PEP new assert idiom

[Raymond Hettinger]

"Paul Rubin" <http://phr.cx@NOSPAM.invalid> wrote in message
news:7xr7n62t0v.fsf at ruckus.brouhaha.com...
> Some time ago I found myself proposing a new "Validate" statement,
> which would work exactly like "assert", except:
>
>   1) it would throw a ValidationError instead of AssertionError
>      if the condition failed
>
>   2) it would NOT be turned into a no-op by the optimizing compiler.
>
> The purpose is runtime validation of user data, rather than sanity
> checking the program logic.  Example: I keep finding myself writing
> code like:
>
>     x = float(raw_input('Enter a positive number: '))
>     assert x > 0, 'number wasn't positive'
>
> This is incorrect because the assert statement is not guaranteed to be
> executed.  I propose to say instead,
>
>    validate x > 0, 'number wasn't positive'
>
> which would do the right thing.

Why do you need a statement for this?
IMO, a plain function will do the trick:

def validate(expression, msg=None):
    if expression: return
    if msg is None:
        raise ValidationError
    else
        raise ValidationError(msg)


Raymond Hettinger