How can I reduce code duplication when using inheritance in relation to exceptions?

Question

I have a class.

class IncorrectOfferDetailsOfferException(Exception):
    def __init__(self, offer):
        self.offerName = offer[0]
        self.offerType = offer[1]
        self.message = "Offer \"" + self.offerName + "\" could not be completed. It appears to be of type \"" + self.offerType + "\", but this may be what is wrong and causing the exception."
        super(IncorrectOfferDetailsOfferException, self).__init__(self.message)

I want to write a more general class to expand it.

class BadOfferException(Exception):
    def __init__(self, offer):
        self.offerName = offer[0]
        self.offerType = offer[1]
        self.message = "Offer \"" + self.offerName + "\" caused a problem."

How can I relate those two together to remove the redundant code and override the more general message text with the more specific one? You know, class inheritance. I'm having a lot of trouble understanding how to use super the right way to do this.

First, for inheritance, the key question isn't how to share code, it's whether one is a subtype of the other. If you do except BadOfferException:, should that catch an IncorrectOfferDetailsOfferException? If so, then make IncorrectOfferDetailsOfferException a subclass of BadOfferException. If not, there are other ways to share code—a mixin class, a class decorator, a helper function… So, decide that first, and then we can help you with the next step. — abarnert
– abarnert, Commented Mar 22, 2018 at 4:34
@abarnert IncorrectOfferDetailsOfferException should be a subclass of BadOfferException. I don't understand how to do that and use the former to summon a specialized version of the latter. — Zeke Marffy
– Zeke Marffy, Commented Mar 22, 2018 at 4:47

salparadise · Accepted Answer · 2018-03-22 04:55:29Z

2

How about a method that is overriden by the sub-type:

class BadOfferException(Exception):
    def __init__(self, offer):
        self.offerName = offer[0]
        self.offerType = offer[1]
        self.message = self._construct_message()
        super(BadOfferException, self).__init__(self.message)

    def _construct_message(self):
        return 'Offer "{}" caused a problem'.format(self.offerName)


class IncorrectOfferDetailsOfferException(BadOfferException):
    def _construct_message(self):
        return 'Offer "{}" could not be completed. It appears to be of type "{}", but this may be what is wrong and causing the exception.'.format(self.offerName, self.offerType)

Then when you raise IncorrectOfferDetailsOfferException it just "works"

answered Mar 22, 2018 at 4:55

salparadise

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1 Comment

Zeke Marffy Over a year ago

I love this. Seems clever and really does get the job done. Thank you. I understand this.

salparadise · Accepted Answer · 2018-03-22 05:03:01Z

2

First, the way you make a subclass is through the base class list at the top:

class BadOfferException(Exception):
    # etc.

class IncorrectOfferDetailsOfferException(BadOfferException):
    # etc.

I just replaced Exception with BadOfferException. (We don't need both, because BadOfferException is already a subclass of Exception) And you have to declare the superclass first, and the subclass afterward, so I switched the order.

Now, you'll want to make the implementation of the superclass to be a little more generic, so the subclass can override only the part it cares about. The only thing that's different is the message, right? So allow that to be passed in:

class BadOfferException(Exception):
    def __init__(self, offer, message=None):
        self.offerName = offer[0]
        self.offerType = offer[1]
        if message is None:
            message = "Offer \"" + self.offerName + "\" caused a problem."
        self.message = message

And now, that's all you have to pass via super. That means we can't use offerName and offerType, since they haven't been defined yet, but that isn't a problem here:

class IncorrectOfferDetailsOfferException(BadOfferException):
    def __init__(self, offer):
        message = "Offer \"" + offer[0] + "\" could not be completed. It appears to be of type \"" + offer[1] + "\", but this may be what is wrong and causing the exception."
        super(IncorrectOfferDetailsOfferException, self).__init__(offer, message)

That's all you need to do.

But while we're at it, you might want to make BadOfferException into a nicer subclass of Exception. People expect to be able to see the args on an Exception instance—it's not mandatory; you can leave it off and it'll just be an empty tuple, but it is nice. You can do that by calling super(BadOfferException, self).__init__(offer).

edited Mar 22, 2018 at 5:03

salparadise

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answered Mar 22, 2018 at 4:55

abarnert

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2 Comments

Zeke Marffy Over a year ago

This is a clean solution.

abarnert Over a year ago

@SmallBlueGarbageCollector I think salparadise's is cleaner, and more powerful, for many use cases. This is the simplest solution, but it's not necessarily the best.

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