[Python-ideas] except expression

[Chris Angelico]

On Sun, Feb 16, 2014 at 10:17 AM, Greg Ewing
<greg.ewing at canterbury.ac.nz> wrote:
>> This does look a tiny bit like a function call, especially if you delete
>> the space between the leading expression and the opening bracket:
>>
>>     # ugly, don't do this
>>     things[i](except IndexError: 42)
>
>
> Yes, that's why I'm leaning towards the paren-less version.

I'm not liking the parenthesized version here, because the 'except'
expression has to know how much of the preceding expression to modify.
With a function call:

expression(args)

the expression is fully evaluated first, and then whatever it returns
gets called; with the except expression, a try block has to be set up
somewhere. This is more similar to if-else than to a function call, in
that the expression before the 'if' isn't evaluated until the
condition has been tested.

Parens could go around the whole thing:

(thing[i] except IndexError: 42)
(1/x if x else "Div by 0")

but not around the except clause:

thing[i] (except IndexError: 42) # Nope
1/x (if x else "Div by 0") # Nope

Incidentally, I'm looking at this being able to chain quite nicely:

((expr except Exception1: default1) except Exception2: default2)

Ideally the peephole optimizer could set up a single try/except
structure for both, but syntactically, I'm seeing this as the way the
operator associates.

I've mailed the first-draft PEP to peps@; is it appropriate to post it
here as well, or should I wait to hear back from peps@?

ChrisA