Clarity vs. code reuse/generality

kj at
Thu Jul 9 04:19:25 CEST 2009

In <mailman.2700.1246895025.8015.python-list at> Tim Rowe <digitig at> writes:

>2009/7/4 kj < at>:

>> Precisely. =A0As I've stated elsewhere, this is an internal helper
>> function, to be called only a few times under very well-specified
>> conditions. =A0The assert statements checks that these conditions
>> are as intended. =A0I.e. they are checks against the module writer's
>> programming errors.

>Good for you. I'm convinced that you have used the assertion
>appropriately, and the fact that so many here are unable to see that
>looks to me like a good case for teaching the right use of assertions.
>For what it's worth, I read assertions at the beginning of a procedure
>as part of the specification of the procedure, and I use them there in
>order to document the procedure. An assertion in that position is for
>me a statement to the user of the procedure "it's your responsibility
>to make sure that you never call this procedure in such a way as to
>violate these conditions". They're part of a contract, as somebody
>(maybe you) pointed out.

>As somebody who works in the safety-critical domain, it's refreshing
>to see somebody teaching students to think about the circumstances in
>which a procedure can legitimately be called. The hostility you've
>received to that idea is saddening, and indicative of why there's so
>much buggy software out there.

Thanks for the encouragement.

When I teach programming, the students are scientists.  For the
stuff they do, correctness of the code trumps everything else.  I
teach them to view assertions as way of translating their assumptions
into code.  And by this I mean not only assumptions about the
correctness of their code (the typical scope of assertions), but
also, more broadly, assumptions about the data that they are dealing
with (which often comes from external sources with abysmal quality

My scientific code is jam-packed with assertions.  I can't count
the number of times that one such lowly assertion saved me from a
silent but potentially disastrous bug.  And yes I find it distressing
that so so few programmers in my line of work use assertions at


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