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 an error in the __new__ method:

class a():
    def __new__(cls, x):
        if x == True:
            return cls
        else:
            raise ValueError

This is what I was hoping would happen:

>>obj1 = a(True)
>>obj2 = a(False)
ValueError Traceback (most recent call last)

obj1 exists but obj2 doesn't.

Any ideas?

@matt b. It's not as semantic if then intent is to stop object creation. It does work though. — aaronasterling
– aaronasterling, Commented Oct 5, 2010 at 21:28
@AaronMcSmooth: Why would raising an exception in __new__ be preferable to raising one in __init__. The result seems the same to me and __init__ is where all the other initialization code goes. Why shouldn't this go there as well? — Arlaharen
– Arlaharen, Commented Oct 5, 2010 at 22:04
@Arlaharen. because it's not initilization code. It's conditional construction code. It seems like __init__ should only throw an exception if it runs into trouble initializing the instance. Here, it's an explicit test that's being done. I'm probably just quibbling though. I would say that if you want to do it in the initializer, just try to use whatever x is and let that raise an exception or go through. — aaronasterling
– aaronasterling, Commented Oct 5, 2010 at 22:38
I really think that the advice one would want to give in general is to treat __init__ as the constructor. If the construction of the object needs to be aborted, for any reason, it works just as well to raise an exception there. Overriding __new__ is very rarely needed and for everyday normal classes I think it should be avoided. Doing so keeps the class implementation simple. — Arlaharen
– Arlaharen, Commented Oct 6, 2010 at 14:06

aaronasterling · Accepted Answer · 2010-10-05 21:14:53Z

23

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 "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in __new__
ValueError
>>> obj1
<__main__.Test object at 0xb7738b2c>
>>> obj2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
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.

answered Oct 5, 2010 at 21:14

aaronasterling

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1 Comment

mertyildiran Over a year ago

If I need x variable too, what should I do?

aaronasterling · Accepted Answer · 2010-10-05 21:35:40Z

9

Just raise an exception in the initializer:

class a(object):
    def __init__(self, x):
        if not x:
            raise Exception()

edited Oct 5, 2010 at 21:35

aaronasterling

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answered Oct 5, 2010 at 21:12

Arlaharen

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2 Comments

Daenyth Over a year ago

__init__ is not the constructor, it is the initializer. You're thinking of __new__

kdubs Over a year ago

what's wrong with doing this in the initializer?

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