What is ast.Dict(keys, values) in Python?

ast.Dict(keys, values) is a class defined in the ast module that is used to express Python dictionaries 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 dictionary, the ast.Dict class is invoked and expresses the dictionary to a node in an ast tree data structure. The ast.Dict class represents the Dict node type in the ast tree. keys and values in the class parameters contain the list of keys and values in the dictionary in their matching order.

Example

import ast
from pprint import pprint
class DictVisitor(ast.NodeVisitor):
    def visit_Dict(self,node):
        print('Node type: Dict\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 = DictVisitor()
tree = ast.parse('{"Name":Ali, "Age":27}', mode='eval')
pprint(ast.dump(tree))
visitor.visit(tree)

Explanation

  • We define a DictVisitor class that extends from the parent class ast.NodeVisitor. We override the predefined visit_Constant and visit_Dict methods in the parent class, which receive the Constant and Dict 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 the visit on the children nodes of the input node.
  • We initialize a visitor object of the class DictVisitor.
  • We define a Python dictionary {"Name":Ali, "Age":27} and send it to the ast.parse() method with mode='eval', which returns the result of the expression after evaluation, and 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 objects visits all the nodes in the tree structure.

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