| :mod:`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> |
| |
| **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) |
| |
| .. deprecated:: 3.8 |
| |
| Class :class:`ast.Constant` is now used for all constants. 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. |
| |
| |
| .. _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:: 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. |
| |
| |
| .. 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 copy_location(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. |
| |
| 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. 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. |
| |
| .. seealso:: |
| |
| `Green Tree Snakes <https://greentreesnakes.readthedocs.io/>`_, an external documentation resource, has good |
| details on working with Python ASTs. |