cpython/Doc/library/ast.rst

:mod:`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>

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

--------------

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` built-in 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`.  An 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.

   .. index:: single: ? (question mark); in AST grammar
   .. index:: single: * (asterisk); in AST grammar

   .. 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
                  end_lineno
                  end_col_offset

      Instances of :class:`ast.expr` and :class:`ast.stmt` subclasses have
      :attr:`lineno`, :attr:`col_offset`, :attr:`lineno`, and :attr:`col_offset`
      attributes.  The :attr:`lineno` and :attr:`end_lineno` are the first and
      last line numbers of source text span (1-indexed so the first line is line 1)
      and the :attr:`col_offset` and :attr:`end_col_offset` are the corresponding
      UTF-8 byte offsets of the first and last tokens that generated the node.
      The UTF-8 offset is recorded because the parser uses UTF-8 internally.

      Note that the end positions are not required by the compiler and are
      therefore optional. The end offset is *after* the last symbol, for example
      one can get the source segment of a one-line expression node using
      ``source_line[node.col_offset : node.end_col_offset]``.

   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.Constant()
      node.operand.value = 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.Constant(5, lineno=0, col_offset=0),
                         lineno=0, col_offset=0)

.. versionchanged:: 3.8

   Class :class:`ast.Constant` is now used for all constants.

.. deprecated:: 3.8

   Old classes :class:`ast.Num`, :class:`ast.Str`, :class:`ast.Bytes`,
   :class:`ast.NameConstant` and :class:`ast.Ellipsis` are still available,
   but they will be removed in future Python releases.  In the meanwhile,
   instantiating them will return an instance of a different class.


.. _abstract-grammar:

Abstract Grammar
----------------

The abstract grammar is currently defined as follows:

.. literalinclude:: ../../Parser/Python.asdl
   :language: none


:mod:`ast` Helpers
------------------

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

.. function:: parse(source, filename='<unknown>', mode='exec', *, type_comments=False, feature_version=None)

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

   If ``type_comments=True`` is given, the parser is modified to check
   and return type comments as specified by :pep:`484` and :pep:`526`.
   This is equivalent to adding :data:`ast.PyCF_TYPE_COMMENTS` to the
   flags passed to :func:`compile()`.  This will report syntax errors
   for misplaced type comments.  Without this flag, type comments will
   be ignored, and the ``type_comment`` field on selected AST nodes
   will always be ``None``.  In addition, the locations of ``# type:
   ignore`` comments will be returned as the ``type_ignores``
   attribute of :class:`Module` (otherwise it is always an empty list).

   In addition, if ``mode`` is ``'func_type'``, the input syntax is
   modified to correspond to :pep:`484` "signature type comments",
   e.g. ``(str, int) -> List[str]``.

   Also, setting ``feature_version`` to a tuple ``(major, minor)``
   will attempt to parse using that Python version's grammar.
   Currently ``major`` must equal to ``3``.  For example, setting
   ``feature_version=(3, 4)`` will allow the use of ``async`` and
   ``await`` as variable names.  The lowest supported version is
   ``(3, 4)``; the highest is ``sys.version_info[0:2]``.

   .. warning::
      It is possible to crash the Python interpreter with a
      sufficiently large/complex string due to stack depth limitations
      in Python's AST compiler.

   .. versionchanged:: 3.8
      Added ``type_comments``, ``mode='func_type'`` and ``feature_version``.


.. function:: unparse(ast_obj)

   Unparse an :class:`ast.AST` object and generate a string with code
   that would produce an equivalent :class:`ast.AST` object if parsed
   back with :func:`ast.parse`.

   .. warning::
      The produced code string will not necessarily be equal to the original
      code that generated the :class:`ast.AST` object.

   .. versionadded:: 3.9


.. function:: literal_eval(node_or_string)

   Safely evaluate an expression node or a string containing a Python literal or
   container display.  The string or node provided may only consist of the
   following Python literal structures: strings, bytes, numbers, tuples, lists,
   dicts, sets, 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.

   .. warning::
      It is possible to crash the Python interpreter with a
      sufficiently large/complex string due to stack depth limitations
      in Python's AST compiler.

   .. versionchanged:: 3.2
      Now allows bytes and set literals.

   .. versionchanged:: 3.9
      Now supports creating empty sets with ``'set()'``.


.. function:: get_docstring(node, clean=True)

   Return the docstring of the given *node* (which must be a
   :class:`FunctionDef`, :class:`AsyncFunctionDef`, :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`.

   .. versionchanged:: 3.5
      :class:`AsyncFunctionDef` is now supported.


.. function:: get_source_segment(source, node, *, padded=False)

   Get source code segment of the *source* that generated *node*.
   If some location information (:attr:`lineno`, :attr:`end_lineno`,
   :attr:`col_offset`, or :attr:`end_col_offset`) is missing, return ``None``.

   If *padded* is ``True``, the first line of a multi-line statement will
   be padded with spaces to match its original position.

   .. versionadded:: 3.8


.. 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 and end 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`, :attr:`col_offset`, :attr:`end_lineno`,
   and :attr:`end_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 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:: 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 :meth:`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.

   .. deprecated:: 3.8

      Methods :meth:`visit_Num`, :meth:`visit_Str`, :meth:`visit_Bytes`,
      :meth:`visit_NameConstant` and :meth:`visit_Ellipsis` are deprecated
      now and will not be called in future Python versions.  Add the
      :meth:`visit_Constant` method to handle all constant nodes.


.. class:: NodeTransformer()

   A :class:`NodeVisitor` subclass that walks the abstract syntax tree and
   allows modification of nodes.

   The :class:`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 Subscript(
                  value=Name(id='data', ctx=Load()),
                  slice=Index(value=Constant(value=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.

   If :class:`NodeTransformer` introduces new nodes (that weren't part of
   original tree) without giving them location information (such as
   :attr:`lineno`), :func:`fix_missing_locations` should be called with
   the new sub-tree to recalculate the location information::

      tree = ast.parse('foo', mode='eval')
      new_tree = fix_missing_locations(RewriteName().visit(tree))

   Usually you use the transformer like this::

      node = YourTransformer().visit(node)


.. function:: dump(node, annotate_fields=True, include_attributes=False, *, indent=None)

   Return a formatted dump of the tree in *node*.  This is mainly useful for
   debugging purposes.  If *annotate_fields* is true (by default),
   the returned string will show the names and the values for fields.
   If *annotate_fields* is false, the result string will be more compact by
   omitting unambiguous field names.  Attributes such as line
   numbers and column offsets are not dumped by default.  If this is wanted,
   *include_attributes* can be set to true.

   If *indent* is a non-negative integer or string, then the tree will be
   pretty-printed with that indent level.  An indent level
   of 0, negative, or ``""`` will only insert newlines.  ``None`` (the default)
   selects the single line representation. Using a positive integer indent
   indents that many spaces per level.  If *indent* is a string (such as ``"\t"``),
   that string is used to indent each level.

   .. versionchanged:: 3.9
      Added the *indent* option.


.. _ast-cli:

Command-Line Usage
------------------

.. versionadded:: 3.9

The :mod:`ast` module can be executed as a script from the command line.
It is as simple as:

.. code-block:: sh

   python -m ast [-m <mode>] [-a] [infile]

The following options are accepted:

.. program:: ast

.. cmdoption:: -h, --help

   Show the help message and exit.

.. cmdoption:: -m <mode>
               --mode <mode>

   Specify what kind of code must be compiled, like the *mode* argument
   in :func:`parse`.

.. cmdoption:: --no-type-comments

   Don't parse type comments.

.. cmdoption:: -a, --include-attributes

   Include attributes such as line numbers and column offsets.

.. cmdoption:: -i <indent>
               --indent <indent>

   Indentation of nodes in AST (number of spaces).

If :file:`infile` is specified its contents are parsed to AST and dumped
to stdout.  Otherwise, the content is read from stdin.


.. seealso::

    `Green Tree Snakes <https://greentreesnakes.readthedocs.io/>`_, an external documentation resource, has good
    details on working with Python ASTs.