On 2/21/2014 3:29 PM, Greg Ewing wrote:

Antoine Pitrou wrote:

   lst = [1, 2]
   value = lst[2] except IndexError: "No value"

the gain in concision is counterbalanced by a loss in
readability,

This version might be more readable:

   value = lst[2] except "No value" if IndexError

since it puts the normal and exceptional values next
to each other, and relegates the exception type (which
is of much less interest) to a trailing aside.

Ternary if teaches us that the normal and alternate values should be on either end, so the present proposal corresponds to that; ternary if also teaches us that nesting affects the right value, in the absence of parentheses, this proposal seems to correspond, although requiring parentheses or not is presently a topic of discussion in a different subthread.

On the other hand, "if" and "except" are opposites in another way: "if" is followed by an expression that, if true, means the left value is used, and if false, means the right value is used. "Except" is followed by an exception that, if thrown, means the right value is used, and if not thrown, means the left value is used... but the word "except" is opposite in meaning to "if", so this actually fits nicely with English definitions.

Here's a challenge: There has been a big thread about None versus (SQL) Null. Show how an except: expression can help the DB API more easily convert from using None to using a new Null singleton, and you'll have a winner :)