try/except KeyError vs "if name in ..."

[Terry Reedy]

On 10/6/2012 7:36 AM, Dave Angel wrote:

> The distinction in performance between the success and failure modes of
> the try/catch isn't nearly as large as one of the other responses might
> lead you to believe.  For example, a for loop generally terminates with
> a raise (of StopIteration exception), and that doesn't convince us to
> replace it with a while loop.

For statement generally loop many times, up to millions of times, 
without an exception being raised, whereas while statements test the 
condition each time around the loop. So the rule 'if failure is rare 
(less than 10-20%) use try', applies here. For if/them versus 
try/except, I don't worry too much about it.

-- 
Terry Jan Reedy