[Python-ideas] except expression

[MRAB]

On 2014-02-18 08:00, Paul Moore wrote:
> On 18 February 2014 07:22, Chris Angelico <rosuav at gmail.com> wrote:
>> On Tue, Feb 18, 2014 at 6:12 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>>> Not in general, but in situations where you need the
>>> conciseness of an except-expression?
>>>
>>> The examples in the PEP all seem to be variations on
>>>
>>>
>>>    value = next(it) except StopIteration as e: e.args[0]
>>>
>>> which seems a bit contrived. It's hard to think of a
>>> situation where you'd want to treat the yielded values
>>> and the return value of a generator in the same way.
>>
>> There's another one based on retrieving a document or its error text,
>> and showing that to a human. Beyond that, I really need other people
>> to make suggestions; there are those here who spend 99% of their time
>> writing Python code and know all the ins and outs of the libraries,
>> but I am not them.
>
> That was about the only even vaguely compelling one to me, and even
> then I'm not convinced. Reproducing it here (btw, spacing out the
> proposed and current syntax would be a huge help):
>
> Set a PyGTK label to a human-readable result from fetching a URL::
>
>      display.set_text(
>          urllib.request.urlopen(url)
>          except urllib.error.HTTPError as e: "Error %d: %s"%(x.getcode(), x.msg)
>          except (ValueError, urllib.error.URLError) as e: "Invalid URL: "+str(e)
>      )
>
>
>      try:
>          display.set_text(urllib.request.urlopen(url))
>      except urllib.error.HTTPError as e:
>          display.set_text("Error %d: %s"%(x.getcode(), x.msg))
>      except (ValueError, urllib.error.URLError) as e:
>          display.set_text("Invalid URL: "+str(e))
>
> However, your "current syntax" is not what I'd write at all - why not
> use a temporary variable, as in:
>
>      try:
>          text = urllib.request.urlopen(url)
>      except urllib.error.HTTPError as e:
>          text = "Error %d: %s"%(x.getcode(), x.msg)
>      except (ValueError, urllib.error.URLError) as e:
>          text = "Invalid URL: "+str(e)
>      display.set_text(text)
>
> That seems much clearer and natural to me than either of your two
> examples. At this point the discussion starts to remind me of the "we
> need multiline lambda" discussions, which fall apart when trying to
> find compelling use cases where a simple def using a named function
> wouldn't be better.
>
> I really think that this proposal needs to focus on the short, simple
> use cases and *not* worry about too much generality. For example:
>
>     sum(x[3] except 0 for x in list_of_tuples)
>
Shouldn't that be:

     sum(x[3] except: 0 for x in list_of_tuples)

(colon added)?
[snip]