Add the "ast" module, containing helpers to ease use of the "_ast" classes.

Doc/ACKS.txt

@@ -157,6 +157,7 @@ docs@python.org), and we'll be glad to correct the problem.
    * Bernhard Reiter
    * Armin Rigo
    * Wes Rishel
+   * Armin Ronacher
    * Jim Roskind
    * Guido van Rossum
    * Donald Wallace Rouse II

Doc/library/_ast.rst

-.. _ast:
-
-Abstract Syntax Trees
-=====================
-
-.. module:: _ast
-   :synopsis: Abstract Syntax Tree classes.
-
-.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>
-
-
-.. versionadded:: 2.5
-
-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 :data:`_ast.PyCF_ONLY_AST`
-as a flag to the :func:`compile` builtin function. The result will be a tree of
-objects whose classes all inherit from :class:`_ast.AST`.
-
-A modified abstract syntax tree can be compiled into a Python code object using
-the built-in :func:`compile` function.
-
-The actual classes are derived from the ``Parser/Python.asdl`` file, which is
-reproduced below. 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.
-
-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``.   Instances of ``_ast.expr`` and
-``_ast.stmt`` subclasses also 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 utf8 byte offset of the first token that generated the
-node.  The utf8 offset is recorded because the parser uses utf8 internally.
-
-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 :func:`compile`.
-
-The constructor of a class ``_ast.T`` parses their 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 a ``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)
-
-
-
-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:
-
-.. literalinclude:: ../../Parser/Python.asdl

Doc/library/ast.rst

+.. _ast:
+
+Abstract Syntax Trees
+=====================
+
+.. module:: ast
+   :synopsis: Abstract Syntax Tree classes and manipulation.
+
+.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>
+.. sectionauthor:: Georg Brandl <georg@python.org>
+
+.. versionadded:: 2.5
+   The low-level ``_ast`` module containing only the node classes.
+
+.. versionadded:: 2.6
+   The high-level ``ast`` module containing all helpers.
+
+
+The :mod:`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 :data:`_ast.PyCF_ONLY_AST`
+as a flag to the :func:`compile` builtin function, or using the :func:`parse`
+helper provided in this module.  The result will be a tree of objects whose
+classes all inherit from :class:`ast.AST`.
+
+A modified abstract syntax tree can be compiled into a Python code object using
+the built-in :func:`compile` function.
+
+Node classes
+------------
+
+.. class:: AST
+
+   This is the base of all AST node classes.  The actual node classes are
+   derived from the :file:`Parser/Python.asdl` file, which is reproduced
+   :ref:`below <abstract-grammar>`.  They are defined in the :mod:`_ast` C
+   module and re-exported in :mod:`ast`.
+
+   There is one class defined for each left-hand side symbol in the abstract
+   grammar (for example, :class:`ast.stmt` or :class:`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,
+   :class:`ast.BinOp` inherits from :class:`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.
+
+   .. attribute:: _fields
+
+      Each concrete class has an attribute :attr:`_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, :class:`ast.BinOp`
+      instances have an attribute :attr:`left` of type :class:`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 :func:`compile`.
+
+   .. attribute:: lineno
+                  col_offset
+
+      Instances of :class:`ast.expr` and :class:`ast.stmt` subclasses have
+      :attr:`lineno` and :attr:`col_offset` attributes.  The :attr:`lineno` is
+      the line number of source text (1-indexed so the first line is line 1) and
+      the :attr:`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 :class:`ast.T` parses its arguments as follows:
+
+   * If there are positional arguments, there must be as many as there are items
+     in :attr:`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 :class:`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)
+
+
+.. _abstract-grammar:
+
+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:
+
+.. literalinclude:: ../../Parser/Python.asdl
+
+
+:mod:`ast` Helpers
+------------------
+
+.. versionadded:: 2.6
+
+Apart from the node classes, :mod:`ast` module defines these utility functions
+and classes for traversing abstract syntax trees:
+
+.. function:: parse(expr, filename='<unknown>', mode='exec')
+
+   Parse an expression into an AST node.  Equivalent to ``compile(expr,
+   filename, mode, PyCF_ONLY_AST)``.
+
+   
+.. function:: literal_eval(node_or_string)
+
+   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 ``None``.
+
+   This can be used for safely evaluating strings containing Python expressions
+   from untrusted sources without the need to parse the values oneself.
+
+
+.. function:: get_docstring(node, clean=True):
+
+   Return the docstring of the given *node* (which must be a
+   :class:`FunctionDef`, :class:`ClassDef` or :class:`Module` node), or ``None``
+   if it has no docstring.  If *clean* is true, clean up the docstring's
+   indentation with :func:`inspect.cleandoc`.
+
+
+.. function:: fix_missing_locations(node)
+
+   When you compile a node tree with :func:`compile`, the compiler expects
+   :attr:`lineno` and :attr:`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*.
+
+
+.. function:: 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.
+
+
+.. function:: copy_location(new_node, old_node)
+
+   Copy source location (:attr:`lineno` and :attr:`col_offset`) from *old_node*
+   to *new_node* if possible, and return *new_node*.
+
+
+.. function:: iter_fields(node)
+
+   Yield a tuple of ``(fieldname, value)`` for each field in ``node._fields``
+   that is present on *node*.
+
+
+.. function:: 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.
+
+
+.. function:: walk(node)
+
+   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't care about the
+   context.
+
+
+.. class:: 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.
+
+   .. method:: visit(node)
+
+      Visit a node.  The default implementation calls the method called
+      :samp:`self.visit_{classname}` where *classname* is the name of the node
+      class, or :meth:`generic_visit` if that method doesn't exist.
+
+   .. method:: generic_visit(node)
+
+      This visitor calls :meth:`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 :meth:`generic_visit` or visits them
+      itself.
+
+   Don't use the :class:`NodeVisitor` if you want to apply changes to nodes
+   during traversal.  For this a special visitor exists
+   (:class:`NodeTransformer`) that allows modifications.
+
+
+.. class:: NodeTransformer()
+
+   A :class:`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 :meth:`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)
+
+
+.. function:: 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 dumped by default.  If this is wanted,
+   *include_attributes* can be set to ``True``.

Doc/library/language.rst

@@ -15,7 +15,7 @@ These modules include:
 .. toctree::
 
    parser.rst
-   _ast.rst
+   ast.rst
    symbol.rst
    token.rst
    keyword.rst

Lib/test/test_ast.py

 import sys, itertools, unittest
 from test import test_support
-import _ast
+import ast
 
 def to_tuple(t):
     if t is None or isinstance(t, (basestring, int, long, complex)):
@@ -123,9 +123,9 @@ eval_tests = [
 class AST_Tests(unittest.TestCase):
 
     def _assert_order(self, ast_node, parent_pos):
-        if not isinstance(ast_node, _ast.AST) or ast_node._fields is None:
+        if not isinstance(ast_node, ast.AST) or ast_node._fields is None:
             return
-        if isinstance(ast_node, (_ast.expr, _ast.stmt, _ast.excepthandler)):
+        if isinstance(ast_node, (ast.expr, ast.stmt, ast.excepthandler)):
             node_pos = (ast_node.lineno, ast_node.col_offset)
             self.assert_(node_pos >= parent_pos)
             parent_pos = (ast_node.lineno, ast_node.col_offset)
@@ -142,29 +142,29 @@ class AST_Tests(unittest.TestCase):
                                     (single_tests, single_results, "single"),
                                     (eval_tests, eval_results, "eval")):
             for i, o in itertools.izip(input, output):
-                ast_tree = compile(i, "?", kind, _ast.PyCF_ONLY_AST)
+                ast_tree = compile(i, "?", kind, ast.PyCF_ONLY_AST)
                 self.assertEquals(to_tuple(ast_tree), o)
                 self._assert_order(ast_tree, (0, 0))
 
     def test_nodeclasses(self):
-        x = _ast.BinOp(1, 2, 3, lineno=0)
+        x = ast.BinOp(1, 2, 3, lineno=0)
         self.assertEquals(x.left, 1)
         self.assertEquals(x.op, 2)
         self.assertEquals(x.right, 3)
         self.assertEquals(x.lineno, 0)
 
         # node raises exception when not given enough arguments
-        self.assertRaises(TypeError, _ast.BinOp, 1, 2)
+        self.assertRaises(TypeError, ast.BinOp, 1, 2)
 
         # can set attributes through kwargs too
-        x = _ast.BinOp(left=1, op=2, right=3, lineno=0)
+        x = ast.BinOp(left=1, op=2, right=3, lineno=0)
         self.assertEquals(x.left, 1)
         self.assertEquals(x.op, 2)
         self.assertEquals(x.right, 3)
         self.assertEquals(x.lineno, 0)
 
         # this used to fail because Sub._fields was None
-        x = _ast.Sub()
+        x = ast.Sub()
 
     def test_pickling(self):
         import pickle
@@ -181,8 +181,99 @@ class AST_Tests(unittest.TestCase):
                     ast2 = mod.loads(mod.dumps(ast, protocol))
                     self.assertEquals(to_tuple(ast2), to_tuple(ast))
 
+
+class ASTHelpers_Test(unittest.TestCase):
+
+    def test_parse(self):
+        a = ast.parse('foo(1 + 1)')
+        b = compile('foo(1 + 1)', '<unknown>', 'exec', ast.PyCF_ONLY_AST)
+        self.assertEqual(ast.dump(a), ast.dump(b))
+
+    def test_dump(self):
+        node = ast.parse('spam(eggs, "and cheese")')
+        self.assertEqual(ast.dump(node),
+            "Module(body=[Expr(value=Call(func=Name(id='spam', ctx=Load()), "
+            "args=[Name(id='eggs', ctx=Load()), Str(s='and cheese')], "
+            "keywords=[], starargs=None, kwargs=None))])"
+        )
+        self.assertEqual(ast.dump(node, annotate_fields=False),
+            "Module([Expr(Call(Name('spam', Load()), [Name('eggs', Load()), "
+            "Str('and cheese')], [], None, None))])"
+        )
+        self.assertEqual(ast.dump(node, include_attributes=True),
+            "Module(body=[Expr(value=Call(func=Name(id='spam', ctx=Load(), "
+            "lineno=1, col_offset=0), args=[Name(id='eggs', ctx=Load(), "
+            "lineno=1, col_offset=5), Str(s='and cheese', lineno=1, "
+            "col_offset=11)], keywords=[], starargs=None, kwargs=None, "
+            "lineno=1, col_offset=0), lineno=1, col_offset=0)])"
+        )
+
+    def test_copy_location(self):
+        src = ast.parse('1 + 1', mode='eval')
+        src.body.right = ast.copy_location(ast.Num(2), src.body.right)
+        self.assertEqual(ast.dump(src, include_attributes=True),
+            'Expression(body=BinOp(left=Num(n=1, lineno=1, col_offset=0), '
+            'op=Add(), right=Num(n=2, lineno=1, col_offset=4), lineno=1, '
+            'col_offset=0))'
+        )
+
+    def test_fix_missing_locations(self):
+        src = ast.parse('write("spam")')
+        src.body.append(ast.Expr(ast.Call(ast.Name('spam', ast.Load()),
+                                          [ast.Str('eggs')], [], None, None)))
+        self.assertEqual(src, ast.fix_missing_locations(src))
+        self.assertEqual(ast.dump(src, include_attributes=True),
+            "Module(body=[Expr(value=Call(func=Name(id='write', ctx=Load(), "
+            "lineno=1, col_offset=0), args=[Str(s='spam', lineno=1, "
+            "col_offset=6)], keywords=[], starargs=None, kwargs=None, "
+            "lineno=1, col_offset=0), lineno=1, col_offset=0), "
+            "Expr(value=Call(func=Name(id='spam', ctx=Load(), lineno=1, "
+            "col_offset=0), args=[Str(s='eggs', lineno=1, col_offset=0)], "
+            "keywords=[], starargs=None, kwargs=None, lineno=1, "
+            "col_offset=0), lineno=1, col_offset=0)])"
+        )
+
+    def test_increment_lineno(self):
+        src = ast.parse('1 + 1', mode='eval')
+        self.assertEqual(ast.increment_lineno(src, n=3), src)
+        self.assertEqual(ast.dump(src, include_attributes=True),
+            'Expression(body=BinOp(left=Num(n=1, lineno=4, col_offset=0), '
+            'op=Add(), right=Num(n=1, lineno=4, col_offset=4), lineno=4, '
+            'col_offset=0))'
+        )
+
+    def test_iter_fields(self):
+        node = ast.parse('foo()', mode='eval')
+        d = dict(ast.iter_fields(node.body))
+        self.assertEqual(d.pop('func').id, 'foo')
+        self.assertEqual(d, {'keywords': [], 'kwargs': None,
+                             'args': [], 'starargs': None})
+
+    def test_iter_child_nodes(self):
+        node = ast.parse("spam(23, 42, eggs='leek')", mode='eval')
+        self.assertEqual(len(list(ast.iter_child_nodes(node.body))), 4)
+        iterator = ast.iter_child_nodes(node.body)
+        self.assertEqual(next(iterator).id, 'spam')
+        self.assertEqual(next(iterator).n, 23)
+        self.assertEqual(next(iterator).n, 42)
+        self.assertEqual(ast.dump(next(iterator)),
+            "keyword(arg='eggs', value=Str(s='leek'))"
+        )
+
+    def test_get_docstring(self):
+        node = ast.parse('def foo():\n  """line one\n  line two"""')
+        self.assertEqual(ast.get_docstring(node.body[0]),
+                         'line one\nline two')
+
+    def test_literal_eval(self):
+        self.assertEqual(ast.literal_eval('[1, 2, 3]'), [1, 2, 3])
+        self.assertEqual(ast.literal_eval('{"foo": 42}'), {"foo": 42})
+        self.assertEqual(ast.literal_eval('(True, False, None)'), (True, False, None))
+        self.assertRaises(ValueError, ast.literal_eval, 'foo()')
+
+
 def test_main():
-    test_support.run_unittest(AST_Tests)
+    test_support.run_unittest(AST_Tests, ASTHelpers_Test)
 
 def main():
     if __name__ != '__main__':

Misc/ACKS

@@ -574,6 +574,7 @@ Andy Robinson
 Kevin Rodgers
 Giampaolo Rodola
 Mike Romberg
+Armin Ronacher
 Case Roole
 Timothy Roscoe
 Jim Roskind

Misc/NEWS

@@ -77,6 +77,8 @@ Extension Modules
 Library
 -------
 
+- Added the ast module.
+
 - Factored out the indentation cleaning from inspect.getdoc() into
   inspect.cleandoc() to ease standalone use.