19.3.1 AST Nodes

Python 2.4

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19.3.1 AST Nodes

The compiler.ast module is generated from a text file that describes each node type and its elements. Each node type is represented as a class that inherits from the abstract base class compiler.ast.Node and defines a set of named attributes for child nodes.

class Node( )

The Node instances are created automatically by the parser generator. The recommended interface for specific Node instances is to use the public attributes to access child nodes. A public attribute may be bound to a single node or to a sequence of nodes, depending on the Node type. For example, the bases attribute of the Class node, is bound to a list of base class nodes, and the doc attribute is bound to a single node.

Each Node instance has a lineno attribute which may be None. XXX Not sure what the rules are for which nodes will have a useful lineno.

All Node objects offer the following methods:

getChildren( ): Returns a flattened list of the child nodes and objects in the order they occur. Specifically, the order of the nodes is the order in which they appear in the Python grammar. Not all of the children are Node instances. The names of functions and classes, for example, are plain strings.

getChildNodes( ): Returns a flattened list of the child nodes in the order they occur. This method is like getChildren(), except that it only returns those children that are Node instances.

Two examples illustrate the general structure of Node classes. The while statement is defined by the following grammar production:

while_stmt:     "while" expression ":" suite
               ["else" ":" suite]

The While node has three attributes: test, body, and else_. (If the natural name for an attribute is also a Python reserved word, it can't be used as an attribute name. An underscore is appended to the word to make it a legal identifier, hence else_ instead of else.)

The if statement is more complicated because it can include several tests.

if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]

The If node only defines two attributes: tests and else_. The tests attribute is a sequence of test expression, consequent body pairs. There is one pair for each if/elif clause. The first element of the pair is the test expression. The second elements is a Stmt node that contains the code to execute if the test is true.

The getChildren() method of If returns a flat list of child nodes. If there are three if/elif clauses and no else clause, then getChildren() will return a list of six elements: the first test expression, the first Stmt, the second text expression, etc.

The following table lists each of the Node subclasses defined in compiler.ast and each of the public attributes available on their instances. The values of most of the attributes are themselves Node instances or sequences of instances. When the value is something other than an instance, the type is noted in the comment. The attributes are listed in the order in which they are returned by getChildren() and getChildNodes().

Node type	Attribute	Value
`Add`	`left`	left operand
	`right`	right operand
`And`	`nodes`	list of operands
`AssAttr`		attribute as target of assignment
	`expr`	expression on the left-hand side of the dot
	`attrname`	the attribute name, a string
	`flags`	XXX
`AssList`	`nodes`	list of list elements being assigned to
`AssName`	`name`	name being assigned to
	`flags`	XXX
`AssTuple`	`nodes`	list of tuple elements being assigned to
`Assert`	`test`	the expression to be tested
	`fail`	the value of the `AssertionError`
`Assign`	`nodes`	a list of assignment targets, one per equal sign
	`expr`	the value being assigned
`AugAssign`	`node`
	`op`
	`expr`
`Backquote`	`expr`
`Bitand`	`nodes`
`Bitor`	`nodes`
`Bitxor`	`nodes`
`Break`
`CallFunc`	`node`	expression for the callee
	`args`	a list of arguments
	`star_args`	the extended *-arg value
	`dstar_args`	the extended **-arg value
`Class`	`name`	the name of the class, a string
	`bases`	a list of base classes
	`doc`	doc string, a string or `None`
	`code`	the body of the class statement
`Compare`	`expr`
	`ops`
`Const`	`value`
`Continue`
`Decorators`	`nodes`	List of function decorator expressions
`Dict`	`items`
`Discard`	`expr`
`Div`	`left`
	`right`
`Ellipsis`
`Exec`	`expr`
	`locals`
	`globals`
`For`	`assign`
	`list`
	`body`
	`else_`
`From`	`modname`
	`names`
`Function`	`decorators`	`Decorators` or `None`
	`name`	name used in def, a string
	`argnames`	list of argument names, as strings
	`defaults`	list of default values
	`flags`	xxx
	`doc`	doc string, a string or `None`
	`code`	the body of the function
`Getattr`	`expr`
	`attrname`
`Global`	`names`
`If`	`tests`
	`else_`
`Import`	`names`
`Invert`	`expr`
`Keyword`	`name`
	`expr`
`Lambda`	`argnames`
	`defaults`
	`flags`
	`code`
`LeftShift`	`left`
	`right`
`List`	`nodes`
`ListComp`	`expr`
	`quals`
`ListCompFor`	`assign`
	`list`
	`ifs`
`ListCompIf`	`test`
`Mod`	`left`
	`right`
`Module`	`doc`	doc string, a string or `None`
	`node`	body of the module, a `Stmt`
`Mul`	`left`
	`right`
`Name`	`name`
`Not`	`expr`
`Or`	`nodes`
`Pass`
`Power`	`left`
	`right`
`Print`	`nodes`
	`dest`
`Printnl`	`nodes`
	`dest`
`Raise`	`expr1`
	`expr2`
	`expr3`
`Return`	`value`
`RightShift`	`left`
	`right`
`Slice`	`expr`
	`flags`
	`lower`
	`upper`
`Sliceobj`	`nodes`	list of statements
`Stmt`	`nodes`
`Sub`	`left`
	`right`
`Subscript`	`expr`
	`flags`
	`subs`
`TryExcept`	`body`
	`handlers`
	`else_`
`TryFinally`	`body`
	`final`
`Tuple`	`nodes`
`UnaryAdd`	`expr`
`UnarySub`	`expr`
`While`	`test`
	`body`
	`else_`
`Yield`	`value`

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