How to catch custom exception in Python [duplicate]

Question

I'm using a python library in which at one point an exception is defined as follows:

raise Exception("Key empty")

I now want to be able to catch that specific exception, but I'm not sure how to do that.

I tried the following

try:
    raise Exception('Key empty')
except Exception('Key empty'):
    print 'caught the specific exception'
except Exception:
    print 'caught the general exception'

but that just prints out caught the general exception.

Does anybody know how I can catch that specific Key empty exception? All tips are welcome!

Answer 1

Define your exception:

class KeyEmptyException(Exception):
    def __init__(self, message='Key Empty'):
        # Call the base class constructor with the parameters it needs
        super(KeyEmptyException, self).__init__(message)

Use it:

try:
    raise KeyEmptyException()
except KeyEmptyException as e:
    print e

Update: based on the discussion in comment OP posted:

But the lib is not under my control. It's open source, so I can edit it, but I would preferably try to catch it without editing the library. Is that not possible?

say library raises an exception as

# this try is just for demonstration 
try:

    try:
        # call your library code that can raise `Key empty` Exception
        raise Exception('Key empty')
    except Exception as e:
        # if exception occurs, we will check if its 
        # `Key empty` and raise our own exception
        if str(e) == 'Key empty':
            raise KeyEmptyException()
        else:
            # else raise the same exception
            raise e
except Exception as e:
    # we will finally check what exception we are getting
    print('Caught Exception', e)