how to decide whether or not we want to create instance in python

1def createInstance():
2    # Do what ever you want to determine if instance can be created
3    return True 
4
5class CustomizeInstance(object):
6    def __new__(cls, a, b):
7        if not createInstance():
8            raise RuntimeError, "Count not create instance"
9        instance = super(CustomizeInstance, cls).__new__(cls, a, b)
10        instance.a = a
11        return instance
12
13    def __init__(self, a, b):
14        pass