cpython/Doc/library/_ast.rst

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.. _ast:
Abstract Syntax Trees
=====================
.. module:: _ast
:synopsis: Abstract Syntax Tree classes.
.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>
The ``_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.
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An abstract syntax tree can be generated by passing :data:`_ast.PyCF_ONLY_AST`
as a flag to the :func:`compile` builtin function. The result will be a tree of
objects whose classes all inherit from :class:`_ast.AST`.
A modified abstract syntax tree can be compiled into a Python code object using
the built-in :func:`compile` function.
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The actual classes are derived from the ``Parser/Python.asdl`` file, which is
reproduced below. There is one class defined for each left-hand side symbol in
the abstract grammar (for example, ``_ast.stmt`` or ``_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,
``_ast.BinOp`` inherits from ``_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.
Each concrete class has an attribute ``_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, ``_ast.BinOp`` instances have an
attribute ``left`` of type ``_ast.expr``. Instances of ``_ast.expr`` and
``_ast.stmt`` subclasses also have lineno and col_offset attributes. The lineno
is the line number of source text (1 indexed so the first line is line 1) and
the col_offset is the utf8 byte offset of the first token that generated the
node. The utf8 offset is recorded because the parser uses utf8 internally.
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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`.
The constructor of a class ``_ast.T`` parses their arguments as follows:
* If there are positional arguments, there must be as many as there are items in
``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 a ``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)
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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