[Python-ideas] except expression

[Greg Ewing]

Steven D'Aprano wrote:
> This suggests that perhaps the parentheses aren't needed unless you have
> multiple except parts:
> 
>     things[i] (except IndexError, KeyError as exc: exc.args)
>     things[i] except IndexError, KeyError as exc: exc.args

Possibly not even necessary when there are multiple except
clauses, if you don't require a comma between them:

    things[i] except IndexError: 42 except KeyError: 17

Parsing may be easier without the parens as well, since
otherwise it looks like a function call when you hit
the '(' until you see the 'except' after it. (I *think*
that Python's parser could be made to handle that, but
I'd have to experiment to find out for sure.)

> Here is a torture-test for the syntax: can we combine it with an 
> if-expression? I think we can, although you may need parentheses to 
> disambiguate the expression:
> 
>     something(x) (except TypeError: a if condition else b)

The paren-less version of this would be

    something(x) except TypeError: a if condition else b

the interpretation of which would depend on what relative
precedence we decide on between 'except' and 'if'.

Parens can still be used to clarify, though:

    (something(x) except TypeError: a) if condition else b

    something(x) except TypeError: (a if condition else b)

> 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.

-- 
Greg