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What is ast.Assert(test, msg) in Python?

Abdul Monum
Abstract Syntax Tree

ast.Assert(test, msg) is a class defined in the ast module that is used to express the assert 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.Assert class is invoked, which expresses the assert statement to a node in an ast tree data structure.

The ast.Assert class represents the Assert node type in the ast tree. In the class parameter, test contains the condition on which the assert statement is called, and msg contains any error messages as a result of the assert statement.

Example

import ast
from pprint import pprint

class AssertVisitor(ast.NodeVisitor):

    def visit_Assert(self, node):
        print('Node type: Assert\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)
    
    def visit_Compare(self, node):
        print('Node type: Compare\nFields:', node._fields)
        ast.NodeVisitor.generic_visit(self, node)



visitor = AssertVisitor()
tree = ast.parse('assert x==y')
pprint(ast.dump(tree))
visitor.visit(tree)

Explanation

  • We define an AssertVisitor class that extends from the parent class ast.NodeVisitor. We override the predefined visit_Assert, visit_Compare, and visit_Name methods in the parent class, which receive the Assert, Compare, 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 AssertVisitor.
  • We write the Python statement assert x==y and send it to the ast.parse() method, which returns the result of the expression after evaluation, and then stores 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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