creating Python classes with arbitrarily substituted attribute name

Question

I apologize for not giving this question a better title; the reason that I am posting it is that I don't even have the correct terminology to know what I am looking for.

I have defined a class with an attribute 'spam':

def SpamClass(object):
    def __init__(self, arg):
        self.spam = arg
    def __str__(self):
        return self.spam

I want to create a (sub/sibling?)class that has exactly the same functionality, but with an attribute named 'eggs' instead of 'spam':

def EggsClass(object):
    def __init__(self, arg):
        self.eggs = arg
    def __str__(self):
        return self.eggs

To generalize, how do I create functionally-identical classes with arbitrary attribute names? When the class has complicated behavior, it seems silly to duplicate code.

Update: I agree that this smells like bad design. To clarify, I'm not trying to solve a particular problem in this stupid way. I just want to know how to arbitrarily name the (non-magic) contents of an object's __dict__ while preserving functionality. Consider something like the keys() method for dict-like objects. People create various classes with keys() methods that behave according to convention, and the naming convention is a Good Thing. But the name is arbitrary. How can I make a class with a spam() method that exactly replaces keys() without manually substituting /keys/spam/ in the source?

Overloading __getattr__ and friends to reference the generic attribute seems inelegant and brittle to me. If a subclass reimplements these methods, it must accommodate this behavior. I would rather have it appear to the user that there is simply a base class with a named attribute that can be accessed naively.

Actually, I can think of a plausible use case. Suppose that you want a mixin class that confers a special attribute and some closely related methods that manipulate or depend upon this attribute. A user may want to name this special attribute differently for different classes (to match names in the real-world problem domain or to avoid name collisions) while reusing the underlying behavior.

If the classes are identical, why do you need different attribute names? — Seth
– Seth, Commented Feb 11, 2011 at 7:19

btilly · Accepted Answer · 2011-02-11 07:32:18Z

4

Here is a way to get the effect I think you want.

Define a generic class with a generic attribute name. Then in each sub class follow the advice in http://docs.python.org/reference/datamodel.html#customizing-attribute-access to make the attribute look externally like it is called whatever you want it called.

Your description of what you do feels like it has a "code smell" to me, I'd suggest reviewing your design very carefully to see whether this is really what you want to do. But you can make it work with my suggestion.

answered Feb 11, 2011 at 7:32

btilly

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Comments

Don · Accepted Answer · 2011-02-11 07:50:05Z

1

You can also create a super-class with all common stuff and then sub-classes with specific attributes.

Or even:

def SuperClass(object):
    specific_attribute = 'unset'

    def __init__(self, arg):
        setattr(self, specific_attribute, arg)

    def __str__(self):
        return getattr(self, specific_attribute)


def EggClass(SuperClass):
    specific_attribute = 'eggs'

answered Feb 11, 2011 at 7:50

Don

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Comments

Maxim Sloyko · Accepted Answer · 2011-02-11 08:46:27Z

1

Have you considered not overcomplicating things and just create one class? (since they are identical anyway)

class FoodClass(object):
    def __init__(self, foodname, arg):
        self.attrs = {foodname: arg}
        self.foodname = foodname

    def __str__(self):
        return self.attrs[foodname]

If you want some nice constructors, just create them separately:

def make_eggs(arg):
    return FoodClass('eggs', arg)

def make_spam(arg):
    return FoodClass('spam', arg)

answered Feb 11, 2011 at 8:46

Maxim Sloyko

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1 Comment

chairmanK Over a year ago

Sorry, but I don't think that this will work. Only __init__ has direct access to arg, but other methods also need to access self.arg. Also, polluting self.__dict__ with this extra attrs dictionary seems kludgy.

marw · Accepted Answer · 2011-02-11 08:52:31Z

0

To create attributes during runtime, just add them in self.__dict__['foo'] = 'I'm foo' in the class code.

answered Feb 11, 2011 at 8:52

marw

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3 Comments

detly Over a year ago

This won't work if a class uses slots, or has some other feature that is incompatible with self.__dict__. setattr(self, 'name', ...) is really the equivalent, but it's still going to be a bad idea...

marw Over a year ago

I used the above only once, in a metaclass that created attributes based on the inheritance. Btw, I did't know that a class can be incompatible with setattr.

detly Over a year ago

it can't, but it can be incompatible with self.__dict__ (by incompatible, I really mean "do unexpected things").

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