ValueError: malformed string when using ast.literal_eval

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It is widely known that using eval() is a potential security risk so the use of ast.literal_eval(node_or_string) is promoted

However In python 2.7 it returns ValueError: malformed string when running this example:

>>> ast.literal_eval("4 + 9")

Whereas in python 3.3 this example works as expected:

>>> ast.literal_eval('4+9')
13

Why does it run on python 3 and not python 2? How can I fix it in python 2.7 without using the risky eval() function?

python-2.7 python-3.x python
5 Answers

An updated version of the answer from @poke that allows negative numbers in py3.x or other unary operators. So "-3" evaluates to -3 for example, rather than an error.

import ast, operator

binOps = {
ast.Add: operator.add,
ast.Sub: operator.sub,
ast.Mult: operator.mul,
ast.Div: operator.truediv,
ast.Mod: operator.mod
}

unOps = {
ast.USub: operator.neg
}

node = ast.parse(s, mode='eval')

def arithmetic_eval(s):
    binOps = {
    ast.Add: operator.add,
    ast.Sub: operator.sub,
    ast.Mult: operator.mul,
    ast.Div: operator.truediv,
    ast.Mod: operator.mod
    }

    unOps = {
    ast.USub: operator.neg
    }

    node = ast.parse(s, mode='eval')

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

    return _eval(node.body)
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