Use literal_eval instead of eval

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import ast
import operator

valid_ops = {
    ast.Add: operator.add,
    ast.Sub: operator.sub,
    ast.Mult: operator.mul,
    ast.Div: operator.truediv,
    ast.USub: operator.neg,  # negative number
}

def safe_eval(expr):
    """Use `ast.literal_eval` instead of `eval`
    via: https://stackoverflow.com/a/20748308
    """

    def _eval(node):
        if isinstance(node, ast.Expression):
            return _eval(node.body)
        if isinstance(node, ast.Str):
            return node.s
        if isinstance(node, ast.Num):
            return node.n
        if isinstance(node, ast.UnaryOp):
            return valid_ops[type(node.op)](node.operand.n)
        if isinstance(node, ast.BinOp):
            return valid_ops[type(node.op)](_eval(node.left), _eval(node.right))
        raise TypeError('Unsupported type {}'.format(node))

    return _eval(ast.parse(expr, mode='eval').body)

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safe_eval('1 + 2 / 3')
Out[3]: 1.6666666666666665
safe_eval('(1 + 2) / 3')
Out[4]: 1.0
safe_eval('-1')
Out[5]: -1