% XXX Label can't be _ast?
% XXX Where should this section/chapter go?
\chapter{Abstract Syntax Trees\label{ast}}

\sectionauthor{Martin v. L\"owis}{martin@v.loewis.de}

The \code{_ast} module helps Python applications to process
trees of the Python abstract syntax grammar. The Python compiler
currently provides read-only access to such trees, meaning that
applications can only create a tree for a given piece of Python
source code; generating byte code from a (potentially modified)
tree is not supported. 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 \code{_ast.PyCF_ONLY_AST}
as a flag to the \function{compile} builtin function. The result will be a tree
of objects whose classes all inherit from \code{_ast.AST}.

The actual classes are derived from the \code{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, \code{_ast.stmt} or \code{_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, \code{_ast.BinOp} inherits from \code{_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 \code{_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, \code{_ast.BinOp}
instances have an attribute \code{left} of type \code{_ast.expr}.

If these attributes are marked as optional in the grammar (using a
question mark), the value might be \code{None}. If the attributes
can have zero-or-more values (marked with an asterisk), the
values are represented as Python lists.

\subsection{Abstract Grammar}

The abstract grammar is currently defined as follows:

\verbatiminput{../../Parser/Python.asdl}