Question

I implemented a python class for which the instances are stored in a h5-file. As such, the __init__()-function of this class first checks if this h5-file yet exists from a previous simulations, and if so raises a raw_input asking to overwrite this file (whereafter the file is overwritten by the new instance) or not, whereafter an error is raised.

I am looking for a way that, when this file (and thus instance) already exists and this is noted in the __init__()-method, i don’t raise an Error whereafter the script stops, buta way to “cancel” the started instantiation and the script just continues … Is there a clean way to implement such “cancel and ocntinue” ?

Answer 1

The right way to do it is to raise an exception. By default, the exception will propagate out from the __init__ to whoever called it, and eventually all the way to the top, halting the script.

But if you want to handle that exception and continue, use a try/catch block at whatever level you want to continue from, as described in Handling Exceptions.

For example:

class ThingyAlreadyExistsError(RuntimeError):
    pass

class Thingy(object):
    def __init__(self, pathname):
        if os.path.exists(pathname):
            yn = raw_input('{} already exists. Overwrite (y/N)?'.format(pathname))
            if yn.lower != 'y':
                raise ThingyAlreadyExistsError(pathname)
        # finish initialization

thingies = []
for pathname in pathnames:
    try:
        thingy = Thingy(pathname)
    except ThingyAlreadyExistsError:
        continue
    thingies.append(thingy)

If you want to catch this before even getting to the __init__, you could always do the check in the __new__ method, or in a @classmethod factory function, or in the for loop, in which case you don’t even need an exception; just don’t initialize. But there’s nothing stopping you from raising an exception inside __init__.