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

smarie sylvain.marie at schneider-electric.com
Tue Jan 16 10:37:29 EST 2018

Le mardi 16 janvier 2018 11:24:34 UTC+1, Steven D'Aprano a écrit :
> That's not a bug, or even a problem ("wrong"), it is the very purpose of 
> assert. Assertions are intended to allow the end user to disable the 
> checks.
> If you, the developer, don't want a check to be disabled, then you 
> shouldn't call it an assertion and use assert.

That is exactly what I'm saying. It seems that we both agree that 
applicative value validation is different from asserts, and that assert 
should not be used for applicative value validation. 
For this reason, I do not suggest to go in the direction the OP is 
mentioning but rather to explicitly separate the 2 concepts by creating a 
new statement for value validation.

> The problem with a statement called "validate" is that it will break a 
> huge number of programs that already include functions and methods using 
> that name.

You definitely make a point here. But that would be the case for absolutely 
*any* language evolution as soon as the proposed statements are plain old 
english words. Should it be a show-stopper ? I dont think so.

> But apart from the use of a keyword, we already have a way to do almost 
> exactly what you want:
>     if not expression: raise ValidationError(message)
> after defining some appropriate ValidationError class. And it is only a 
> few key presses longer than the proposed:
>     validate expression, ValidationError, message

This is precisely what is not good in my opinion: here you do not separate 
<validation means> from <validation intent>. Of course if <validation 
means> is just a "x > 0" statement, it works, but now what if you rely on a 
3d-party provided validation function (or even yours) such as e.g. 
"is_foo_compliant" ? 

if not is_foo_compliant(x): raise ValidationError(message)

What if this third part method raises an exception instead of returning 
False in some cases ? 

    if not is_foo_compliant(x): raise ValidationError(message)
    raise MyValidationError(message)

What if you want to compose this third party function with *another* one 
that returns False but not exceptions ? Say, with an OR ? (one or the other 
should work). Yields:

    if not is_foo_compliant(x): raise ValidationError(message)
    if not is_bar_compliant(x):
        raise MyValidationError(message)

It starts to be quite ugly, messy... while the applicative intent is clear 
and could be expressed directly as:

    validate is_foo_compliant(x) or is_bar_compliant(x) 

The goal is really to let developers express their applicative intent 
=(what should be checked and what is the outcome if anything goes wrong), 
and give them confidence that the statement will always fail the same way, 
whatever the failure modes /behaviour of the checkers used in the 
statement. This is what valid8 <https://smarie.github.io/python-valid8/> 
proposes today with assert_valid, but obviously again having it built-in in 
the language would be much more concise. Note that the class and function 
decorators (@validate_arg, @validate_field...) would remain useful anyway.

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