| .. _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> |
| |
| |
| 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. |
| |
| .. 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 |
| ------------------ |
| |
| 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, bytes, numbers, tuples, lists, dicts, |
| sets, 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. |
| |
| .. versionchanged:: 3.2 |
| Now allows bytes and set literals. |
| |
| |
| .. 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``. |