What is ast.Constant(value) in Python?
ast.Constant is a class defined in the ast module that is used to express constant literals in Python, such as a number or string in the form of an parse() method of ast is called on a Python source code that contains constant literals, the ast.Constant class is invoked and converts the constant literal to a node in an ast tree data structure. The ast.Constant class represents the Constant node type in the ast tree.
Example
import astfrom pprint import pprintclass ConstantVisitor(ast.NodeVisitor):def visit_Constant(self,node):print('Node type: Constant\nFields: ', node._fields)ast.NodeVisitor.generic_visit(self, node)visitor = ConstantVisitor()tree = ast.parse('400', mode='eval')pprint(ast.dump(tree))visitor.visit(tree)
Explanation
- We define a
ConstantVisitorclass that extends from the parent classast.NodeVisitor. We override the predefinedvisit_Constantmethods in the parent class that receive theConstantnode, respectively. In the method, we print the type and the fields inside the node and call thegeneric_visit()method to invoke the propagation of the visit on the children nodes of the input node. - We initialize a
visitorobject of the classConstantVisitor. - We define a Python expression
400and send it to theast.parse()method withmode='eval', which returns the result of the expression after evaluation, and store it intree. - The
ast.dump()method returns a formatted string of the tree structure intree. - The
visitmethod available to thevisitorobjects visits all the nodes in the tree structure.
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