Safe evaluation of mathematical expressions in Python.

safe_eval.py

import ast
import operator as op


DEFINED_OPS = {
    ast.Add: op.add,
    ast.Sub: op.sub,
    ast.Mult: op.mul,
    ast.Div: op.truediv,
    ast.USub: op.neg,
}

def safe_eval(node):
    if isinstance(node, ast.Num):
        return node.n
    elif isinstance(node, ast.BinOp):
        left = safe_eval(node.left)
        right = safe_eval(node.right)
        return DEFINED_OPS[type(node.op)](left, right)
    elif isinstance(node, ast.UnaryOp):
        return DEFINED_OPS[type(node.op)](safe_eval(node.operand))
    else:
        print(type(node))
        raise TypeError(node)

def eval_expr(exp:str):
    return safe_eval(ast.parse(exp, mode='eval').body)


if __name__ == '__main__':
    assert eval_expr('1 + 1') == 2
    assert eval_expr('1 + 1 * 5') == 6
    assert eval_expr('(1 + 1) * 5') == 10
    assert eval_expr('((1 + 1) + 2) * 5') == 20
    assert eval_expr('((1 + - 1) + 2) * 5') == 10