32.2. "ast" — Abstract Syntax Trees
***********************************

New in version 2.5: The low-level "_ast" module containing only the
node classes.

New in version 2.6: The high-level "ast" module containing all
helpers.

**Source code:** Lib/ast.py

======================================================================

The "ast" 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
"ast.PyCF_ONLY_AST" as a flag to the "compile()" built-in function, or
using the "parse()" helper provided in this module.  The result will
be a tree of objects whose classes all inherit from "ast.AST".  An
abstract syntax tree can be compiled into a Python code object using
the built-in "compile()" function.


32.2.1. Node classes
====================

class ast.AST

   This is the base of all AST node classes.  The actual node classes
   are derived from the "Parser/Python.asdl" file, which is reproduced
   below.  They are defined in the "_ast" C module and re-exported in
   "ast".

   There is one class defined for each left-hand side symbol in the
   abstract grammar (for example, "ast.stmt" or "ast.expr").  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, "ast.BinOp" inherits from
   "ast.expr".  For production rules with alternatives (aka “sums”),
   the left-hand side class is abstract: only instances of specific
   constructor nodes are ever created.

   _fields

      Each concrete class has an attribute "_fields" 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,
      "ast.BinOp" instances have an attribute "left" of type
      "ast.expr".

      If these attributes are marked as optional in the grammar (using
      a question mark), the value might be "None".  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
      "compile()".

   lineno
   col_offset

      Instances of "ast.expr" and "ast.stmt" subclasses have "lineno"
      and "col_offset" attributes.  The "lineno" is the line number of
      source text (1-indexed so the first line is line 1) and the
      "col_offset" 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.

   The constructor of a class "ast.T" parses its arguments as follows:

   * If there are positional arguments, there must be as many as
     there are items in "T._fields"; they will be assigned as
     attributes of these names.

   * If there are keyword arguments, they will set the attributes of
     the same names to the given values.

   For example, to create and populate an "ast.UnaryOp" node, you
   could use

      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

   or the more compact

      node = ast.UnaryOp(ast.USub(), ast.Num(5, lineno=0, col_offset=0),
                         lineno=0, col_offset=0)

   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.


32.2.2. Abstract Grammar
========================

The module defines a string constant "__version__" which is the
decimal Subversion revision number of the file shown below.

The abstract grammar is currently defined as follows:

   -- 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 & 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 < 4 < 3 and (x < 4) < 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)
   }


32.2.3. "ast" Helpers
=====================

New in version 2.6.

Apart from the node classes, "ast" module defines these utility
functions and classes for traversing abstract syntax trees:

ast.parse(source, filename='<unknown>', mode='exec')

   Parse the source into an AST node.  Equivalent to "compile(source,
   filename, mode, ast.PyCF_ONLY_AST)".

ast.literal_eval(node_or_string)

   Safely evaluate an expression node or a Unicode or *Latin-1*
   encoded string containing a Python literal or container display.
   The string or node provided may only consist of the following
   Python literal structures: strings, numbers, tuples, lists, dicts,
   booleans, and "None".

   This can be used for safely evaluating strings containing Python
   values from untrusted sources without the need to parse the values
   oneself.  It is not capable of evaluating arbitrarily complex
   expressions, for example involving operators or indexing.

ast.get_docstring(node, clean=True)

   Return the docstring of the given *node* (which must be a
   "FunctionDef", "ClassDef" or "Module" node), or "None" if it has no
   docstring.  If *clean* is true, clean up the docstring’s
   indentation with "inspect.cleandoc()".

ast.fix_missing_locations(node)

   When you compile a node tree with "compile()", the compiler expects
   "lineno" and "col_offset" 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*.

ast.increment_lineno(node, n=1)

   Increment the line number of each node in the tree starting at
   *node* by *n*. This is useful to “move code” to a different
   location in a file.

ast.copy_location(new_node, old_node)

   Copy source location ("lineno" and "col_offset") from *old_node* to
   *new_node* if possible, and return *new_node*.

ast.iter_fields(node)

   Yield a tuple of "(fieldname, value)" for each field in
   "node._fields" that is present on *node*.

ast.iter_child_nodes(node)

   Yield all direct child nodes of *node*, that is, all fields that
   are nodes and all items of fields that are lists of nodes.

ast.walk(node)

   Recursively yield all descendant nodes in the tree starting at
   *node* (including *node* itself), in no specified order.  This is
   useful if you only want to modify nodes in place and don’t care
   about the context.

class ast.NodeVisitor

   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 "visit()" method.

   This class is meant to be subclassed, with the subclass adding
   visitor methods.

   visit(node)

      Visit a node.  The default implementation calls the method
      called "self.visit_*classname*" where *classname* is the name of
      the node class, or "generic_visit()" if that method doesn’t
      exist.

   generic_visit(node)

      This visitor calls "visit()" on all children of the node.

      Note that child nodes of nodes that have a custom visitor method
      won’t be visited unless the visitor calls "generic_visit()" or
      visits them itself.

   Don’t use the "NodeVisitor" if you want to apply changes to nodes
   during traversal.  For this a special visitor exists
   ("NodeTransformer") that allows modifications.

class ast.NodeTransformer

   A "NodeVisitor" subclass that walks the abstract syntax tree and
   allows modification of nodes.

   The "NodeTransformer" 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 "None", 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 ("foo") to "data['foo']":

      class RewriteName(NodeTransformer):

          def visit_Name(self, node):
              return copy_location(Subscript(
                  value=Name(id='data', ctx=Load()),
                  slice=Index(value=Str(s=node.id)),
                  ctx=node.ctx
              ), node)

   Keep in mind that if the node you’re operating on has child nodes
   you must either transform the child nodes yourself or call the
   "generic_visit()" 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:

      node = YourTransformer().visit(node)

ast.dump(node, annotate_fields=True, include_attributes=False)

   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 "True".
