[Python-ideas] except expression

[Chris Angelico]

On Fri, Feb 21, 2014 at 12:45 AM, M.-A. Lemburg <mal at egenix.com> wrote:
> I also find it disturbing that people are actually considering
> to use this expression form as a way to do quick&dirty suppression
> of exceptions.
>
> The intended use case should really be limited to providing default
> values for functions or methods that are expected to return a value.
>
> Abusing the fact that procedures in Python return None (simply
> because we don't have procedures and need to use functions instead)
> to make use of except expressions would make code less readable.

I absolutely agree here. Nothing in the proposal is _ever_ advocating
that. None of the examples is even showing that. The fact that you
happen to be able to is just an effect of simple and straight-forward
rules.

> x = data[1] except IndexError return None # is readable

That's the intent.

> f = open('x.txt', 'r') except IOError return None # is probably not a good idea

Well, that'd be okay if the rest of your code is like this:

if f: data = f.read(...)

For instance, you might have a rule that a config file will be read if
it exists, but otherwise you query the console. So you might do:

foo = f.readline() if f else input("Enter the foobinator: ")

> os.remove('/') except IOError return None # is really bad style

Yes. This one I don't like, and style guides should condemn it. Or
just leave it up to common sense: Don't be stupid. :)

ChrisA