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What is ast.While(test, body, orelse) in Python?
Abdul Monum
ast.While is a class defined in the ast module that expresses the while loop in Python in the form of an Abstract Syntax Tree.
When the parse() method of ast is called on a Python source code that contains a while loop, the ast.While class is invoked, which expresses the try statement to a node in an ast tree data structure. The ast.While class represents the While node type in the ast tree.
Class parameters
test: Contains the condition of the loop, such as aComparenode.body: List of nodes in the body of awhileloop.orelse: List of nodes in the body ofelsenodes.
Example
import ast
from pprint import pprint
class WhileVisitor(ast.NodeVisitor):
def visit_While(self, node):
print('Node type: While\nFields:', node._fields)
ast.NodeVisitor.generic_visit(self, node)
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_Pass(self, node):
print('Node type: Pass\nFields:', node._fields)
ast.NodeVisitor.generic_visit(self, node)
visitor = WhileVisitor()
tree = ast.parse("""
while (a < 10):
pass
else:
...
""")
pprint(ast.dump(tree))
visitor.visit(tree)
Explanation
- We define a
WhileVisitorclass that extends from the parent classast.NodeVisitor. We override the predefinedvisit_While,visit_Compare,visit_Name, andvisit_Passmethods in the parent class, which receive theWhile,Compare,Name, andPassnodes, respectively. - In the method, we print the type and the fields inside the node and call the
generic_visit()method, which invokes the propagation ofvisiton the children nodes of the input node. - We initialize a visitor object of the class
WhileVisitor. - We write Python code that contains a
tryandexceptblock. We pass the code to theast.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 a tree. - The
visitmethod available to the visitor object visits all the nodes in the tree structure.
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