PEP 308: A PEP Writer's Experience - PRO

[David Eppstein]

On 2/11/03 12:53 AM +0100 holger krekel <pyth at devel.trillke.net> wrote:
>> > Subsequently
>> >
>> >     result = hasattr(obj, 'method') and obj.method()
>> >
>> > is not obvious to them but
>> >
>> >     result = obj.method() if hasatttr(obj, 'method') else None
>> >
>> > is
>>
>> I fail to see why forcing people to avoid an idiom that's obvious to
>> them (because it's not the "right" one) is a benefit to their
>> productivity or the readability of their code.
>
> because it easily leads to
>
>     if obj.method() if hasattr(obj, 'method') else None:
>
> which you have a harder time to justify as nice, not?

Different expressions are better in different contexts -- the "and" version 
clearly works better as an if condition, but I think the ternary version 
works better in an assignment.

If a programmer is intent on using a ternary in an if, I'd at least prefer 
something like your example above that stands out as being unstylish and in 
need of rewriting.  Consider the alternative: a programmer intending a 
ternary who writes it as
    if hasattr(obj, 'method') and obj.method() or some_default_value:
It would be much harder to notice that this is buggy (in the case where 
obj.method returns false but some_default_value is true), because of the 
naturality of finding ands and ors in an if condition.

--
David Eppstein       UC Irvine Dept. of Information & Computer Science
eppstein at ics.uci.edu http://www.ics.uci.edu/~eppstein/