Clarity vs. code reuse/generality

[Terry Reedy]

Steven D'Aprano wrote:
> On Fri, 10 Jul 2009 12:27:25 -0400, Charles Yeomans wrote:
> 
>>> (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.
> 
> So is design-by-contract just another way of saying "let's hope the data 
> is valid, because if it's not, we're screwed"?
> 
> Perhaps Python should have a new statement, `assume`, used just like 
> `assert` except it never actually performs the test or raises an error.

The manual says quite clearly

"The simple form, assert expression, is equivalent to
if __debug__:
    if not expression: raise AssertionError"

It should be used when and only when one actually wants the double 
condition check, and not as an abbreviation of the second conditional only.

tjr