How do I abort object instance creation in Python?

Question

I want to set up a class that will abort during instance creation based on the value of the the argument passed to the class. I\'ve tried a few things, one of them being raising

Answer 1

When you override __new__, dont forget to call to super!

>>> class Test(object):
...     def __new__(cls, x):
...         if x:
...             return super(Test, cls).__new__(cls)
...         else:
...             raise ValueError
... 
>>> obj1 = Test(True)
>>> obj2 = Test(False)
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 6, in __new__
ValueError
>>> obj1
<__main__.Test object at 0xb7738b2c>
>>> obj2
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'obj2' is not defined

Simply returning the class does nothing when it was your job to create an instance. This is what the super class's __new__ method does, so take advantage of it.