[Python-ideas] Repurpose `assert' into a general-purpose check

[Ivan Pozdeev]

On 28.11.2017 10:22, Elazar wrote:

> Just a note : in typechecked code (such as mypy's source code) assert 
> is used to guide the checker:
>
> assert isinstance(x, CallableType)
> return x.args  # checker knows it's valid
>
> So the assert becomes a kind of type annotation. The runtime check 
> helps during tests, but is not that important - failure will be caught 
> relatively soon. And I believe that the ability to remove the check at 
> runtime is important, since isinstance calls have non-negligible 
> impact on performance in mypy.
> (but other contributors here can correct me on this).
>
This results in two different interfaces. In normal mode, it enforces 
types while in -O, accepts anything that passes the duck test.
>
> Elazar
>
>
> בתאריך יום ג׳, 28 בנוב׳ 2017, 09:12, מאת Ivan Pozdeev via Python-ideas 
> ‏<python-ideas at python.org <mailto:python-ideas at python.org>>:
>
>     On 28.11.2017 8:59, Steven D'Aprano wrote:
>     > On Tue, Nov 28, 2017 at 07:35:45AM +0300, Ivan Pozdeev via
>     Python-ideas wrote:
>     >
>     >> Actually, the way I'm using them,
>     >>
>     >>      assert condition, "error message", type
>     >>
>     >> would probably be the most expressive way.
>     > I disagree that is expressive -- I call it *misleading*. I see
>     something
>     > which looks like an assertion (that is, a checked comment, a
>     contract, a
>     > check on an internal piece of logic etc) but it is actually
>     being used
>     > as a test.
>     >
>     >
>     >> I can do anything in any Turing-complete language without any
>     changes to
>     >> the language. That's no reason to never change anything, is it.
>     > "We can change this" is not a reason to change this. There needs
>     to be a
>     > *good* reason to change, and you have given no good reasons for this
>     > change.
>     >
>     >
>     >> The rationale basically is:
>     >> * As it was intended, the statement has no practical use --
>     basically a
>     >> rudiment, due to disappear eventually
>     > Nonsense. I make extensive use of assert as a way of checking
>     > assertions, and I will fight tooth and nail against any proposal to
>     > either remove it or to misuse it for public input tests instead of
>     > assertions.
>     I invite you to show me a single use case for those "assertions"
>     because
>     after ~20 years of experience in coding (that included fairly large
>     projects), I've yet to see one.
>
>     Any, every check that you make at debug time either
>     * belongs in production as well (all the more because it's harder to
>     diagnose there), or
>     * belongs in a test -- something coded independently from the program
>     (if your code as a whole cannot be trusted, how any specific part
>     of it
>     can?), or
>     * isn't needed at all because a fault will inevitably surface
>     somewhere
>     down the line (as some exception or an incorrect result that a
>     test will
>     catch).
>
>     Finally, I've got much experience using existing code outside its
>     original use cases, where the original author's assumptions may no
>     longer hold but the specific logic can be gauded to produce the
>     desired
>     result. Coding these assumptions in would undermine that goal.
>
>     So, I see "debug assertions" as either intentionally compromizing
>     correctness for performance (a direct opposite of Python's design
>     principles), or as an inferiour, faulty, half-measure rudiment from
>     times when CI wasn't a thing (thus not something that should be taught
>     and promoted as a best practice any longer).
>     >
>     >> * It can instead be reused as syntax sugar to cover a very
>     common use case
>     > There is no need for such syntactic sugar. It would be harmful
>     > to use assert for something which is not an assertion.
>     >
>     >
>     >
>
>     --
>     Regards,
>     Ivan
>
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-- 
Regards,
Ivan

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