[Python-ideas] PEP-3151 pattern-matching

[Mike Graham]

On Fri, Apr 8, 2011 at 10:13 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On Sat, Apr 9, 2011 at 12:04 AM, Mike Graham <mikegraham at gmail.com> wrote:
>> Neither of these seem justified for a rare case (this sort of patter
>> is fairly rare, most notably this one example) when there's nothing
>> that awful about the current solution.
>
> Actually, there is an awful lot of code in the wild that gives
> incorrect and wildly misleading error messages *because* correctly
> checking the errno attribute is so rare. PEP 3151 would improve the
> quality of information provided by a lot of tracebacks and error
> messages without many of those developers needing to even set finger
> to keyboard.

But Nick, that's different from what I'm saying is rare.

I'm saying that the situation where we need an if->raise on a constant
parameter is rare (this being almost the only common case). The issue
of whether people handle that case being rare is separate. Neither the
"except Foo as e if f(e):" syntax nor the "except foo(some_errno):"
pattern propose something that stops people from doing stupid stuff
like "except IOError" with no check.

Mike