What is ast.withitem(context_expr, optional_vars) in Python?
Python ast module
The ast
This module helps applications to process trees of syntax and find out what the current syntax grammar looks like programmatically.
Abstract syntax trees are great tools for program analysis and program transformation systems.
The ast.withitem method
ast.withitem(context_expr, optional_vars) is a class defined in the ast module that expresses a single context in the with block 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 with block, the ast.with and ast.withitem classes are invoked.
Each context expression in the with block is expressed as a withitem node in an ast tree data structure.
The
ast.Withclass represents aWithnode which contains a list ofwithitemnodes.
Parameters
-
context_expr: contains a single context manager in awithblock. -
optional_vars: node representing theas foopart ofwithstatement. Often aName,Tuple, or aListnode. It is aNonenode if there is noas foopart. -
body: list of nodes in the body of thewithblock.
Code
The following Python code illustrates an example of the ast.withitem class.
import ast
from pprint import pprint
class WithItemVisitor(ast.NodeVisitor):
def visit_withitem(self, node):
print('\nNode type: withitem\nFields: ', node._fields)
self.generic_visit(node)
def visit_With(self, node):
print('\nNode type: With\nFields: ', node._fields)
self.generic_visit(node)
def visit_Name(self,node):
print('\nNode type: Name\nFields: ', node._fields)
ast.NodeVisitor.generic_visit(self, node)
visitor = WithItemVisitor()
tree = ast.parse("""
with a as b, c as d:
...
""")
pprint(ast.dump(tree))
visitor.visit(tree)Explanation
-
We define a
WithItemVisitorclass that extends from theast.NodeVisitorparent class. We override the predefinedvisit_withitem,visit_With, andvisit_Namemethods in the parent class, which receive thewithitem,With, andNamenodes, respectively. -
In these methods, we print the type and the fields inside the node and call the
generic_visit()method to visit the children nodes of the input node. -
We initialize a visitor object of the class
WithItemVisitorin line 18. -
We write a Python
withblock and send it to theast.parse()method, which returns the result of the expression after evaluation, and then stores this result intree. -
The
ast.dump()method returns a formatted string of the tree structure intree. You can observe the string returned by thedumpfunction in the output of the code. -
The inbuilt
visitmethod available to thevisitorobject visits all the nodes in the tree structure.
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