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 -- http://mail.python.org/mailman/listinfo/python-list
