"Andreas 'ads' Scherbaum" <adsmail(at)wars-nicht(dot)de> writes:
> This leads back to my initial question: intended behaviour or just a
> bug? And how can one validate an ISBN without raising an error?
Judging from the comments, is_valid (and the internal validity bit)
were a bad design decision that the author later regretted, but felt
he couldn't change for compatibility reasons. I'm not sure why not
... we make bigger incompatible changes than that all the time.
The way to validate an ISBN is exactly the same as it is for every
other data type: feed the string to the input function and see if
it throws an error.
regards, tom lane