[Python-ideas] except expression

[Andrew Barnert]

On Feb 20, 2014, at 5:45, "M.-A. Lemburg" <mal at egenix.com> wrote:

> On 20.02.2014 02:18, Chris Angelico wrote:
>> On Thu, Feb 20, 2014 at 11:15 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
>>> result = 1/x except ZeroDivisionError -> NaN
>>> 
>>> For the record, I could just as easily live with the colon instead of the
>>> arrow.
>> 
>> Time to open up this branch of the discussion... colon or arrow?
>> 
>> For the purposes of this debate, I'm comparing these two notations,
>> and nothing else:
>> 
>> result = 1/x except ZeroDivisionError -> NaN
>> result = 1/x except ZeroDivisionError: NaN
> 
> I'm -1 on both of them.
> 
> The colon should stay reserved for starting new blocks of statements.

So you don't like the colon in lambdas, dict displays, or slices?

[snip]

> 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.

Agreed. Especially after Nick Coghlan demonstrated that we already have a more readable and more concise way to do it without abusing anything:

    with suppress(IOError): os.remove('/')

Who sees that and says, "I like that it's one line, but if only it could be a longer line, with more keywords, and misleadingly imply a useful value"?

[snip]

> Sometimes I wish we had expression objects in Python to wrap
> expressions without evaluating them - sort of like lambdas
> without arguments but with a nicer syntax. These could then
> be used to implement a default() builtin.

There's no "sort of" about it; you want lambda with a nicer syntax.

I sympathize with that. Why do you think Haskell gets away with catch being a function when other functional languages don't? Maybe it's this:

    catch \-> expensive_call \-e> (dangerous_default e)

vs. this:

    catch(function() { expensive_call() }, function(e) { dangerous_default(e) }

That being said, I don't know that it would have the same benefits in Python. In a language that already encourages defining functions all over the place like JavaScript or OCaml, the verbose syntax is painful. But Python isn't like that. Haskell also has a one-character compose operator, which I love in Haskell and would love in JS, but I don't miss it in Python. (We don't even have a compose function in the stdlib.) Python lets you use functional style when it's the obvious way to express something, but doesn't force it on you when it isn't.