On Sat, May 05, 2018 at 08:04:45AM +0000, Eloi Gaudry wrote:
> Hi folks,
> I intend to add a runtime assertion feature in python.
I'm very interested in this idea, but I'm afraid your draft PEP isn't
very clear about this feature. Comments below.
> Rationale
> There is no runtime assert relying on python grammar available. For
> diagnostics and/or debugging reasons, it would be valuable to add such
> a feature.
Why not this?
if DEBUG_ENABLED:
run_check(expression)
where run_check is an ordinary function. If DEBUG_ENABLED is false, the
only runtime overhead is the name lookup and bool test, the expression
is not evaluated.
What features does your suggestion offer over this simple technique?
[...]
> A brief example why avoiding evaluating the expression is needed to
> avoid any overhead in case the runtime assert should be ignored.
> ::
> runtime_assert( 999 in { i:None for i in range( 10000000 ) } )
You say that you have been using this feature in production for two
years. You should give real-life examples, not fake examples like the
above. That example would be better written as:
runtime_assert(True)
since no part of the test actually depends on either the enormous dict
or range objects that you build only to throw away.
> Usage
> ::
>
> runtime_assert( expr )
>
> #would result in
> if expr and runtime_assert_active:
> print RuntimeAssertionError()
Surely the test should be the other way around?
if runtime_assert_active and expr:
print(RuntimeAssertionError())
otherwise the expression is evaluated regardless of whether the runtime
assertions are active or not.
Please use Python 3 syntax for the PEP, as Python 2 is in maintenance
mode and will absolutely not get any new language features.
Some more observations:
I like the idea of more control over runtime checking, but your PEP
should justify why this is a good idea and why "just write more unit
tests" isn't the solution.
What happens if the caller has defined their own function or variable
called runtime_assert?
Can the caller customise the assertion error message?
If you are changing the grammar, why not make runtime_assert a statement?
Can the caller set an application-wide global across all modules, or do
they have to set
runtime_assert_active = True
in every module they want to use runtime_assert?
--
Steve
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