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46 lines
2.1 KiB
TeX
46 lines
2.1 KiB
TeX
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% XXX Label can't be _ast?
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% XXX Where should this section/chapter go?
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\chapter{Abstract Syntax Trees\label{ast}}
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\sectionauthor{Martin v. L\"owis}{martin@v.loewis.de}
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The \code{_ast} module helps Python applications to process
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trees of the Python abstract syntax grammar. The Python compiler
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currently provides read-only access to such trees, meaning that
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applications can only create a tree for a given piece of Python
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source code; generating byte code from a (potentially modified)
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tree is not supported. The abstract syntax itself might change with
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each Python release; this module helps to find out programmatically
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what the current grammar looks like.
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An abstract syntax tree can be generated by passing \code{_ast.PyCF_ONLY_AST}
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as a flag to the \function{compile} builtin function. The result will be a tree
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of objects whose classes all inherit from \code{_ast.AST}.
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The actual classes are derived from the \code{Parser/Python.asdl} file,
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which is reproduced below. There is one class defined for each left-hand
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side symbol in the abstract grammar (for example, \code{_ast.stmt} or \code{_ast.expr}).
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In addition, there is one class defined for each constructor on the
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right-hand side; these classes inherit from the classes for the left-hand
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side trees. For example, \code{_ast.BinOp} inherits from \code{_ast.expr}.
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For production rules with alternatives (aka "sums"), the left-hand side
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class is abstract: only instances of specific constructor nodes are ever
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created.
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Each concrete class has an attribute \code{_fields} which gives the
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names 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, \code{_ast.BinOp}
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instances have an attribute \code{left} of type \code{_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 \code{None}. If the attributes
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can have zero-or-more values (marked with an asterisk), the
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values are represented as Python lists.
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\subsection{Abstract Grammar}
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The abstract grammar is currently defined as follows:
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\verbatiminput{../../Parser/Python.asdl}
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