[Python-ideas] Allow "assigning" to Ellipse to discard values.

[Neil Girdhar]

Is this true?  I thought that b can be immediately deleted early since it 
can no longer be referenced.  C++ has guaranteed object lifetimes.  I 
didn't think Python was the same way.

On Tuesday, February 10, 2015 at 4:52:10 PM UTC-5, Chris Kaynor wrote:
>
> On Tue, Feb 10, 2015 at 1:33 PM,  <rand... at fastmail.us <javascript:>> 
> wrote: 
> > Is cpython capable of noticing that the variable is never referenced (or 
> > is only assigned) after this point and dropping it early? Might be a 
> > good improvement to add if not. 
>
> I'm not sure that such would be viable in the general case, as 
> assignment of classes could have side-effects. Consider the following 
> code (untested): 
>
> class MyClass: 
>     def __init__(self): 
>         global a 
>         a = 1 
>     def __del__(self): 
>         global a 
>         a = 2 
>
> def myFunction(): 
>     b = MyClass() # Note that b is not referenced anywhere - it is 
> local and not used in the scope or a internal scope. 
>     print(a) 
>
> myFunction() 
>
> Without the optimization (existing behavior), this will print 1. With 
> the optimization, MyClass could be garbage collected early (as it 
> cannot be referenced), and thus the code would print 2 (in CPython, 
> but it could also be a race condition or otherwise undetermined 
> whether 1 or 2 will be printed). 
>
> I'm not saying that it is a good idea to write such a class, but a 
> change to how variables are assigned would be backwards-incompatible, 
> and could result in very odd behavior in somebody's code. 
>
> Chris 
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