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[Python-ideas] except expression

M.-A. Lemburg mal at egenix.com
Mon Feb 17 16:00:28 CET 2014 
On 17.02.2014 15:33, Rob Cliffe wrote:
> 
> On 17/02/2014 10:00, M.-A. Lemburg wrote:
>> On 16.02.2014 05:04, Chris Angelico wrote:
>> The colon in there breaks the basic Python concept of having
>> colons end headers which start a new block of statements:
>>
>> http://docs.python.org/reference/compound_stmts.html
>>
>> I think it would be better to have the PEP focus on a solution
>> that doesn't introduce more lambda like syntax to the language :-)
>>
>> Please also add examples where the expression notation is
>> used as part of a larger expression, e.g.
>>
>> value = lst[2] except IndexError: 0, 1
>> value = lst[2] except IndexError: 0 if x else -1
>> value = lst[2] except IndexError: 0 if lst[1] except IndexError: False else -1
>> value = lst[2] except IndexError: 0 + lst[0]
>> d = {'a': lst[2] except IndexError: 0,
>>       'b': lst[1] except IndexError: 0 if lst[0] else 1,
>>       'c': lst[0],
>>      }
>>
>>
> Just wanted to point out the ambiguity in some of these, at least to a human reader.

This ambiguity is the reason why I pointed at these examples :-)

> If I am not mistaken, the proposed precedence for "except" (higher than "lambda", lower than
> everything else including "if - else") means that:
> 
> The first one means
>     value = lst[2] except IndexError: (0,1)
> not
>     value = (lst[2] except IndexError: 0), 1

The second is what Python would do. You can use if-else for testing
this:

>>> 1 if 2 else 3, 4
(1, 4)

> The second means
>     value = lst[2] except IndexError: (0 if x else -1)
> not
>     value = (lst[2] except IndexError: 0) if x else -1

Yes.

> The third means
>     value = lst[2] except IndexError: (0 if (lst[1] except IndexError: False) else -1)
> rather than anything else (this is getting a bit mind-boggling).

Yes; but I'm not entirely sure if the Python parser could even
handle this case. It also fails on this example:

>>> 1 if 2 if 3 else 4 else 5
  File "<stdin>", line 1
    1 if 2 if 3 else 4 else 5
            ^
SyntaxError: invalid syntax

> The fourth means
>     value = lst[2] except IndexError: (0 + lst[0])
> rather than
>     value = (lst[2] except IndexError: 0) + lst[0]

Yes.

> I wonder if the actual meaning coincides in all cases with your intention when you wrote them?
> No offence intended, MAL, I'm not trying to slag you off, just pointing out some of the issues that
> arise. :-)

Looking at such issues was the intention of posting the list :-)

> And wondering if there is a case for "except" having a higher precedence than "if - else".  I don't
> have an opinion yet, I'm just thinking aloud.
>
> The last one is I believe unambiguous except for the value associated with the key 'b', but to my
> mind is another reason for not allowing commas in the proposed "except expression" syntax -
> confusion with commas as separators between dictionary literals (sorry I keep plugging this!). 
> There is already ISTM a clash between comma as separator between dictionary items and between tuple
> elements:
> 
>>>> { 'a' : (1,3), 'b' : 2 }
> { 'a': (1, 3), 'b': 2 }
>>>> { 'a' : 1,3, 'b' : 2 } # Python 2.7.3
> SyntaxError: invalid syntax
> 
> so I think it would be a mistake to overload the comma further.

Commas are indeed too overloaded already.

Guess what these evaluate to :-)

x = lambda y: y, 2
x = 2,
x = 1, 2,
x = {1:2,}

-- 
Marc-Andre Lemburg
eGenix.com

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