[Python-Dev] PEP 463: Exception-catching expressions

[Brett Cannon]

On Fri, Feb 21, 2014 at 8:52 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:

> On Fri, 21 Feb 2014 14:15:59 +1100
> Chris Angelico <rosuav at gmail.com> wrote:
> >
> > A number of functions and methods have parameters which will cause
> > them to return a specified value instead of raising an exception.  The
> > current system is ad-hoc and inconsistent, and requires that each
> > function be individually written to have this functionality; not all
> > support this.
>
> While I sympathize with the motivation, I really don't like the end
> result:
>
> >     lst = [1, 2]
> >     value = lst[2] except IndexError: "No value"
>
> is too much of a break from the usual stylistic conventions, and looks
> like several statements were stuffed on a single line.
>
> In other words, the gain in concision is counterbalanced by a loss in
> readability, a compromise which doesn't fit in Python's overall design
> principles.
>
> (compare with "with", which solves actual readability issues due to the
> distance between the "try" and the "finally" clause, and also promotes
> better resource management)
>

While I like the general concept, I agree that it looks too much like a
crunched statement; the use of the colon is a non-starter for me. I'm sure
I'm not the only one whose brain has been trained to view a colon in Python
to mean "statement", period. This goes against that syntactic practice and
just doesn't work for me.

I'm -1 with the current syntax, but it can go into the + range if a better
syntax can be chosen.
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