Question

I want to create a class in python, which should work like this:

1. Data assigned, maybe bound to a variable (eg a = exampleclass(data) or just exampleclass(data))

2. Upon being inserted data, it should automatically determine some properties of the data, and if some certain properties are fullfilled, it will automatically…

3. … change class to another class

The part 3 is the part that i have problem with. How do i really change the class inside of the class? for example:

If I have two classes, one is Small_Numbers, and the other is Big_numbers; now I want any small_number smaller than 1000 to be transferred into a Big_number and vice versa, testcode:

a = Small_number(50)
type(a) # should return Small_number.
b = Small_number(234234)
type(b) # should return Big_number.
c = Big_number(2)
type(c) # should return Small_number.

Is this possible to do?

Answer 1

Using a factory method is the usual way to solve this, especially since instantiating a class is indistinguishable from calling a function in Python.

However, if you really want, you can assign to self.__class__:

THRESHOLD = 1000

class Small(object):
    def __init__(self, n):
        if n < THRESHOLD:
            self.n = n
        else:
            self.__class__ = Big
            self.__init__(n)

class Big(object):
    def __init__(self, n):
        if n < THRESHOLD:
            self.__class__ = Small
            self.__init__(n)
        else:
            self.n = n

This works as expected:

>>> a = Small(100)
>>> type(a)
<class 'Small'>
>>> b = Small(1234)
>>> type(b)
<class 'Big'>
>>> c = Big(2)
>>> type(c)
<class 'Small'>

If assigning to self.__class__ seems too strange, then you can override __new__ instead. This method is called before __init__ is called and it can be used to pick the class to instantiate:

THRESHOLD = 1000

class Switcher(object):
    def __new__(cls, n):
        if n < THRESHOLD:
            new_cls = Small
        else:
            new_cls = Big
        instance = super(Switcher, new_cls).__new__(new_cls, n)
        if new_cls != cls:
            instance.__init__(n)
        return instance

class Small(Switcher):
    def __init__(self, n):
        self.n = n

class Big(Switcher):
    def __init__(self, n):
        self.n = n