[Python-ideas] PEP 505: None-aware operators

[Steven D'Aprano]

On Thu, Jul 19, 2018 at 08:57:50PM -0400, Michael Selik wrote:

> Try/except also looks decent.
> 
>     try:
>         x = foo['bar'][0]
>     except TypeError:
>         x = 'default'

Consider the case that foo['bar'] is supposed to return either a 
collection (something that can be indexed) or None. But due to a bug, it 
returns a float 1.234. Now the try...except will *wrongly* catch the 
exception from 1.234[0] and return the default.

The problem here is that the try...except block is NOT equivalent to the 
None-aware pattern:

obj = foo['bar']  # avoid time-of-check-to-time-of-use bugs
if obj is None:
    x = 'default'
else:
   x = obj[0]

except in the special case that we can guarantee that foo['bar'] can 
only ever return a collection or None and will never, ever be buggy.



-- 
Steve