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What is ast.Compare(left, ops, comparators) in Python?

Abdul Monum
Abstract Syntax Tree

ast.Compare(left, ops, comparators) is a class defined in the ast module that expresses a comparison condition 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 a comparison of two or more Python objects, the ast.Compare class is invoked, which expresses the comparison expression to a node in an ast tree data structure. The ast.Compare class represents the Compare node type in the ast tree. In the class parameter, left contains the first value in the comparison, ops is the list of operators in the comparison expression, and comparators is the list of values after the first value in the comparison.

Example

import ast
from pprint import pprint

class CompareVisitor(ast.NodeVisitor):

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

    
    

visitor = CompareVisitor()
tree = ast.parse('20 <= a < 30')
pprint(ast.dump(tree))
visitor.visit(tree)

Explanation

  • We define a CompareVisitor class that extends from the parent class ast.NodeVisitor. We override the predefined visit_Compare, visit_Constant, and visit_Name methods in the parent class, which receive the Compare, Constant, 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 CompareVisitor.
  • We write the Python statement '20 <= a < 30 and send it to the ast.parse() method, which returns the result of the expression after evaluation, and store the result 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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