all Python 3astterm

ast.expr_context

variables

The abstract class of a name's context: Load (read), Store (write), Del (delete). It is the ctx field of Name, Attribute, Subscript, List, Tuple, Starred.

Syntax

isinstance(node.ctx, ast.Store)

ctx tells assignment from reading without inspecting the parent: in a[i] = b the Subscript itself is Store, while a and i inside it are Load. Walking the tree by ctx easily finds every variable the program changes. The subclasses AugLoad, AugStore and Param still exist, but the parser has not produced them for years.

Examples

import ast
target = ast.parse('a[i] = b').body[0].targets[0]
print(type(target.ctx).__name__, type(target.value.ctx).__name__)  # → Store Load

import ast
tree = ast.parse('x = 1\ny += x\ndel z')
for n in ast.walk(tree):
    if isinstance(n, ast.Name):
        print(n.id, type(n.ctx).__name__)  # → x Store / y Store / z Del / x Load

ast.expr_context

Syntax

Examples

See also