[Python-ideas] Possible enhancement to typing

Steve Barnes gadgetsteve at live.co.uk
Mon Nov 6 02:39:53 EST 2017

On 06/11/2017 07:13, Steven D'Aprano wrote:
> On Sun, Nov 05, 2017 at 07:18:30PM +0000, Steve Barnes wrote:
>> If a group of iterators were to be added to the typing module it would
>> be reasonably simple to automatically add and assert to any decorated
>> modules to ensure that such modules were always called with the
>> documented types.
> "Iterators"?
>> I am thinking of decorators such as:
>>    - @typing.assert_params_mismatch - this would provide a wrapper that
>> had auto-generated asserts that all the parameters were of designated types.
>>    - @typing.debug_assert_params_mismatch - this would provide a wrapper
>> that had auto-generated asserts that all the parameters were of
>> designated types only if a DEBUG environmental variable was set or similar.
> That's what assert does: assert only runs when __DEBUG__ is true. That's
> not controlled by an environment variable, but by the -O flag to the
> interpreter.
Good point.
> So your assert_params_mismatch and debug_assert_params_mismatch are
> effectively the same thing.
> But using assert to check to perform argument checks is often an abuse
> of assert. To be more specific, using assert to check the value of
> public arguments in library code (where the arguments come from outside the
> library) is wrong, since you (the library author) cannot guarantee
> that your type tests will even run.
> Using asserts for argument checking inside application code is more of a
> grey area, with arguments for and against using assert.
> But in my opinion, the deciding factor is nearly always that an
> AssertionError is the wrong sort of exception. Outside of some fairly
> limited circumstances, most of which don't involve type-checking
> function arguments, using assert robs the caller of some useful
> information: the *kind* of error. (TypeError, ValueError, etc.)
I see your point here.

> See here for further discussion:
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fimport-that.dreamwidth.org%2F676.html&data=02%7C01%7Cgadgetsteve%40live.co.uk%7C5e419db490b84018f2d208d524e612b4%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636455492928665911&sdata=EFZIvxGxO18vk5s97RnSHH9kAuLS3sXBxDNS9VbLWlw%3D&reserved=0
> In general, I don't think we want to encourage such runtime type
> testing. Obviously there are exceptions -- library code should
> probably type check arguments, applications perhaps not -- and
> we're not exactly discouraging it either. There are already a number of
> third-party libraries that provide argument type tests at runtime, and
> I think that's probably the right place for them.
I'll have to look out for them.
> [...]
>> I also think that this might increase the uptake of typing by giving
>> some clear benefits outside of documentation and static type checking.
> Problem is, the benefits of runtime type checking aren't clear. But the
> costs certainly are: if you want slow code, do lots and lots of runtime
> type checks.

Too much time spent writing safety critical code on my part then! I'll 
drop the idea.
Steve (Gadget) Barnes
Any opinions in this message are my personal opinions and do not reflect 
those of my employer.

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