How to get another class based upon type of object?

Question

I have a use case where I want to get another class based upon the type of object.

For one solution, I have created a dict where I am maintaining this relation and using type to match among classes.

For example, consider the below code

class classA(object):
    def __init__(self):
        print "A called"

class classB(object):
    def __init__(self):
        print "B called"

MAP = {
   A: classA,
   B: classB
}

class C(object):
    pass

class A(C):
    pass

class B(C):
    pass

obj can be object of A or B.

new_cls = MAP.get(type(obj), 'None')
new_obj = new_cls()

Is there a better way to achieve this?

Mahmoud Hossam · Answer 1 · 2019-08-12T11:30:55.860

2

why not to:

class C(object):
    pass

class A(C):
    def call:
        print("A")

class B(C):
    def call:
        print("B")
obj.call()

or use the inheritance.

But wait , what if i can't do this and really need to know which class is this??

you can use this obj.__class__.__name__ this will work for old style and new style Classes

edited Aug 12 '19 at 11:30

answered Aug 12 '19 at 11:03

Mahmoud Hossam

139
10

call is just an example, I may want to get some other class reference also – Deepanshu Arora Aug 12 '19 at 11:04
@DeepanshuArora you'll need to show a more elaborate example in your original question so that it can then be more completely answered. – lurker Aug 12 '19 at 11:05
@lurker edited the example. does it makes sense now? – Deepanshu Arora Aug 12 '19 at 11:07

score 1 · Accepted Answer · answered Aug 12 '19 at 12:09

1

Looks like you're trying to reinvent type-based polymorphic dispatch...

I assume you don't have the hand on classes A and B (=> they belong to the stdlib or a third-part one that you don't want to fork), else the very obvious solution would be to simply implement call (or any other method) on those classes directly.

In this context, your solution is actually quite pythonic. The other possible pythonic solution would be using functools.singledispatch, and the last resort evil ugly but-sometimes-a-lifesaver solution to monkeypatch A and B.

answered Aug 12 '19 at 12:09

bruno desthuilliers

75,974
6
88
118

can you check my edited question, does this holds true also if instead of methods, i require different class types to create an object – Deepanshu Arora Aug 12 '19 at 13:19
@DeepanshuArora python has a notion of "callable object" (https://docs.python.org/3/library/functions.html#callable). Functions are callable, methods are callable, classes are callable (that's how you instanciate them), and you can define your own callables. My answer applies to all callable objects. – bruno desthuilliers Aug 12 '19 at 13:27

How to get another class based upon type of object?

2 Answers2