Clarity vs. code reuse/generality

Charles Yeomans charles at declareSub.com
Fri Jul 10 18:27:25 CEST 2009


On Jul 9, 2009, at 10:57 PM, Steven D'Aprano wrote:

> On Fri, 10 Jul 2009 03:28:04 +0100, Nobody wrote:
>
>> On Thu, 09 Jul 2009 04:57:15 -0300, Gabriel Genellina wrote:
>>
>>> Nobody says you shouldn't check your data. Only that "assert" is not
>>> the right way to do that.
>>
>> "assert" is not the right way to check your *inputs*. It's a  
>> perfectly
>> reasonable way to check data which "should" be valid, as well as a  
>> way
>> to document what variables are supposed to contain.
>
> Where are those variables coming from?
>
> The distinction really boils down to this:
>
> * asserts should never fail. If there is any chance that an assertion
> might fail outside of test suites, then don't use assert.
>
>
> You can't control what input the caller provides to a function, so  
> unless
> the data is generated inside the function, a caller might provide
> something unexpected. Therefore, assert is inappropriate for checking
> function parameters, even if that function is "private" and only  
> called
> by your own functions.
>
> (1) So-called "private" or "internal" functions have a habit of  
> becoming
> public, and then you have asserts in public functions.
>
> (2) If public() calls _private(x), and _private uses assert to check  
> the
> value of x, there is a risk that if public() is buggy and supplies an
> invalid x, the assertion will never be checked and _private() will  
> return
> incorrect results.
>
> (3) assert is absolutely unsuitable for enforcing pre-conditions and  
> post-
> conditions, unless such conditions are mere "guidelines", because  
> assert
> can be switched off at runtime.


Unless, of course, you want to switch off such checking at runtime, as  
you might when using a design-by-contract approach.

Charles Yeomans



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