Declaring None type exception

I want to define a custom Exception class for the NoneType error in python inheriting from Exception class. I expect the code below to do such a thing for me but no success. Is there any way to do this?

    class NoneTypeException(Exception):
        pass

    try:
         image = cv2.imread("/path/")
         gray = cv2.cvtColor(image.copy(), cv2.COLOR_BGR2GRAY) #exception should be raised here
         ...

    except NoneTypeException:
        raise NoneTypeException("Image not found") # and should be caught here

I want to the try except block raise the NoneTypeException. Any way?

Solution

While you can declare an exception to represent whatever you want, existing code won't raise it for you. In specific, operations on None already raise some well-defined errors and you cannot replace them. Strictly speaking, you cannot get the desired behaviour.

If you know that a certain section of code is vulnerable to None values, you can catch the generic Exception and raise a specific one. This hinges on the assumption that None is the only bogus value you might get:

class NoneTypeException(Exception):
    pass

try:
     image = cv2.imread("/path/")
     gray = cv2.cvtColor(image.copy(), cv2.COLOR_BGR2GRAY)
     ...
except AttributeError as err:  # handle generic exception from action on ``None``
    raise NoneTypeException("Image not found")

Take note that this is rarely what you should be doing - the None value is a symptom, not the cause of the error. It would be more appropriate to raise a FileNotFoundError.

Since None can trigger errors anywhere, it is easier to protect against it at its origin. Checking for identity with None is very cheap:

try:
    image = cv2.imread("/path/")
    if image is None:  # verify value immediately
        raise NoneTypeException()
    # safely work with image
    gray = cv2.cvtColor(image.copy(), cv2.COLOR_BGR2GRAY)
    ...
except NoneTypeException:
    raise FileNotFoundError('image not found')