_visit()</span></tt></a> or visits them
itself.</p>
</dd></dl>

<p>Don&#8217;t use the <a class="reference internal" href="#ast.NodeVisitor" title="ast.NodeVisitor"><tt class="xref py py-class docutils literal"><span class="pre">NodeVisitor</span></tt></a> if you want to apply changes to nodes
during traversal.  For this a special visitor exists
(<a class="reference internal" href="#ast.NodeTransformer" title="ast.NodeTransformer"><tt class="xref py py-class docutils literal"><span class="pre">NodeTransformer</span></tt></a>) that allows modifications.</p>
</dd></dl>

<dl class="class">
<dt id="ast.NodeTransformer">
<em class="property">class </em><tt class="descclassname">ast.</tt><tt class="descname">NodeTransformer</tt><a class="headerlink" href="#ast.NodeTransformer" title="Permalink to this definition">¶</a></dt>
<dd><p>A <a class="reference internal" href="#ast.NodeVisitor" title="ast.NodeVisitor"><tt class="xref py py-class docutils literal"><span class="pre">NodeVisitor</span></tt></a> subclass that walks the abstract syntax tree and
allows modification of nodes.</p>
<p>The <a class="reference internal" href="#ast.NodeTransformer" title="ast.NodeTransformer"><tt class="xref py py-class docutils literal"><span class="pre">NodeTransformer</span></tt></a> will walk the AST and use the return value of
the visitor methods to replace or remove the old node.  If the return value
of the visitor method is <tt class="docutils literal"><span class="pre">None</span></tt>, the node will be removed from its
location, otherwise it is replaced with the return value.  The return value
may be the original node in which case no replacement takes place.</p>
<p>Here is an example transformer that rewrites all occurrences of name lookups
(<tt class="docutils literal"><span class="pre">foo</span></tt>) to <tt class="docutils literal"><span class="pre">data['foo']</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">RewriteName</span><span class="p">(</span><span class="n">NodeTransformer</span><span class="p">):</span>

    <span class="k">def</span> <span class="nf">visit_Name</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">node</span><span class="p">):</span>
        <span class="k">return</span> <span class="n">copy_location</span><span class="p">(</span><span class="n">Subscript</span><span class="p">(</span>
            <span class="n">value</span><span class="o">=</span><span class="n">Name</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="s">&#39;data&#39;</span><span class="p">,</span> <span class="n">ctx</span><span class="o">=</span><span class="n">Load</span><span class="p">()),</span>
            <span class="nb">slice</span><span class="o">=</span><span class="n">Index</span><span class="p">(</span><span class="n">value</span><span class="o">=</span><span class="n">Str</span><span class="p">(</span><span class="n">s</span><span class="o">=</span><span class="n">node</span><span class="o">.</span><span class="n">id</span><span class="p">)),</span>
            <span class="n">ctx</span><span class="o">=</span><span class="n">node</span><span class="o">.</span><span class="n">ctx</span>
        <span class="p">),</span> <span class="n">node</span><span class="p">)</span>
</pre></div>
</div>
<p>Keep in mind that if the node you&#8217;re operating on has child nodes you must
either transform the child nodes yourself or call the <tt class="xref py py-meth docutils literal"><span class="pre">generic_visit()</span></tt>
method for the node first.</p>
<p>For nodes that were part of a collection of statements (that applies to all
statement nodes), the visitor may also return a list of nodes rather than
just a single node.</p>
<p>Usually you use the transformer like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">node</span> <span class="o">=</span> <span class="n">YourTransformer</span><span class="p">()</span><span class="o">.</span><span class="n">visit</span><span class="p">(</span><span class="n">node</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>

<dl class="function">
<dt id="ast.dump">
<tt class="descclassname">ast.</tt><tt class="descname">dump</tt><big>(</big><em>node</em>, <em>annotate_fields=True</em>, <em>include_attributes=False</em><big>)</big><a class="headerlink" href="#ast.dump" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a formatted dump of the tree in <em>node</em>.  This is mainly useful for
debugging purposes.  The returned string will show the names and the values
for fields.  This makes the code impossible to evaluate, so if evaluation is
wanted <em>annotate_fields</em> must be set to False.  Attributes such as line
numbers and column offsets are not dumped by default.  If this is wanted,
<em>include_attributes</em> can be set to <tt class="docutils literal"><span class="pre">True</span></tt>.</p>
</dd></dl>

</div>
</div>


          </div>
        </div>
      </div>
      <div class="sphinxsidebar">
        <div class="sphinxsidebarwrapper">
  <h3><a href="../contents.html">Table Of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#">31.2. <tt class="docutils literal"><span class="pre">ast</span></tt> &#8212; Abstract Syntax Trees</a><ul>
<li><a class="reference internal" href="#node-classes">31.2.1. Node classes</a></li>
<li><a class="reference internal" href="#abstract-grammar">31.2.2. Abstract Grammar</a></li>
<li><a class="reference internal" href="#ast-helpers">31.2.3. <tt class="docutils literal"><span class="pre">ast</span></tt> Helpers</a></li>
</ul>
</li>
</ul>

  <h4>Previous topic</h4>
  <p class="topless"><a href="parser.html"
                        title="previous chapter">31.1. <tt class="docutils literal"><span class="pre">parser</span></tt> &#8212; Access Python parse trees</a></p>
  <h4>Next topic</h4>
  <p class="topless"><a href="symtable.html"
                        title="next chapter">31.3. <tt class="docutils literal"><span class="pre">symtable</span></tt> &#8212; Access to the compiler&#8217;s symbol tables</a></p>
<h3>This Page</h3>
<ul class="this-page-menu">
  <li><a href="../bugs.html">Report a Bug</a></li>
  <li><a href="../_sources/library/ast.txt"
         rel="nofollow">Show Source</a></li>
</ul>

<div id="searchbox" style="display: none">
  <h3>Quick search</h3>
    <form class="search" action="../search.html" method="get">
      <input type="text" name="q" />
      <input type="submit" value="Go" />
      <input type="hidden" name="check_keywords" value="yes" />
      <input type="hidden" name="area" value="default" />
    </form>
    <p class="searchtip" style="font-size: 90%">
    Enter search terms or a module, class or function name.
    </p>
</div>
<script type="text/javascript">$('#searchbox').show(0);</script>
        </div>
      </div>
      <div class="clearer"></div>
    </div>
    <div class="related">
      <h3>Navigation</h3>
      <ul>
        <li class="right" style="margin-right: 10px">
          <a href="../genindex.html" title="General Index"
             >index</a></li>
        <li class="right" >
          <a href="../py-modindex.html" title="Python Module Index"
             >modules</a> |</li>
        <li class="right" >
          <a href="symtable.html" title="31.3. symtable — Access to the compiler’s symbol tables"
             >next</a> |</li>
        <li class="right" >
          <a href="parser.html" title="31.1. parser — Access Python parse trees"
             >previous</a> |</li>
        <li><img src="../_static/py.png" alt=""
                 style="vertical-align: middle; margin-top: -1px"/></li>
        <li><a href="http://www.python.org/">Python</a> &raquo;</li>
        <li>
          <a href="../index.html">Python 2.7.5 documentation</a> &raquo;
        </li>

          <li><a href="index.html" >The Python Standard Library</a> &raquo;</li>
          <li><a href="language.html" >31. Python Language Services</a> &raquo;</li> 
      </ul>
    </div>
    <div class="footer">
    &copy; <a href="../copyright.html">Copyright</a> 1990-2019, Python Software Foundation.
    <br />
    The Python Software Foundation is a non-profit corporation.
    <a href="http://www.python.org/psf/donations/">Please donate.</a>
    <br />
    Last updated on Jul 03, 2019.
    <a href="../bugs.html">Found a bug</a>?
    <br />
    Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.1.3.
    </div>

  </body>
</html>

Filemanager

DIR : / usr/ share/ doc/ python-docs-2.7.5/ html/ library/
Name Type Size Permission Actions
2to3.html File 49.27 KB 0644
__builtin__.html File 10.26 KB 0644
__future__.html File 13.79 KB 0644
__main__.html File 7.05 KB 0644
_winreg.html File 59.21 KB 0644
abc.html File 23.9 KB 0644
aepack.html File 13.16 KB 0644
aetools.html File 14.91 KB 0644
aetypes.html File 18.88 KB 0644
aifc.html File 22.4 KB 0644
al.html File 17.34 KB 0644
allos.html File 33.72 KB 0644
anydbm.html File 16.33 KB 0644
archiving.html File 9.26 KB 0644
argparse.html File 237.62 KB 0644
array.html File 29.29 KB 0644
ast.html File 34.98 KB 0644
asynchat.html File 31.43 KB 0644
asyncore.html File 36.51 KB 0644
atexit.html File 16.8 KB 0644
audioop.html File 31.36 KB 0644
autogil.html File 8.19 KB 0644
base64.html File 19.67 KB 0644
basehttpserver.html File 34.04 KB 0644
bastion.html File 11.04 KB 0644
bdb.html File 36.68 KB 0644
binascii.html File 20.67 KB 0644
binhex.html File 10.58 KB 0644
bisect.html File 23.24 KB 0644
bsddb.html File 26.43 KB 0644
bz2.html File 26.08 KB 0644
calendar.html File 37.79 KB 0644
carbon.html File 48.94 KB 0644
cd.html File 27.96 KB 0644
cgi.html File 49.92 KB 0644
cgihttpserver.html File 13.1 KB 0644
cgitb.html File 11.41 KB 0644
chunk.html File 14.66 KB 0644
cmath.html File 25.63 KB 0644
cmd.html File 26.09 KB 0644
code.html File 24.58 KB 0644
codecs.html File 100.64 KB 0644
codeop.html File 14.84 KB 0644
collections.html File 133.96 KB 0644
colorpicker.html File 7.52 KB 0644
colorsys.html File 11.04 KB 0644
commands.html File 14.36 KB 0644
compileall.html File 16.83 KB 0644
compiler.html File 67.75 KB 0644
configparser.html File 62.13 KB 0644
constants.html File 12.83 KB 0644
contextlib.html File 19.39 KB 0644
cookie.html File 39.07 KB 0644
cookielib.html File 83.82 KB 0644
copy.html File 12.19 KB 0644
copy_reg.html File 13.76 KB 0644
crypt.html File 10.04 KB 0644
crypto.html File 7.59 KB 0644
csv.html File 67.37 KB 0644
ctypes.html File 238.78 KB 0644
curses.ascii.html File 22.29 KB 0644
curses.html File 146.63 KB 0644
curses.panel.html File 14.39 KB 0644
custominterp.html File 7.62 KB 0644
datatypes.html File 16.84 KB 0644
datetime.html File 226.59 KB 0644
dbhash.html File 15.48 KB 0644
dbm.html File 12.07 KB 0644
debug.html File 10.15 KB 0644
decimal.html File 194.44 KB 0644
development.html File 14.17 KB 0644
difflib.html File 84.83 KB 0644
dircache.html File 11.41 KB 0644
dis.html File 69.95 KB 0644
distutils.html File 8.05 KB 0644
dl.html File 16.33 KB 0644
doctest.html File 165.54 KB 0644
docxmlrpcserver.html File 16.43 KB 0644
dumbdbm.html File 14.02 KB 0644
dummy_thread.html File 9.43 KB 0644
dummy_threading.html File 8.37 KB 0644
easydialogs.html File 30.55 KB 0644
email-examples.html File 45.65 KB 0644
email.charset.html File 26.8 KB 0644
email.encoders.html File 11.86 KB 0644
email.errors.html File 15.77 KB 0644
email.generator.html File 20.77 KB 0644
email.header.html File 26.92 KB 0644
email.html File 44.24 KB 0644
email.iterators.html File 11.52 KB 0644
email.message.html File 63.16 KB 0644
email.mime.html File 27.93 KB 0644
email.parser.html File 30.45 KB 0644
email.util.html File 24.46 KB 0644
errno.html File 37.99 KB 0644
exceptions.html File 56.13 KB 0644
fcntl.html File 22.67 KB 0644
filecmp.html File 22.3 KB 0644
fileformats.html File 9.14 KB 0644
fileinput.html File 24.28 KB 0644
filesys.html File 10.2 KB 0644
fl.html File 49.92 KB 0644
fm.html File 11.91 KB 0644
fnmatch.html File 14.58 KB 0644
formatter.html File 34.06 KB 0644
fpectl.html File 16.01 KB 0644
fpformat.html File 10.59 KB 0644
fractions.html File 22.61 KB 0644
framework.html File 33.34 KB 0644
frameworks.html File 7.14 KB 0644
ftplib.html File 43.99 KB 0644
functions.html File 183.14 KB 0644
functools.html File 27.17 KB 0644
future_builtins.html File 13.04 KB 0644
gc.html File 25.75 KB 0644
gdbm.html File 15.96 KB 0644
gensuitemodule.html File 11.51 KB 0644
getopt.html File 23.66 KB 0644
getpass.html File 10.65 KB 0644
gettext.html File 78.76 KB 0644
gl.html File 22.09 KB 0644
glob.html File 13.26 KB 0644
grp.html File 10.49 KB 0644
gzip.html File 18.99 KB 0644
hashlib.html File 18.2 KB 0644
heapq.html File 31.61 KB 0644
hmac.html File 10.46 KB 0644
hotshot.html File 18.65 KB 0644
htmllib.html File 25.32 KB 0644
htmlparser.html File 39.11 KB 0644
httplib.html File 62.95 KB 0644
i18n.html File 9.52 KB 0644
ic.html File 17.17 KB 0644
idle.html File 20.9 KB 0644
imageop.html File 14.76 KB 0644
imaplib.html File 51.99 KB 0644
imgfile.html File 11.71 KB 0644
imghdr.html File 11.3 KB 0644
imp.html File 34.34 KB 0644
importlib.html File 8.26 KB 0644
imputil.html File 31.81 KB 0644
index.html File 72.78 KB 0644
inspect.html File 50.71 KB 0644
internet.html File 24.87 KB 0644
intro.html File 8.93 KB 0644
io.html File 98.13 KB 0644
ipc.html File 13.41 KB 0644
itertools.html File 115.91 KB 0644
jpeg.html File 12.74 KB 0644
json.html File 67.04 KB 0644
keyword.html File 7.68 KB 0644
language.html File 11.03 KB 0644
linecache.html File 10.59 KB 0644
locale.html File 55.14 KB 0644
logging.config.html File 63.36 KB 0644
logging.handlers.html File 69.64 KB 0644
logging.html File 95.64 KB 0644
mac.html File 21.79 KB 0644
macos.html File 14.76 KB 0644
macosa.html File 12.96 KB 0644
macostools.html File 15.52 KB 0644
macpath.html File 7.76 KB 0644
mailbox.html File 156.75 KB 0644
mailcap.html File 13.21 KB 0644
markup.html File 18.77 KB 0644
marshal.html File 17.98 KB 0644
math.html File 39.24 KB 0644
md5.html File 13.97 KB 0644
mhlib.html File 21.54 KB 0644
mimetools.html File 19.25 KB 0644
mimetypes.html File 28.39 KB 0644
mimewriter.html File 15.02 KB 0644
mimify.html File 13.36 KB 0644
miniaeframe.html File 12.2 KB 0644
misc.html File 6.87 KB 0644
mm.html File 9.03 KB 0644
mmap.html File 28.36 KB 0644
modulefinder.html File 15.31 KB 0644
modules.html File 8.46 KB 0644
msilib.html File 52.43 KB 0644
msvcrt.html File 19.37 KB 0644
multifile.html File 24.3 KB 0644
multiprocessing.html File 365.71 KB 0644
mutex.html File 11.23 KB 0644
netdata.html File 16.98 KB 0644
netrc.html File 12.3 KB 0644
new.html File 12.12 KB 0644
nis.html File 10.64 KB 0644
nntplib.html File 41.92 KB 0644
numbers.html File 37.75 KB 0644
numeric.html File 13.55 KB 0644
operator.html File 82 KB 0644
optparse.html File 222.56 KB 0644
os.html File 214.25 KB 0644
os.path.html File 38.34 KB 0644
ossaudiodev.html File 41.5 KB 0644
othergui.html File 9.08 KB 0644
parser.html File 39.36 KB 0644
pdb.html File 33.96 KB 0644
persistence.html File 14.87 KB 0644
pickle.html File 102.27 KB 0644
pickletools.html File 10.63 KB 0644
pipes.html File 18.01 KB 0644
pkgutil.html File 25.11 KB 0644
platform.html File 28.37 KB 0644
plistlib.html File 17.03 KB 0644
popen2.html File 25.43 KB 0644
poplib.html File 22.32 KB 0644
posix.html File 14.41 KB 0644
posixfile.html File 19.76 KB 0644
pprint.html File 29.92 KB 0644
profile.html File 63.56 KB 0644
pty.html File 9.48 KB 0644
pwd.html File 11.43 KB 0644
py_compile.html File 11.12 KB 0644
pyclbr.html File 14.71 KB 0644
pydoc.html File 11.48 KB 0644
pyexpat.html File 71.53 KB 0644
python.html File 12.27 KB 0644
queue.html File 24.22 KB 0644
quopri.html File 11.9 KB 0644
random.html File 37.83 KB 0644
re.html File 134.74 KB 0644
readline.html File 28.24 KB 0644
repr.html File 20.43 KB 0644
resource.html File 26.48 KB 0644
restricted.html File 11.65 KB 0644
rexec.html File 37.41 KB 0644
rfc822.html File 42.22 KB 0644
rlcompleter.html File 13.51 KB 0644
robotparser.html File 12.27 KB 0644
runpy.html File 19.34 KB 0644
sched.html File 18.54 KB 0644
scrolledtext.html File 9.32 KB 0644
select.html File 39.67 KB 0644
sets.html File 36.92 KB 0644
sgi.html File 9.71 KB 0644
sgmllib.html File 30.77 KB 0644
sha.html File 12.09 KB 0644
shelve.html File 27.02 KB 0644
shlex.html File 32.1 KB 0644
shutil.html File 40.22 KB 0644
signal.html File 31.14 KB 0644
simplehttpserver.html File 18.41 KB 0644
simplexmlrpcserver.html File 31.39 KB 0644
site.html File 23.64 KB 0644
smtpd.html File 12.46 KB 0644
smtplib.html File 42.13 KB 0644
sndhdr.html File 10.02 KB 0644
socket.html File 106.34 KB 0644
socketserver.html File 59.83 KB 0644
someos.html File 15.11 KB 0644
spwd.html File 10.33 KB 0644
sqlite3.html File 139.5 KB 0644
ssl.html File 65.62 KB 0644
stat.html File 32.31 KB 0644
statvfs.html File 10.6 KB 0644
stdtypes.html File 260.4 KB 0644
string.html File 106.65 KB 0644
stringio.html File 18.81 KB 0644
stringprep.html File 16.13 KB 0644
strings.html File 14.93 KB 0644
struct.html File 40.88 KB 0644
subprocess.html File 84.91 KB 0644
sun.html File 6.84 KB 0644
sunau.html File 27.1 KB 0644
sunaudio.html File 17.79 KB 0644
symbol.html File 7.66 KB 0644
symtable.html File 22.94 KB 0644
sys.html File 98.7 KB 0644
sysconfig.html File 23.84 KB 0644
syslog.html File 17.92 KB 0644
tabnanny.html File 10.63 KB 0644
tarfile.html File 78.68 KB 0644
telnetlib.html File 25.48 KB 0644
tempfile.html File 29.42 KB 0644
termios.html File 16.01 KB 0644
test.html File 52.62 KB 0644
textwrap.html File 27.25 KB 0644
thread.html File 20.47 KB 0644
threading.html File 76.69 KB 0644
time.html File 56.93 KB 0644
timeit.html File 36.27 KB 0644
tix.html File 46.96 KB 0644
tk.html File 23.64 KB 0644
tkinter.html File 67.67 KB 0644
token.html File 19.62 KB 0644
tokenize.html File 18.45 KB 0644
trace.html File 25.54 KB 0644
traceback.html File 33.44 KB 0644
ttk.html File 101.75 KB 0644
tty.html File 9.06 KB 0644
turtle.html File 211.74 KB 0644
types.html File 27.59 KB 0644
undoc.html File 23.16 KB 0644
unicodedata.html File 18.55 KB 0644
unittest.html File 202.85 KB 0644
unix.html File 10.55 KB 0644
urllib.html File 58.68 KB 0644
urllib2.html File 100.58 KB 0644
urlparse.html File 40.41 KB 0644
user.html File 11.83 KB 0644
userdict.html File 29.73 KB 0644
uu.html File 11.03 KB 0644
uuid.html File 28.19 KB 0644
warnings.html File 46.6 KB 0644
wave.html File 22.22 KB 0644
weakref.html File 36.52 KB 0644
webbrowser.html File 23.07 KB 0644
whichdb.html File 8.85 KB 0644
windows.html File 9.33 KB 0644
winsound.html File 18.75 KB 0644
wsgiref.html File 81.04 KB 0644
xdrlib.html File 29.94 KB 0644
xml.dom.html File 89.04 KB 0644
xml.dom.minidom.html File 40.42 KB 0644
xml.dom.pulldom.html File 12.71 KB 0644
xml.etree.elementtree.html File 93.22 KB 0644
xml.html File 16.49 KB 0644
xml.sax.handler.html File 38.63 KB 0644
xml.sax.html File 20.22 KB 0644
xml.sax.reader.html File 39.09 KB 0644
xml.sax.utils.html File 14.26 KB 0644
xmlrpclib.html File 60.79 KB 0644
zipfile.html File 53.14 KB 0644
zipimport.html File 20.42 KB 0644
zlib.html File 25.46 KB 0644