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What is ast.Raise(exc, cause) in Python?

Abdul Monum
Abstract Syntax Tree

ast.Raise(exc, cause) is a class defined in the ast module that is used to express the raise statement in Python in the form of an Abstract Syntax Treetree representation of source code.

When the parse() method of ast is called on a Python source code that contains the assert keyword, the ast.Raise class is invoked, which expresses the raise statement to a node in an ast tree data structure.

The ast.Raise class represents the Raise node type in the ast tree. In the class parameter, exc contains the exception object that needs to be raised. The cause is an optional parameter when the from keyword is part of the raise statement. For example, when a statement like raise x from y is encountered, then the ast.Raise class is invoked with the cause parameter, which will hold y.

Example

import ast
from pprint import pprint

class RaiseVisitor(ast.NodeVisitor):

    def visit_Raise(self, node):
        print('Node type: Raise\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 = RaiseVisitor()
tree = ast.parse('raise NotImplementedError')
pprint(ast.dump(tree))
visitor.visit(tree)

Explanation

  • We define a RaiseVisitor class that extends from the parent class ast.NodeVisitor. We override the predefined visit_Raise and visit_Name methods in the parent class, which receive the Raise and Name nodes, respectively. In the method, we print the type and the fields inside the node and call the generic_visit() method, which invokes the propagation of visit on the children nodes of the input node.
  • We initialize a visitor object of the class RaiseVisitor.
  • We write the Python statement raise NotImplementedError and send it to the ast.parse() method, which returns the result of the expression after evaluation, then store it in tree.
  • The ast.dump() method returns a formatted string of the tree structure in tree.
  • The visit method available to the visitor object visits all the nodes in the tree structure.

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