Optimizing methods away or not?

Marc 'BlackJack' Rintsch bj_666 at gmx.net
Sun Dec 14 10:19:45 CET 2008


On Sun, 14 Dec 2008 07:41:55 +0000, Steven D'Aprano wrote:

> I have a class with a method meant to verify internal program logic (not
> data supplied by the caller). Because it is time-consuming but optional,
> I treat it as a complex assertion statement, and optimize it away if
> __debug__ is false:
> 
> class Parrot:
>     def __init__(self, *args):
>         print "Initialising instance..."
>         if __debug__:
>             self.verify()  # check internal program state, not args
>     if __debug__:
>         def verify(self):
>             print "Verifying..."
> 
> 
> If I run Python normally, I can do this:
> 
>>>> p = Parrot()
> Initialising instance...
> Verifying...
>>>> p.verify()
> Verifying...
> 
> 
> and if I run Python with the -O flag, I get this:
> 
>>>> p = Parrot()
> Initialising instance...
>>>> p.verify()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: Parrot instance has no attribute 'verify'
> 
> 
> This is the behaviour I want, but I haven't seen it before in other
> code. What do people think? Is it a good idea or a bad?
> 
> If you think it is a bad idea to have verify disappear under
> optimization, would you change your mind if it were called __verify
> instead?
> 
> 
> One possible alternative is to do something like this:
> 
> class Parrot:
>     def __init__(self, *args):
>         print "Initialising instance..."
>         if __debug__:
>             self.verify()
>     def verify(self):
>         if __debug__:
>             print "Verifying..."
>             return None
>         # this is optional
>         else:
>             warnings.warn("verify() is a null op")
> 
> 
> which now means that Parrot instances will always have a verify method,
> even if it does nothing. I'm not sure I like that. What do others think?
> Which do you consider better design?

None of it.  Why not simply:

class Parrot:
    def __init__(self, *args):
        print "Initialising instance..."
        assert self.verify()
    
    def _verify(self):
        print "Verifying..."
        return None

If you compile with -O the ``assert`` is optimized away.  But you still 
can call `_verify()` at specific points even in optimized code if you 
want or need.

Ciao,
	Marc 'BlackJack' Rintsch



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