[Python-Dev] PEP 463: Exception-catching expressions
v+python at g.nevcal.com
Sat Feb 22 02:10:50 CET 2014
On 2/21/2014 3:29 PM, Greg Ewing wrote:
> Antoine Pitrou wrote:
>>> lst = [1, 2]
>>> value = lst except IndexError: "No value"
>> the gain in concision is counterbalanced by a loss in
> This version might be more readable:
> value = lst 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
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
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
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