276 lines
10 KiB
ReStructuredText
276 lines
10 KiB
ReStructuredText
:mod:`ast` --- Abstract Syntax Trees
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====================================
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.. module:: ast
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:synopsis: Abstract Syntax Tree classes and manipulation.
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.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>
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.. sectionauthor:: Georg Brandl <georg@python.org>
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**Source code:** :source:`Lib/ast.py`
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--------------
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The :mod:`ast` module helps Python applications to process trees of the Python
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abstract syntax grammar. The abstract syntax itself might change with each
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Python release; this module helps to find out programmatically what the current
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grammar looks like.
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An abstract syntax tree can be generated by passing :data:`ast.PyCF_ONLY_AST` as
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a flag to the :func:`compile` built-in function, or using the :func:`parse`
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helper provided in this module. The result will be a tree of objects whose
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classes all inherit from :class:`ast.AST`. An abstract syntax tree can be
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compiled into a Python code object using the built-in :func:`compile` function.
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Node classes
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------------
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.. class:: AST
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This is the base of all AST node classes. The actual node classes are
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derived from the :file:`Parser/Python.asdl` file, which is reproduced
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:ref:`below <abstract-grammar>`. They are defined in the :mod:`_ast` C
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module and re-exported in :mod:`ast`.
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There is one class defined for each left-hand side symbol in the abstract
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grammar (for example, :class:`ast.stmt` or :class:`ast.expr`). In addition,
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there is one class defined for each constructor on the right-hand side; these
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classes inherit from the classes for the left-hand side trees. For example,
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:class:`ast.BinOp` inherits from :class:`ast.expr`. For production rules
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with alternatives (aka "sums"), the left-hand side class is abstract: only
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instances of specific constructor nodes are ever created.
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.. attribute:: _fields
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Each concrete class has an attribute :attr:`_fields` which gives the names
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of all child nodes.
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Each instance of a concrete class has one attribute for each child node,
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of the type as defined in the grammar. For example, :class:`ast.BinOp`
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instances have an attribute :attr:`left` of type :class:`ast.expr`.
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If these attributes are marked as optional in the grammar (using a
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question mark), the value might be ``None``. If the attributes can have
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zero-or-more values (marked with an asterisk), the values are represented
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as Python lists. All possible attributes must be present and have valid
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values when compiling an AST with :func:`compile`.
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.. attribute:: lineno
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col_offset
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Instances of :class:`ast.expr` and :class:`ast.stmt` subclasses have
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:attr:`lineno` and :attr:`col_offset` attributes. The :attr:`lineno` is
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the line number of source text (1-indexed so the first line is line 1) and
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the :attr:`col_offset` is the UTF-8 byte offset of the first token that
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generated the node. The UTF-8 offset is recorded because the parser uses
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UTF-8 internally.
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The constructor of a class :class:`ast.T` parses its arguments as follows:
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* If there are positional arguments, there must be as many as there are items
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in :attr:`T._fields`; they will be assigned as attributes of these names.
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* If there are keyword arguments, they will set the attributes of the same
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names to the given values.
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For example, to create and populate an :class:`ast.UnaryOp` node, you could
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use ::
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node = ast.UnaryOp()
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node.op = ast.USub()
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node.operand = ast.Num()
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node.operand.n = 5
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node.operand.lineno = 0
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node.operand.col_offset = 0
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node.lineno = 0
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node.col_offset = 0
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or the more compact ::
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node = ast.UnaryOp(ast.USub(), ast.Num(5, lineno=0, col_offset=0),
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lineno=0, col_offset=0)
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.. _abstract-grammar:
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Abstract Grammar
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----------------
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The abstract grammar is currently defined as follows:
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.. literalinclude:: ../../Parser/Python.asdl
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:language: none
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:mod:`ast` Helpers
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------------------
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Apart from the node classes, the :mod:`ast` module defines these utility functions
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and classes for traversing abstract syntax trees:
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.. function:: parse(source, filename='<unknown>', mode='exec')
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Parse the source into an AST node. Equivalent to ``compile(source,
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filename, mode, ast.PyCF_ONLY_AST)``.
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.. warning::
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It is possible to crash the Python interpreter with a
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sufficiently large/complex string due to stack depth limitations
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in Python's AST compiler.
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.. function:: literal_eval(node_or_string)
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Safely evaluate an expression node or a string containing a Python literal or
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container display. The string or node provided may only consist of the
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following Python literal structures: strings, bytes, numbers, tuples, lists,
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dicts, sets, booleans, and ``None``.
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This can be used for safely evaluating strings containing Python values from
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untrusted sources without the need to parse the values oneself. It is not
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capable of evaluating arbitrarily complex expressions, for example involving
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operators or indexing.
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.. warning::
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It is possible to crash the Python interpreter with a
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sufficiently large/complex string due to stack depth limitations
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in Python's AST compiler.
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.. versionchanged:: 3.2
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Now allows bytes and set literals.
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.. function:: get_docstring(node, clean=True)
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Return the docstring of the given *node* (which must be a
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:class:`FunctionDef`, :class:`AsyncFunctionDef`, :class:`ClassDef`,
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or :class:`Module` node), or ``None`` if it has no docstring.
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If *clean* is true, clean up the docstring's indentation with
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:func:`inspect.cleandoc`.
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.. versionchanged:: 3.5
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:class:`AsyncFunctionDef` is now supported.
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.. versionchanged:: 3.7
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The docstring is now exported from the node docstring field, instead of
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the first body statement.
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.. function:: fix_missing_locations(node)
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When you compile a node tree with :func:`compile`, the compiler expects
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:attr:`lineno` and :attr:`col_offset` attributes for every node that supports
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them. This is rather tedious to fill in for generated nodes, so this helper
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adds these attributes recursively where not already set, by setting them to
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the values of the parent node. It works recursively starting at *node*.
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.. function:: increment_lineno(node, n=1)
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Increment the line number of each node in the tree starting at *node* by *n*.
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This is useful to "move code" to a different location in a file.
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.. function:: copy_location(new_node, old_node)
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Copy source location (:attr:`lineno` and :attr:`col_offset`) from *old_node*
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to *new_node* if possible, and return *new_node*.
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.. function:: iter_fields(node)
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Yield a tuple of ``(fieldname, value)`` for each field in ``node._fields``
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that is present on *node*.
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.. function:: iter_child_nodes(node)
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Yield all direct child nodes of *node*, that is, all fields that are nodes
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and all items of fields that are lists of nodes.
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.. function:: walk(node)
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Recursively yield all descendant nodes in the tree starting at *node*
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(including *node* itself), in no specified order. This is useful if you only
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want to modify nodes in place and don't care about the context.
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.. class:: NodeVisitor()
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A node visitor base class that walks the abstract syntax tree and calls a
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visitor function for every node found. This function may return a value
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which is forwarded by the :meth:`visit` method.
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This class is meant to be subclassed, with the subclass adding visitor
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methods.
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.. method:: visit(node)
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Visit a node. The default implementation calls the method called
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:samp:`self.visit_{classname}` where *classname* is the name of the node
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class, or :meth:`generic_visit` if that method doesn't exist.
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.. method:: generic_visit(node)
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This visitor calls :meth:`visit` on all children of the node.
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Note that child nodes of nodes that have a custom visitor method won't be
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visited unless the visitor calls :meth:`generic_visit` or visits them
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itself.
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Don't use the :class:`NodeVisitor` if you want to apply changes to nodes
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during traversal. For this a special visitor exists
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(:class:`NodeTransformer`) that allows modifications.
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.. class:: NodeTransformer()
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A :class:`NodeVisitor` subclass that walks the abstract syntax tree and
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allows modification of nodes.
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The :class:`NodeTransformer` will walk the AST and use the return value of
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the visitor methods to replace or remove the old node. If the return value
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of the visitor method is ``None``, the node will be removed from its
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location, otherwise it is replaced with the return value. The return value
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may be the original node in which case no replacement takes place.
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Here is an example transformer that rewrites all occurrences of name lookups
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(``foo``) to ``data['foo']``::
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class RewriteName(NodeTransformer):
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def visit_Name(self, node):
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return copy_location(Subscript(
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value=Name(id='data', ctx=Load()),
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slice=Index(value=Str(s=node.id)),
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ctx=node.ctx
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), node)
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Keep in mind that if the node you're operating on has child nodes you must
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either transform the child nodes yourself or call the :meth:`generic_visit`
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method for the node first.
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For nodes that were part of a collection of statements (that applies to all
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statement nodes), the visitor may also return a list of nodes rather than
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just a single node.
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Usually you use the transformer like this::
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node = YourTransformer().visit(node)
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.. function:: dump(node, annotate_fields=True, include_attributes=False)
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Return a formatted dump of the tree in *node*. This is mainly useful for
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debugging purposes. The returned string will show the names and the values
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for fields. This makes the code impossible to evaluate, so if evaluation is
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wanted *annotate_fields* must be set to ``False``. Attributes such as line
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numbers and column offsets are not dumped by default. If this is wanted,
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*include_attributes* can be set to ``True``.
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.. seealso::
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`Green Tree Snakes <https://greentreesnakes.readthedocs.io/>`_, an external documentation resource, has good
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details on working with Python ASTs.
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