Cancel instance create

Terry Reedy tjreedy at
Sat Sep 6 19:35:23 CEST 2008

Fredrik Lundh wrote:
> Aigars Aigars wrote:
>> I want MyClass to perform some tests and if them fail, I do not want 
>> instance to be created.

If you do not want the instance created at all, you would have to write 
a custom .__new__ method, but that is tricky, something I would have to 
look up how to do, and most likely not needed.  Fredrik's solution below 
is much easier and the one I would use if at all possible.

>> But in code I wrote instance is created and also has parameters, that 
>> it should not have in case of tests failure.
>> Is there a way to perform tests in MyClass.__init__ and set instance 
>> to None without any parameters?
> if you want construction to fail, raise an exception.
>     ...
>     def __init__(self):

At this point, you have an instance of your class bound to local name 
'self'.  Usually, its only individual attributes are (in 3.0, anyway, as 
far as I can tell) .__class__ and an empty .__dict__.  Of course, it 
inherits class and superclass attributes, but it is otherwise blank. 
(The main exception would be if you were inheriting from an immutable 
class that set attributes in .__new__, but then you would not be writing 

>         self.param = "spam"
>         Test = False
>         if Test: # please don't use explicit tests for truth
>             print "Creating instance..."
>         else:
>             raise ValueError("some condition failed")
> (pick an exception class that matches the actual error, or create your 
> own class if necessary)

Once the exception gets disposed of (if not before), the blank instance 
gets unbound from 'self' and since it does not get bound to anything 
else, it becomes eligible for garbage collection.


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