I'm trying to make my class appear as a different object to circumvent lazy type checking in a package I'm using. More specifically, I'm trying to make my object appear as an instance of another object (tuple
in my case) when in reality it is not even a derivation of that.
In order to achieve this, I plan to overwrite the __isinstance__
method which, according to the docs, should do exactly what I desire. However, it appears that I didn't understand how do to do that exactly, because my attempts have been unsuccessful.
Here's an SSCCE that should make isinstance
return False
in all cases but doesn't.
class FalseInstance(type):
def __instancecheck__(self, instance):
return False
class Foo(metaclass=FalseInstance):
pass
g = Foo()
isinstance(g, Foo)
> True
What am I doing wrong?
Aside from the issues with __metaclass__
and the fast-path for an exact type match, __instancecheck__
works in the opposite direction from what you're trying to do. A class's __instancecheck__
checks whether other objects are considered virtual instances of that class, not whether instances of that class are considered virtual instances of other classes.
If you want your objects to lie about their type in isinstance
checks (you really shouldn't), then the way to do that is to lie about __class__
, not implement __instancecheck__
.
class BadIdea(object):
@property
def __class__(self):
return tuple
print(isinstance(BadIdea(), tuple)) # prints True
Incidentally, if you want to get the actual type of an object, use type
rather than checking __class__
or isinstance
.
If you add a print inside FalseInstance.__instancecheck__
you will see that it is not even invoked. However, if you call isinstance('str', Foo)
then you'll see that FalseInstance.__instancecheck__
does get invoked.
This is due to an optimization in isinstance
's implementation that immediately returns True
if type(obj) == given_class
:
int
PyObject_IsInstance(PyObject *inst, PyObject *cls)
{
_Py_IDENTIFIER(__instancecheck__);
PyObject *checker;
/* Quick test for an exact match */
if (Py_TYPE(inst) == (PyTypeObject *)cls)
return 1;
.
.
.
}
来源:https://stackoverflow.com/questions/52168971/instancecheck-overwrite-shows-no-effect-what-am-i-doing-wrong