Python Library Reference

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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
Dict	items
Discard	expr
Div	left
	right
Ellipsis
Exec	expr
	locals
	globals
For	assign
	list
	body
	else_
From	modname
	names
Function	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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Python Library Reference

Previous: 19.3 Python Abstract Syntax Up: 19.3 Python Abstract Syntax Next: 19.3.2 Assignment nodes

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Release 2.2, documentation updated on December 21, 2001.