What is ast.Slice(lower, upper, step) in Python?
ast.Slice(lower, upper, step) is a class defined in the ast module that expresses Python slicing in the form of an 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
SliceVisitorclass that extends from the parent classast.NodeVisitor. We override the predefinedvisit_Slice,visit_Subscript, andvisit_Namemethods in the parent class, which receive theSlice,Subscript, andNamenodes, respectively. In the method, we print the type and the fields inside the node and call thegeneric_visit()method, which invokes the propagation of visit on the children nodes of the input node. - We initialize a
visitorobject of the classSliceVisitor. - We write the Python statement
a[2:5]and send it to theast.parse()method, which returns the result of the expression after evaluation, and then store the result intree. - The
ast.dump()method returns a formatted string of the tree structure intree. You can observe the - The
visitmethod available to thevisitorobject visits all the nodes in the tree structure.
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