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[Python-Dev] Requesting pronouncement on PEP 463: Exception-catching expressions

Chris Angelico rosuav at gmail.com
Wed Mar 12 21:49:47 CET 2014 
On Thu, Mar 13, 2014 at 7:14 AM, Barry Warsaw <barry at python.org> wrote:
> Interestingly enough, where ternaries are most useful are in the same
> situations where I think exception expressions would be most useful, in the
> setting of a variable or attribute to one of two different values.  In both
> branches of the conditional, the left hand side is the same target, and what
> differs is the value being assigned to the LHS.
>
> Looking over exception examples from a large code base, I find it rare (but
> not nonexistent) that exceptions are used in the same situation.  But that
> could be because I'm so comfortable with .get() and other helpers.  Ternary
> expressions seem more compelling.  But there's definitely some overlap, which
> is perhaps why the colon seems so jarring to me.

When you say "looking over", do you mean with the Mk 1 Human Eyeball,
or with the script from my research?

https://github.com/Rosuav/ExceptExpr/blob/master/find_except_expr.py

If you've used the script and have some stats to offer, I'd love to
see them. So far, all I have is the stdlib, which isn't necessarily
representative.

Bear in mind, there isn't a .get() for every situation. You can use
hasattr() in place of AttributeError, which isn't quite the same
thing, but most of the OSError-raising APIs don't (as far as I know)
offer a way to return a default instead of raising. So even if you do
generally use dict.get(), there'll likely be some that the script
would catch.

ChrisA
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