markmin2html.py 文件源码

def safe_eval(node_or_string, env):
    """
    Safely evaluate an expression node or a string containing a Python
    expression.  The string or node provided may only consist of the following
    Python literal structures: strings, numbers, tuples, lists, dicts, booleans,
    and None.
    """
    _safe_names = {'None': None, 'True': True, 'False': False}
    _safe_names.update(env)
    if isinstance(node_or_string, basestring):
        node_or_string = ast.parse(node_or_string, mode='eval')
    if isinstance(node_or_string, ast.Expression):
        node_or_string = node_or_string.body

    def _convert(node):
        if isinstance(node, ast.Str):
            return node.s
        elif isinstance(node, ast.Num):
            return node.n
        elif isinstance(node, ast.Tuple):
            return tuple(map(_convert, node.elts))
        elif isinstance(node, ast.List):
            return list(map(_convert, node.elts))
        elif isinstance(node, ast.Dict):
            return dict((_convert(k), _convert(v)) for k, v
                        in zip(node.keys, node.values))
        elif isinstance(node, ast.Name):
            if node.id in _safe_names:
                return _safe_names[node.id]
        elif isinstance(node, ast.BinOp) and \
                isinstance(node.op, (ast.Add, ast.Sub)) and \
                isinstance(node.right, ast.Num) and \
                isinstance(node.right.n, complex) and \
                isinstance(node.left, ast.Num) and \
                isinstance(node.left.n, (int, long, float)):
            left = node.left.n
            right = node.right.n
            if isinstance(node.op, ast.Add):
                return left + right
            else:
                return left - right
        raise ValueError('malformed string')

    return _convert(node_or_string)