Python dynamic multiple inheritance

Question

I want to choose from which class I inherit at runtime, either class A or B, depending on an argument to my init function in AorB. I already tried the following code, but the methods are not overloaded the way I want them to be overloaded: AorB("B").a() returns A.a() instead of B.a(). How do I choose from which class I inherit at runtime?

Update: From the reaction below I tried the following code. Now I want to inherit AorB in class C, which doesn't work yet:

class A(object):
    def a(self):
        return "I'm A.a"

    def b(self):
        return "I'm A.b"


class B(object):
    def a(self):
        return "I'm B.a"

    def c(self):
        return "I'm B.c"


def AorB(classname, cache={}):
    if not classname in cache:
        Base = globals()[classname]

        class AorB(Base):
            def __init__(self):
                print(classname)
                Base.__init__(self)
        cache[classname] = AorB
    return cache[classname]()

class C(AorB):                                                                  
    def __init__(self, classname):                                             
        AorB.__init__(classname)

if __name__ == "__main__":
    a = AorB("A")
    print("A.a:", a.a())
    print("A.b:", a.b())

    b = AorB("B")
    print("B.a:", b.a())
    print("B.c:", b.c())

    c = C("B")
    print("C.a:", c.a())
    print("C.c:", c.c())

yields

Traceback (most recent call last):
  File "classtest.py", line 28, in <module>
    class C(AorB):
TypeError: Error when calling the metaclass bases
    function() argument 1 must be code, not str

instead of:

A
('A.a:', "I'm A.a")
('A.b:', "I'm A.b")
B
('B.a:', "I'm B.a")
('B.c:', "I'm B.c")
B
('C.a:', "I'm B.a")
('C.c:', "I'm B.c")

Why not have them both and then decide at run-time? or remove the reason for inheritance and have all functions/methods have A/B sides? — Inbar Rose
– Inbar Rose, Commented Mar 5, 2013 at 14:15
I want to make connection to a device, which is possible via different connection types. The commands send to the device are identical for all connection types, however, the write and read functions differ (also some other functions). I would like to create a device class like: calculator = Calculator("IP","10.0.0.1"), or calculator = Calculator("USB","/dev/ttyUSB0") — Vasco
– Vasco, Commented Mar 5, 2013 at 15:19

unutbu · Accepted Answer · 2013-03-05 15:39:48Z

6

class A(object):
    def a(self):
        return "I'm A.a"

    def b(self):
        return "I'm A.b"


class B(object):
    def a(self):
        return "I'm B.a"

    def c(self):
        return "I'm B.c"


def make_AorB(Base, classname):
    class AorB(Base):
        def __init__(self):
            print(classname)
            Base.__init__(self)
    return AorB


def make_C(Base, classname):
    class C(Base):
        def __init__(self):
            Base.__init__(self)

        def d(self):
            return "I'm C.d"
    return C


def make_factory(getbase, make_cls):
    def factory(classname):
        if not classname in factory.cache:
            Base = getbase(classname)
            factory.cache[classname] = make_cls(Base, classname)
        return factory.cache[classname]()
    factory.cache = {}
    return factory


AorB = make_factory(lambda classname: globals()[classname], make_AorB)
C = make_factory(lambda classname: AorB.cache[classname], make_C)

if __name__ == "__main__":
    a = AorB("A")
    print(a.__class__, a.__class__.__bases__)
    print("A.a:", a.a())
    print("A.b:", a.b())

    b = AorB("B")
    print(b.__class__, b.__class__.__bases__)
    print("B.a:", b.a())
    print("B.c:", b.c())

    c = C("B")
    print(c.__class__, c.__class__.__bases__)
    print("C.a:", c.a())
    print("C.c:", c.c())
    print("C.d:", c.d())

yields

A
(<class '__main__.AorB'>, (<class '__main__.A'>,))
('A.a:', "I'm A.a")
('A.b:', "I'm A.b")
B
(<class '__main__.AorB'>, (<class '__main__.B'>,))
('B.a:', "I'm B.a")
('B.c:', "I'm B.c")
B
(<class '__main__.C'>, (<class '__main__.AorB'>,))
('C.a:', "I'm B.a")
('C.c:', "I'm B.c")
('C.d:', "I'm C.d")

edited Mar 5, 2013 at 15:39

answered Mar 5, 2013 at 11:18

unutbu

886k197 gold badges1.9k silver badges1.7k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Vasco Over a year ago

Thanks, for which purpose is the 'cache' part?

Vasco Over a year ago

How can I inherit the AorB class? See update in original post

unutbu Over a year ago

The purpose of the cache is to reuse the same classes if they have already been created. For example, if you say a = AorB("A") and then a2 = AorB("A"), then a and a2 will be instances of the same class. If there were no cache, then a and a2 would be instances of different classes (although both classes would be named AorB.

Jon Clements · Accepted Answer · 2013-03-05 13:53:32Z

0

I'm not 100% sure (of your use-case), but you may be able to use the variation of type to do something like the following (where something is some conditional):

def produce_C(kls, *args, **kwdargs):
    return type('C', (globals()[kls],), {})(*args, **kwdargs)

Which is going to confuse the type system though... (possibly amend the class name to be C_from_A or C_from_B - but ugh)

edited Mar 5, 2013 at 13:53

answered Mar 5, 2013 at 13:42

Jon Clements

143k34 gold badges254 silver badges288 bronze badges

Comments

jsbueno · Accepted Answer · 2013-03-06 13:24:22Z

0

Here is the problem with your code (after the update) C can't inherit from AorB, which is a function:

class C(AorB):
    def __init__(self, classname):
        AorB.__init__(classname)

Since you want to bass the baseclass in a call to C, you can just make C a function that calls AorB in turn:

def C(basename):
    return AorB(basename)

answered Mar 6, 2013 at 13:24

jsbueno

113k11 gold badges159 silver badges239 bronze badges

Collectives™ on Stack Overflow

Python dynamic multiple inheritance

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related