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 <div class="section" id="module-ast">
<h1>31.2. Abstract Syntax Trees<a class="headerlink" href="#module-ast" title="Permalink to this headline">¶</a></h1>

New in version 2.5: The low-level <tt class="docutils literal">_ast</tt> module containing only the node classes.

New in version 2.6: The high-level <tt class="docutils literal">ast</tt> module containing all helpers.
The <tt class="xref docutils literal">ast</tt> 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.
An abstract syntax tree can be generated by passing <tt class="xref docutils literal">ast.PyCF_ONLY_AST</tt> as
a flag to the <a title="compile" class="reference external" href="functions.html#compile"><tt class="xref docutils literal">compile()</tt></a> built-in function, or using the <a title="ast.parse" class="reference internal" href="#ast.parse"><tt class="xref docutils literal">parse()</tt></a>
helper provided in this module. The result will be a tree of objects whose
classes all inherit from <a title="ast.AST" class="reference internal" href="#ast.AST"><tt class="xref docutils literal">ast.AST</tt></a>. An abstract syntax tree can be
compiled into a Python code object using the built-in <a title="compile" class="reference external" href="functions.html#compile"><tt class="xref docutils literal">compile()</tt></a> function.
<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">
class <tt class="descclassname">ast.</tt><tt class="descname">AST</tt><a class="headerlink" href="#ast.AST" title="Permalink to this definition">¶</a></dt>
<dd>This is the base of all AST node classes. The actual node classes are
derived from the <tt class="docutils literal">Parser/Python.asdl</tt> file, which is reproduced
<a class="reference internal" href="#abstract-grammar">below</a>. They are defined in the <tt class="xref docutils literal">_ast</tt> C
module and re-exported in <tt class="xref docutils literal">ast</tt>.
There is one class defined for each left-hand side symbol in the abstract
grammar (for example, <tt class="xref docutils literal">ast.stmt</tt> or <tt class="xref docutils literal">ast.expr</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 docutils literal">ast.BinOp</tt> inherits from <tt class="xref docutils literal">ast.expr</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.
<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>Each concrete class has an attribute <a title="ast.AST._fields" class="reference internal" href="#ast.AST._fields"><tt class="xref docutils literal">_fields</tt></a> which gives the names
of all child nodes.
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 docutils literal">ast.BinOp</tt>
instances have an attribute <tt class="xref docutils literal">left</tt> of type <tt class="xref docutils literal">ast.expr</tt>.
If these attributes are marked as optional in the grammar (using a
question mark), the value might be <tt class="xref docutils literal">None</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 title="compile" class="reference external" href="functions.html#compile"><tt class="xref docutils literal">compile()</tt></a>.
</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>Instances of <tt class="xref docutils literal">ast.expr</tt> and <tt class="xref docutils literal">ast.stmt</tt> subclasses have
<a title="ast.AST.lineno" class="reference internal" href="#ast.AST.lineno"><tt class="xref docutils literal">lineno</tt></a> and <a title="ast.AST.col_offset" class="reference internal" href="#ast.AST.col_offset"><tt class="xref docutils literal">col_offset</tt></a> attributes. The <a title="ast.AST.lineno" class="reference internal" href="#ast.AST.lineno"><tt class="xref docutils literal">lineno</tt></a> is
the line number of source text (1-indexed so the first line is line 1) and
the <a title="ast.AST.col_offset" class="reference internal" href="#ast.AST.col_offset"><tt class="xref docutils literal">col_offset</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.</dd></dl>

The constructor of a class <tt class="xref docutils literal">ast.T</tt> parses its arguments as follows:
<ul class="simple">
<li>If there are positional arguments, there must be as many as there are items
in <tt class="xref docutils literal">T._fields</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>
For example, to create and populate an <tt class="xref docutils literal">ast.UnaryOp</tt> node, you could
use
<div class="highlight-python"><div class="highlight"><pre>node = ast.UnaryOp()
node.op = ast.USub()
node.operand = ast.Num()
node.operand.n = 5
node.operand.lineno = 0
node.operand.col_offset = 0
node.lineno = 0
node.col_offset = 0
</pre></div>
</div>
or the more compact
<div class="highlight-python"><div class="highlight"><pre>node = ast.UnaryOp(ast.USub(), ast.Num(5, lineno=0, col_offset=0),
 lineno=0, col_offset=0)
</pre></div>
</div>

New in version 2.6: 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.
</dd></dl>

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

module Python version "$Revision: 62047 $"
{
	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)

-- 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)
	     | ListComp(expr elt, 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)

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

boolop = And | Or

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. <tt class="xref docutils literal">ast</tt> Helpers<a class="headerlink" href="#ast-helpers" title="Permalink to this headline">¶</a></h2>

New in version 2.6.
Apart from the node classes, <tt class="xref docutils literal">ast</tt> module defines these utility functions
and classes for traversing abstract syntax trees:
<dl class="function">
<dt id="ast.parse">
<tt class="descclassname">ast.</tt><tt class="descname">parse</tt><big>(</big>expr, filename='&lt;unknown&gt;', mode='exec'<big>)</big><a class="headerlink" href="#ast.parse" title="Permalink to this definition">¶</a></dt>
<dd>Parse an expression into an AST node. Equivalent to <tt class="docutils literal">compile(expr,
filename, mode, ast.PyCF_ONLY_AST)</tt>.</dd></dl>

<dl class="function">
<dt id="ast.literal_eval">
<tt class="descclassname">ast.</tt><tt class="descname">literal_eval</tt><big>(</big>node_or_string<big>)</big><a class="headerlink" href="#ast.literal_eval" title="Permalink to this definition">¶</a></dt>
<dd>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="xref docutils literal">None</tt>.
This can be used for safely evaluating strings containing Python expressions
from untrusted sources without the need to parse the values oneself.
</dd></dl>

<dl class="function">
<dt id="ast.get_docstring">
<tt class="descclassname">ast.</tt><tt class="descname">get_docstring</tt><big>(</big>node, clean=True<big>)</big><a class="headerlink" href="#ast.get_docstring" title="Permalink to this definition">¶</a></dt>
<dd>Return the docstring of the given node (which must be a
<tt class="xref docutils literal">FunctionDef</tt>, <tt class="xref docutils literal">ClassDef</tt> or <tt class="xref docutils literal">Module</tt> node), or <tt class="xref docutils literal">None</tt>
if it has no docstring. If clean is true, clean up the docstring&#8217;s
indentation with <a title="inspect.cleandoc" class="reference external" href="inspect.html#inspect.cleandoc"><tt class="xref docutils literal">inspect.cleandoc()</tt></a>.</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>node<big>)</big><a class="headerlink" href="#ast.fix_missing_locations" title="Permalink to this definition">¶</a></dt>
<dd>When you compile a node tree with <a title="compile" class="reference external" href="functions.html#compile"><tt class="xref docutils literal">compile()</tt></a>, the compiler expects
<tt class="xref docutils literal">lineno</tt> and <tt class="xref docutils literal">col_offset</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 node.</dd></dl>

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

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

<dl class="function">
<dt id="ast.iter_fields">
<tt class="descclassname">ast.</tt><tt class="descname">iter_fields</tt><big>(</big>node<big>)</big><a class="headerlink" href="#ast.iter_fields" title="Permalink to this definition">¶</a></dt>
<dd>Yield a tuple of <tt class="docutils literal">(fieldname, value)</tt> for each field in <tt class="docutils literal">node._fields</tt>
that is present on node.</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>node<big>)</big><a class="headerlink" href="#ast.iter_child_nodes" title="Permalink to this definition">¶</a></dt>
<dd>Yield all direct child nodes of node, that is, all fields that are nodes
and all items of fields that are lists of nodes.</dd></dl>

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

<dl class="class">
<dt id="ast.NodeVisitor">
class <tt class="descclassname">ast.</tt><tt class="descname">NodeVisitor</tt><a class="headerlink" href="#ast.NodeVisitor" title="Permalink to this definition">¶</a></dt>
<dd>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 title="ast.NodeVisitor.visit" class="reference internal" href="#ast.NodeVisitor.visit"><tt class="xref docutils literal">visit()</tt></a> method.
This class is meant to be subclassed, with the subclass adding visitor
methods.
<dl class="method">
<dt id="ast.NodeVisitor.visit">
<tt class="descname">visit</tt><big>(</big>node<big>)</big><a class="headerlink" href="#ast.NodeVisitor.visit" title="Permalink to this definition">¶</a></dt>
<dd>Visit a node. The default implementation calls the method called
<tt class="docutils literal">self.visit_classname</tt> where classname is the name of the node
class, or <a title="ast.NodeVisitor.generic_visit" class="reference internal" href="#ast.NodeVisitor.generic_visit"><tt class="xref docutils literal">generic_visit()</tt></a> if that method doesn&#8217;t exist.</dd></dl>

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

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

<dl class="class">
<dt id="ast.NodeTransformer">
class <tt class="descclassname">ast.</tt><tt class="descname">NodeTransformer</tt><a class="headerlink" href="#ast.NodeTransformer" title="Permalink to this definition">¶</a></dt>
<dd>A <a title="ast.NodeVisitor" class="reference internal" href="#ast.NodeVisitor"><tt class="xref docutils literal">NodeVisitor</tt></a> subclass that walks the abstract syntax tree and
allows modification of nodes.
The <a title="ast.NodeTransformer" class="reference internal" href="#ast.NodeTransformer"><tt class="xref docutils literal">NodeTransformer</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="xref docutils literal">None</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.
Here is an example transformer that rewrites all occurrences of name lookups
(<tt class="docutils literal">foo</tt>) to <tt class="docutils literal">data['foo']</tt>:
<div class="highlight-python"><div class="highlight"><pre>class RewriteName(NodeTransformer):

def visit_Name(self, node):
 return copy_location(Subscript(
 value=Name(id=&#39;data&#39;, ctx=Load()),
 slice=Index(value=Str(s=node.id)),
 ctx=node.ctx
 ), node)
</pre></div>
</div>
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 docutils literal">generic_visit()</tt>
method for the node first.
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.
Usually you use the transformer like this:
<div class="highlight-python"><div class="highlight"><pre>node = YourTransformer().visit(node)
</pre></div>
</div>
</dd></dl>

<dl class="function">
<dt id="ast.dump">
<tt class="descclassname">ast.</tt><tt class="descname">dump</tt><big>(</big>node, annotate_fields=True, include_attributes=False<big>)</big><a class="headerlink" href="#ast.dump" title="Permalink to this definition">¶</a></dt>
<dd>Return a formatted dump of the tree in node. 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 annotate_fields must be set to False. Attributes such as line
numbers and column offsets are not dumped by default. If this is wanted,
include_attributes can be set to <tt class="xref docutils literal">True</tt>.</dd></dl>

</div>
</div>

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 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference external" href="#">31.2. Abstract Syntax Trees</a><ul>
<li><a class="reference external" href="#node-classes">31.2.1. Node classes</a></li>
<li><a class="reference external" href="#abstract-grammar">31.2.2. Abstract Grammar</a></li>
<li><a class="reference external" href="#ast-helpers">31.2.3. <tt class="docutils literal">ast</tt> Helpers</a></li>
</ul>
</li>
</ul>

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 title="next chapter">31.3. <tt class="docutils literal">symtable</tt> &#8212; Access to the compiler&#8217;s symbol tables</a>
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