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:
 ...

and this is not:

x = X()

How can I ensure such functionality?

Answer 1

All answers so far do not provide what (I think) OP wants directly.
(I think) OP wants something like this:

>>> with X() as x:
 ...  # ok

>>> x = X()  # ERROR

Traceback (most recent call last):
  File "run.py", line 18, in <module>
    x = X()
  File "run.py", line 9, in __init__
    raise Exception("Should only be used with `with`")
Exception: Should only be used with `with`

This is what I come up with, it may not be very robust, but I think it's closest to OP's intention.

import inspect
import linecache

class X():
    
    def __init__(self):
        if not linecache.getline(__file__,
            inspect.getlineno(inspect.currentframe().f_back)).lstrip(
        ).startswith("with "):
            raise Exception("Should only be used with `with`")

    def __enter__(self):
        return self
    
    def __exit__(self, *exc_info):
        pass

This will give the exact same output as I showed above as long as with is in the same line with X() when using context manager.