[Python-ideas] except expression

[Terry Reedy]

On 2/13/2014 4:24 AM, Nick Coghlan wrote:
> General comment: like Raymond, I'm inclined to favour a nice expression
> friendly exception handling syntax, precisely because of the
> proliferation of relatively ad hoc alternative solutions (in particular,
> the popularity of being able to pass in default values to handle empty
> iterables).

Leaving aside syntax, the idea makes sense to me as follows:

In Python, 'a or b' is not a just a logic operator but is generalized to 
mean, for instance, the following: 'a, unless a is falsey, in which 
case, b instead. The proposal is to introduce a syntax that means the 
same with 'unless a is falsey' replaced by 'unless a raises an exception 
of class C'. The class 'C' make 'a new_op b' inadequate.

The two alternations are related when exception C results from an input 
that is at least conceptually falsey. Some such cases can be handled by 
'or' (see below). Iterators, however, are seen as truthy even when empty 
(conceptually falsey).  And that cannot be fixed since some iterators 
cannot know if they are empty until __next__ is called and bool is not 
supposed to raise.

> One use case, for example, is handing IndexError when retrieving an item
> from a sequence (which currently has no nice standard spelling, and
> isn't amenable to the "pass in a default answer" solution because it
> isn't a normal function call).

This is easy, if not obvious

 >>> (seq or ['default'])[0]
'default'

(Python's precedence rules make the parentheses optional).

Doing something similar instead of dict.get is messier but conceptually 
the same.

 >>> {}.get('key', 'default')
'default'
 >>> {} or {'key':'default'}['key']
'default'

The general scheme is f(a or b, *args), where b is the input to f that 
gives the default as the output. Sort of inverse(partial(f, args))(default).

This does not work if the exception-raising inputs are not all seen as 
falsey (as with empty iterables) or if one cannot invert the partial 
function to get b. In either case, we have to input a and replace 
exceptions with b.

-- 
Terry Jan Reedy