[Python-ideas] "else" expression ":"

[Shane Green]

Ah, yes, I should clarify that when I suggested: 

elif val < 0: 
  return -1
elif value != 0:
  raise ValueError() 

I did NOT mean to propose it as the syntactical translation of anything; I was suggesting that beginning programmers should use that instead of the first approach.  I would say that, between: 

	explicit is always better than implicit; 
	in the face of ambiguity, refuse the temptation to buses; 
	there should be one, and preferably only one, way to do it; and 
	special cases aren't special enough to break rules. 

Well, I like the way this one works now… 







Shane Green 
www.umbrellacode.com
408-692-4666 | shane at umbrellacode.com

On Apr 13, 2013, at 7:36 PM, Steven D'Aprano <steve at pearwood.info> wrote:

> On 14/04/13 04:24, Peter Norvig wrote:
>> Beginners will often write code like this:
>> 
>> if val > 0:
>>     return +1
>> elif val < 0:
>>     return -1
>> elif val == 0:
>>     return 0
>> 
>> Now if you did this in Java, the compiler would produce an error saying
>> that there is an execution path that does not return a value.
> 
> There's one difference between the languages right there: there is no such
> case for Python. If you pass something that doesn't match any of the three
> cases, say a NAN, the function will return None.
> 
> 
>> Python does
>> not give an error message, but it would be considered more idiomatic (and
>> slightly more efficient) to have just "else:" in the third clause.
> 
> Also incorrect, in a language which supports NANs, as Python does. (And Java,
> I believe, which may be why Java correctly tells you that there is a path
> with no return result.)
> 
> 
> 
>> Here's an idea to address this.  What do you think of the syntax
>> 
>>      "else" expression ":"
> 
> 
> I don't think it will help beginners, and for more experienced programmers,
> I don't think it is of much benefit over an explicit
> 
> else:
>    assert expression, "message if the assert fails"
> 
> (or an explicit ValueError test, if more appropriate).
> 
> 
> [...]
>> I have to say, I'm uncertain.  I'm not sure this is even a good idea at
>> all, and I'm not sure if it should translate into  "assert expression" or
>> whether it should be "if not expression: raise ValueError". What do you
>> think?
> 
> I think that there's no one right answer. For some code, an assertion will
> be correct, and for others, an explicit test and ValueError (or some other
> exception!) will be correct. Neither is so obviously more common that Python
> should introduce syntax to favour one over the other.
> 
> 
> -- 
> Steven
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