What is ast.NodeVisitor in Python?
Python ast module
The
Python comes with abstract syntax grammar, which is subject to change with every Python release. This module helps applications process trees of syntax and find out programmatically what the current syntax grammar looks like.
Abstract syntax trees are great tools for program analysis and program transformation systems.
What is the ast.NodeVisitor method?
ast.NodeVisitor is a base class defined in the ast module that allows us to visit the nodes in an Abstract Syntax Tree. It calls a visitor function at every node, which returns a value that is forwarded by the visit() method.
The visit() method
visit() visits the node and calls the method self.visit_classname, where classname is the name of the node class. If the self.visit_classname method does not exist, generic_visit() is called.
The generic_visit() method
generic_visit() calls the visit() method on all the children of the respective node.
The children nodes of nodes that have a custom visitor method will not be called unless
generic_visitis explicitly called on them.
This class is meant to be inherited by subclasses that will add their visitor methods.
Code
The following Python code illustrates an example of how we use the NodeVisitor class:
import astfrom pprint import pprintclass AwaitVisitor(ast.NodeVisitor):def visit_Await(self, node):print('Node type: Await\nFields: ', node._fields)self.generic_visit(node)def visit_Call(self,node):print('Node type: Call\nFields: ', node._fields)ast.NodeVisitor.generic_visit(self, node)def visit_Name(self,node):print('Node type: Name\nFields: ', node._fields)ast.NodeVisitor.generic_visit(self, node)visitor = AwaitVisitor()tree = ast.parse("""async def someFunc():await other_func()""")pprint(ast.dump(tree))visitor.visit(tree)
Explanation
In this example, we use the parse() function to parse a piece of code that contains the definition of an asynchronous function. This function waits for another function’s execution in the await statement.
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We define an
AwaitVisitorclass that extends from the parent classast.NodeVisitor. This is the subclass that we create in line 4. -
We override the predefined
visit_Await,visit_Call, andvisit_Namemethods in the parent class, which receive theAwait,Call, andNamenodes, respectively. -
In these methods, we print the type and the fields inside the node and call the
generic_visit()method to visit the children nodes of the input node. -
We initialize a visitor object of the class
AwaitVisitorin line 18. -
We write a definition of a Python asynchronous function and send it to the
ast.parse()method, which returns the result of the expression after evaluation, and then stores this result intree. -
The
ast.dump()method returns a formatted string of the tree structure intree. You can observe the string returned by thedumpfunction in the output of the code. -
Then we call the inbuilt
visitmethod available to thevisitorobject, which visits all the nodes in the tree structure.You can observe how the visitor visits each node in the AST in the code output.
