Untrusted code execution

[Random832]

On Wed, Apr 6, 2016, at 12:04, Chris Angelico wrote:
> On Thu, Apr 7, 2016 at 1:41 AM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> > type might also be a concern since it can be used to assemble
> > arbitrary classes.
> 
> Sadly, this means denying the ability to interrogate an object for its
> type. And no, this won't do:
> 
> def safe_type(obj): return type(obj)
> 
> because all you need is safe_type(safe_type(1)) and you've just
> regained access to the original 'type' type.

tpdict = {}
class typeproxy:
    def __new__(cls, t):
        if t in tpdict: return tpdict[t] # so is-comparison works
        tpdict[t] = self = object.__new__(cls)
        self._type = t
        return self
    def __instancecheck__(self, obj):
        return isinstance(obj, self._type)
    def __subclasscheck__(self, cls2):
        if isinstance(cls2, typeproxy): cls2 = cls2._type
        return issubclass(self._type, cls2)
    def __call__(self, obj):
        if isinstance(obj, type):
            return typeproxy(type(obj))
        else:
            return type(obj)

safe_type = typeproxy(type)