[Python-ideas] except expression

[MRAB]

On 2014-02-15 23:46, Chris Angelico wrote:
> 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)
>
You'll also need to note that:

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

is not the same as:

     (expr except Exception1: default1 except Exception2: default2)

The first will also catch Exception2 if default1 raises it, whereas the
second will catch Exception2 only if expr raises it and it hasn't been
caught by the preceding 'except'.

> 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@?
>