Bounds checking

[Steven D'Aprano]

On Fri, 18 Mar 2011 15:35:40 -0700, Martin De Kauwe wrote:

>> Don't check for bounds, fix any bug in the code that would set your
>> values out of bounds and use asserts while debugging.
>>
>>
> whilst that is a nice idea in practice this just is not a practical
> solution.

Sorry, are you trying to say that it is not practical to write correct 
code that isn't buggy? Well, you're honest, at least, still I can't help 
but feel that you're admitting defeat before even starting.

 
>> Otherwise if you really need dynamic checks, it will cost you cpu, for
>> sure.
> 
> Yes I agree and I hadn't decided whether to add it or not as there
> aren't any current issues. However I can see that the check would
> overall be safer. I was just wondering if there was some super smartie
> pants solution :P

Make each of the attributes a computed attribute with a setter that 
raises an exception if you try to store a negative value. That way each 
attribute is responsible for ensuring that itself is never negative.

Here's a toy example:


>>> class Demo(object):
...     def __init__(self, x):
...         self.x = x
...     def _getx(self):
...         return self._x
...     def _setx(self, value):
...         if value < 0:
...             raise ValueError('attempt to set x to negative value')
...         self._x = value
...     x = property(_getx, _setx)
...
>>> d = Demo(-42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __init__
  File "<stdin>", line 8, in _setx
ValueError: attempt to set x to negative value
>>> d = Demo(42)
>>> d.x
42
>>> d.x = -23
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 8, in _setx
ValueError: attempt to set x to negative value
>>> d.x = 23
>>> d.x
23


-- 
Steven