Question

I am working on Python 2.6.5.

Given a Abstract Syntax Tree, I want to obtain its children.

Most StackOverflow posts discuss ast.NodeVisitor and the methods defined in it: visit(), generic_visit().
However, visit() and generic_visit() do not give the children, rather they directly apply the function recursively on them.

Can someone please write a short code or so to demonstrate it?
Does there exist a predefined function in python library for the same?

Answer 1

The attaributes containing the node’s children depend on the type of syntax the node represents. Every node class also has a special _fields attribute, that lists the attribute names for the child nodes that class has. For instance,

>>> ast.parse('5+a')
<_ast.Module object at 0x02C1F730>
>>> ast.parse('5+a').body
[<_ast.Expr object at 0x02C1FF50>]
>>> ast.parse('5+a').body[0]
<_ast.Expr object at 0x02C1FBF0>
>>> ast.parse('5+a').body[0]._fields
('value',)
>>> ast.parse('5+a').body[0].value
<_ast.BinOp object at 0x02C1FF90>
>>> ast.parse('5+a').body[0].value._fields
('left', 'op', 'right')
>>> ast.parse('5+a').body[0].value.left
<_ast.Num object at 0x02C1FB70>

and so on.

Edit, to clarify what’s going on

Before going any further, take a glance at the CPython Abstract Grammar

Consider:

>>> type(ast.parse('5+a'))
<class '_ast.Module'>

In fact, if you look at the grammar, the first production rule is for Module. It appears to take a sequence of statements, as an argument called body.

>>> ast.parse('5+a')._fields
('body',)
>>> ast.parse('5+a').body
[<_ast.Expr object at 0x02E965B0>]

The _fields attribute of the AST is just “body”, and the body attribute is a sequence of AST nodes. Back to the grammar, looking in the production rules for stmt, we see that Expr takes a single expr, named value

>>> ast.parse('5+a').body[0].value
<_ast.BinOp object at 0x02E96330>

If we look up the definition for BinOp, we see that it takes 3 different arguments, left, op and right. You should be able to proceed from there, I hope.