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Date: Wed, 18 Jul 2012 12:33:01 -0400
From: Dave Angel <d@davea.name>
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To: Steven D'Aprano <steve+comp.lang.python@pearwood.info>
Subject: Re: Encapsulation, inheritance and polymorphism
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On 07/18/2012 08:58 AM, Steven D'Aprano wrote:
> <SNIP>
>
>
> 2) To check your internal reasoning in a function or method.
>
> For example:
>
> def foo(something):
>     n = len(something)
>     x = math.sqrt(x)
>     # We expect that x must be less than half of n. 
>     # E.g. n=100 gives 10 < 50, which is correct.
>     assert x < n//2
>     process(n, x)
>
>
> <SNIP>
> For bonus points, can you see the mistake? The stated condition is wrong. 
> Without the assert, the process() code could go off and potentially 
> silently do the wrong thing, but the assert guards against that 
> possibility. And once I've had a bug report that my app raises an 
> AssertionError, I will go back and think more carefully about the 
> assertion that sqrt(n) is always less than half of n.
>
>

There are actually two bugs in that function.  One is in the assertion,
but more importantly, there's a typo earlier.  One that would give a
traceback, so it would be caught quickly.

UnboundLocalError: local variable 'x' referenced before assignment

Once you change the argument of sqrt() to n, then you come to the
problem you were expecting;  if n has a value of 1, 2, or 3, 4, or 5 the
assertion will fire.

-- 

DaveA