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ast.NodeTransformer

helper functions

A NodeVisitor subclass for changing a tree: the value returned by visit_* replaces the node, and None removes it.

Syntax

class T(ast.NodeTransformer):
    def visit_Name(self, node): return node
tree = T().visit(tree)

A forgotten return in a visit_* method is a common bug: the method returns None and the node silently disappears from the tree. For statements you may return a list of nodes to replace a single one. New nodes have no lineno, so compile() fails with TypeError until you call ast.fix_missing_locations(tree). As with NodeVisitor, you must visit children yourself via generic_visit.

Examples

import ast
class Oops(ast.NodeTransformer):
    def visit_Expr(self, node):
        node.value = ast.Constant(0)
tree = Oops().visit(ast.parse('print(1)\nx = 2'))
print(ast.unparse(tree))  # → x = 2 — print исчез

import ast
class Upper(ast.NodeTransformer):
    def visit_Constant(self, node):
        if isinstance(node.value, str):
            return ast.Constant(node.value.upper())
        return node
tree = Upper().visit(ast.parse("print('hi')"))
try:
    compile(tree, '<ast>', 'exec')
except TypeError:
    print('нет lineno')  # → нет lineno
exec(compile(ast.fix_missing_locations(tree), '<ast>', 'exec'))  # → HI

See also

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