How to check if an object is created with `with` statement?

Question

I would like to ensure that the class is only instantiated within a \"with\" statement.

i.e. this one is ok:

with X() as x:
 ...

an

Answer 1

OP's question was believed to be an XY problem, and the current chosen answer was indeed (too?) hacky.

I don't really know the OP's original "X problem", but I'd assume the motivation was NOT literally about to "prevent x = X() ASSIGNMENT from working". Instead, it could be about to force the API user to always use x as a context manager, so that its __exit__(...) would always be triggered, which is the whole point of designing class X to be a context manager in the first place. At least, that was the reason brought me to this Q&A post.

class Holder(object):
    def __init__(self, **kwargs):
        self._data = allocate(...)  # Say, it allocates 1 GB of memory, or a long-lived connection, etc.
    def do_something(self):
        do_something_with(self._data)
    def tear_down(self):
        unallocate(self._data)

    def __enter__(self):
        return self
    def __exit__(self, *args):
        self.tear_down()

# This is desirable
with Holder(...) as holder:
    holder.do_something()

# This might not free the resource immediately, if at all
def foo():
    holder = Holder(...)
    holder.do_something()

That said, after learning all the conversations here, I ended up just leave my Holder class as-is, well, I just added one more docstring for my tear_down():

    def tear_down(self):
        """You are expect to call this eventually; or you can simply use this class as a context manager."""
        ...

After all, we are all consenting adults here...