[issue32590] Proposal: add an "ensure(arg)" builtin for parameter validation

[Xavier G. Domingo]

Change by Xavier G. Domingo :


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nosy: +xgdomingo

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[issue32590] Proposal: add an "ensure(arg)" builtin for parameter validation

[Nick Coghlan]

Nick Coghlan  added the comment:

I agreed that this idea isn't really baked enough for an RFE yet, so I'm 
marking this as "postponed" for now.

The main requirement for getting it back out of "postponed" state would be 
having someone that's sufficiently interested to write a PEP and wrangle some 
related python-ideas threads.

--
resolution:  -> postponed
stage:  -> resolved
status: open -> closed

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[issue32590] Proposal: add an "ensure(arg)" builtin for parameter validation

[Sylvain Marie]

Sylvain Marie  added the comment:

Very much interested by the topic.

For reference I tried to summarize the status of inline type and value 
validation here: https://smarie.github.io/python-valid8/why_validation/

And I proposed a solution with a library here 
https://smarie.github.io/python-valid8 (with a somewhat smart handling of the 
ValueError / TypeError inheritance dilemma, for what's worth)

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nosy: +smarie

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[issue32590] Proposal: add an "ensure(arg)" builtin for parameter validation

[Brett Cannon]

Brett Cannon  added the comment:

I agree w/ David that this idea is premature. Plus I say it's easier to make it 
so -O is more fine-grained so you can leave in asserts but get __debug__ set to 
False.

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nosy: +brett.cannon

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[issue32590] Proposal: add an "ensure(arg)" builtin for parameter validation

[R. David Murray]

R. David Murray  added the comment:

I'm not clear why we want a bug tracker issue for something that is still only 
an idea.  We generally *send* people to python-ideas when they propose 
half-baked ideas here :)  (Not actually saying your idea is half-baked, but it 
clearly isn't fully baked since there's no PEP.)  

There is literally no point in discussing this proposal here.  Normally for an 
issue like this (where we've redirected someone to python-ideas for discussion) 
we would close the issue and tell them to come back and re-open it if there is 
a positive consensus on python-ideas.  Since you are designating this as PEP 
level, there seems to be no reason to keep the issue open.

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nosy: +r.david.murray

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[issue32590] Proposal: add an "ensure(arg)" builtin for parameter validation

[Steven D'Aprano]

Steven D'Aprano  added the comment:

Since this is intended for argument checking, i.e. testing preconditions, the 
Eiffel term "require" seems more appropriate.

https://www.eiffel.org/doc/eiffel/ET:%20Design%20by%20Contract%20(tm),%20Assertions%20and%20Exceptions#Preconditions

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[issue32590] Proposal: add an "ensure(arg)" builtin for parameter validation

[Steven D'Aprano]

Steven D'Aprano  added the comment:

Oh I can just imagine the bike-shedding on this one :-)

I'm not really convinced this is needed, but for what it's worth I think that 
given the intended use-case (checking function parameters) ValueError would be 
a more appropriate default exception type. The given argument has the wrong 
value, hence ValueError.

On the other hand, it is more common (at least for me) to check arguments with 
isinstance, so maybe the default should be TypeError.

Definitely needs a PEP.

--
nosy: +steven.daprano
stage: needs patch -> 

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[issue32590] Proposal: add an "ensure(arg)" builtin for parameter validation

[Nick Coghlan]

New submission from Nick Coghlan :

This proposal is an outcome of repeated requests on python-ideas that assert 
statements be made unconditional, so they can be legitimately used for 
parameter validation, rather than solely as a form of inline self-test.

Rather than changing the assert statement, an alternative option would be to 
provide a new builtin (suggested name: "ensure") that raises ValidationError (a 
new subclass of AssertionError) if the first argument is false.

As a function, the new builtin could accept parameters for:

- the message to be reported on failure
- the exception type to be raised on failure

And since it would only be a new builtin rather than a new keyword, existing 
uses of the name "ensure" would be unaffected (except to the extent that 
linters may start warning about shadowing a builtin).

(Since it's a suggestion for a new builtin, actually doing this would require a 
PEP, which I'm not planning to write, I just wanted to get the suggestion 
explicitly on the record rather than leaving it buried in mailing list archives)

--
messages: 310219
nosy: ncoghlan
priority: normal
severity: normal
stage: needs patch
status: open
title: Proposal: add an "ensure(arg)" builtin for parameter validation
type: enhancement

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