Working on the Tree

ast.NodeVisitor is the primary tool for ‘scanning’ the tree. To use it, subclass it and override methods visit_Foo, corresponding to the node classes (see Meet the Nodes).

For example, this visitor will print the names of any functions defined in the given code, including methods and functions defined within other functions:

class FuncLister(ast.NodeVisitor):
    def visit_FunctionDef(self, node):



If you want child nodes to be visited, remember to call self.generic_visit(node) in the methods you override.

Alternatively, you can run through a list of all the nodes in the tree using ast.walk(). There are no guarantees about the order in which nodes will appear. The following example again prints the names of any functions defined within the given code:

for node in ast.walk(tree):
    if isinstance(node, ast.FunctionDef):

You can also get the direct children of a node, using ast.iter_child_nodes(). Remember that many nodes have children in several sections: for example, an If has a node in the test field, and list of nodes in body and orelse. ast.iter_child_nodes() will go through all of these.

Finally, you can navigate directly, using the attributes of the nodes. For example, if you want to get the last node within a function’s body, use node.body[-1]. Of course, all the normal Python tools for iterating and indexing work. In particular, isinstance() is very useful for checking what nodes are.

Inspecting nodes

The ast module has a couple of functions for inspecting nodes:

Modifying the tree

The key tool is ast.NodeTransformer. Like ast.NodeVisitor, you subclass this and override visit_Foo methods. The method should return the original node, a replacement node, or None to remove that node from the tree.

The ast module docs have this example, which rewrites name lookups, so foo becomes data['foo']:

class RewriteName(ast.NodeTransformer):

    def visit_Name(self, node):
        return ast.copy_location(ast.Subscript(
            value=ast.Name(id='data', ctx=ast.Load()),
        ), node)

tree = RewriteName().visit(tree)

When replacing a node, the new node doesn’t automatically have the lineno and col_offset parameters. The example above doesn’t deal with this completely: it copies the location to the Subscript node, but not to any of the newly created children of that node. See Fixing locations.

Be careful when removing nodes. You can quite easily remove a node from a required field, such as the test field of an If node. Python won’t complain about the invalid AST until you try to compile() it, when a TypeError is raised.