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What is ast.Slice(lower, upper, step) in Python?

Abdul Monum
Abstract Syntax Tree

ast.Slice(lower, upper, step) is a class defined in the ast module that expresses Python slicing 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 slice operation, the ast.Slice class is invoked, which expresses the slice operation to a node in an ast tree data structure. The ast.Slice class represents the Slice node type in the ast tree. In the class parameter, lower contains the starting index where the slicing of the object starts, upper contains the index up to which the slicing operation takes place, and step is an optional parameter that indicates the increments to be taken between each index for slicing.

Example

import ast
from pprint import pprint

class SliceVisitor(ast.NodeVisitor):

    def visit_Slice(self, node):
        print('Node type: Slice\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_Subscript(self, node):
        print('Node type: Subscript\nFields:', node._fields)
        ast.NodeVisitor.generic_visit(self, node)
    


visitor = SliceVisitor()
tree = ast.parse('a[2:5]')
pprint(ast.dump(tree))
visitor.visit(tree)

Explanation

  • We define a SliceVisitor class that extends from the parent class ast.NodeVisitor. We override the predefined visit_Slice, visit_Subscript, and visit_Name methods in the parent class, which receive the Slice, Subscript, 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 SliceVisitor.
  • We write the Python statement a[2:5] and send it to the ast.parse() method, which returns the result of the expression after evaluation, and then store the result in tree.
  • The ast.dump() method returns a formatted string of the tree structure in tree. You can observe the
  • The visit method available to the visitor object visits all the nodes in the tree structure.

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