[Python-ideas] Why is design-by-contracts not widely adopted?

Jonathan Fine jfine2358 at gmail.com
Fri Sep 28 05:27:51 EDT 2018

I like this discussion. I'd like to add another theme, namely what
should happen when there is an error. (This is prompted by race
hazards when performing file system operations.)

Suppose fn_a() calls fn_b(), and fn_b() raises an exception. What then
should fn_a() do? It may be that this exception has left part or all
of the system in an inconsistent (invalid) state.

At this level of abstraction, it's not possible to sensibly answer
this question. Sometimes the whole system should be stopped. Other
times, an invalidation of an object is enough. And sometimes, a
rollback of the transaction is what's wanted.

Here's a well-known example (overflow exception in Ariane 5), which to
me shows that these problems can be very hard to get right.

According to wikipedia (above) this failure resulted in "the first
example of large-scale static code analysis by abstract

I expect that in some situations design-by-contract will help here, by
encouraging a focus on providing a more complete specification of
behaviour. It would be good to have some real-life Python examples.
I'd afraid I don't have any (although I've looked either).


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