Python AST parser

ast_parser.py

import ast 
import operator as op

# Supported tokens and their operators
operators = { 
    ast.Add: op.add,
    ast.Sub: op.sub,
    ast.Mult: op.mul,
    ast.Div: op.truediv,
}

def evaluate(expr):
    return limited_eval(ast.parse(expr, mode='eval').body)

def limited_eval(node):
    if isinstance(node, ast.Num):  # <number>
        return node.n
    elif isinstance(node, ast.BinOp):  # <left> <operator> <right>
        return operators[type(node.op)](limited_eval(node.left), limited_eval(node.right))
    elif isinstance(node, ast.UnaryOp):  # <operator> <operand> e.g., -1
        return operators[type(node.op)](limited_eval(node.operand))
    else:
        raise TypeError("Operations are limited to a permitted subset", node, operators.keys())


assert evaluate("14 + 2 * 3 - 6 / 2") == 17
assert evaluate("(1 + 2) * 3") == 9
assert evaluate("__import__('os').remove('foobar.file')")