NodeTransformer

Learn transformation with ast.NodeTransformer.

What is NodeTransformer?

ast.NodeTransformer is the transforming sibling of NodeVisitor. Its visit_ methods take a node and return a modified or new node.

For example inside visit_Constant writing return ast.Constant(value=node.value * 2) doubles every constant. After transformation ast.fix_missing_locations(tree) repairs line numbers; this is required for compile and exec. This pattern is the basis of code optimization and transformation tools.

Changing nodes with visit_

In a NodeTransformer a visit_ method can do three things: return the same node (no change), return a new node (replace) or return None (delete).

When creating a new node you use constructors like ast.Constant(value=...). After transformation ast.fix_missing_locations(tree) repairs line information. Skipping this step makes compile fail. That is why this call always sits between transformation and compilation.

Exercise Transform with NodeTransformer

Import the module with import ast. Write a NumberDoubler(ast.NodeTransformer) class. In visit_Constant(self, node) if isinstance(node.value, int) return ast.Constant(value=node.value * 2); otherwise return node. Build the tree with ast.parse("x = 5"), transform with NumberDoubler().visit(tree), call ast.fix_missing_locations(tree). Run exec(tree, ns) in an empty ns = {} dict and print ns["x"].

Show example solution

import ast

class NumberDoubler(ast.NodeTransformer):
    def visit_Constant(self, node):
        if isinstance(node.value, int):
            return ast.Constant(value=node.value * 2)
        return node

tree = ast.parse("x = 5")
NumberDoubler().visit(tree)
ast.fix_missing_locations(tree)
ns = {}
exec(tree, ns)
print(ns["x"])

Test yourself

What does a NodeTransformer return?
```
class D(ast.NodeTransformer):
    def visit_Constant(self, node):
        return ast.Constant(value=0)
```
- Nothing (None)
- A list
- A new or modified node
- The original node unchanged
Show answer

A new or modified node — answer: a new or modified node. A transformer returns a node from visit_ methods.

Complete the NodeTransformer definition

class Doubler(____):
    def visit_Constant(self, node):
        return node

Show answer

class Doubler(ast.NodeTransformer):
    def visit_Constant(self, node):
        return node

Match NodeTransformer concepts

NodeTransformer

Changes nodes
visit_Constant

Constant node
fix_missing_locations

Repairs lines
return node

Stays the same

Next lesson: compile

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