/Doc/library/ast.rst
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- .. _ast:
- Abstract Syntax Trees
- =====================
- .. module:: ast
- :synopsis: Abstract Syntax Tree classes and manipulation.
- .. sectionauthor:: Martin v. Lรถwis <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`. 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.
- .. 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)
- .. versionadded:: 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.
- .. _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, ast.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 :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.
- .. 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 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 not dumped by default. If this is wanted,
- *include_attributes* can be set to ``True``.