Tertiary Operation

Carsten Haese carsten at uniqsys.com
Tue Oct 17 16:44:22 CEST 2006


On Tue, 2006-10-17 at 09:48, Tim Chase wrote:
> [...]
> Either of the following should suffice:
> 
> 	# return a non-empty string
> 	x is None and "None" or str(x)

This one can be "optimized" to just str(x) since str(None)=="None".

>[...]
> There are more baroque ways of writing the terniary operator in 
> python (although I understand 2.5 or maybe python 3k should have 
> a true way of doing this).

Python 2.5 does already.

>   My understanding is that one common 
> solution is something like
> 
> 	{True: "", False: str(x)}[x is None]

As Fredrik pointed out in not so many words, this is not a good
solution. Besides being ugly, the major problem with this solution is
that both branches are evaluated regardless of the outcome of the
condition. This is not good if the expression is unsafe to calculate
under the wrong condition, or if the expressions are expensive to
calculate, or if the expressions have side effects.

The "condition and result1 or result2" hack at least prevents the
evaluation of the non-applicable expression due to the short-circuiting
nature of the "and" and "or" operators.

-Carsten





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