[Python-ideas] except expression

[Chris Angelico]

On Wed, Feb 19, 2014 at 3:09 AM, MRAB <python at mrabarnett.plus.com> wrote:
> The question is whether it should be OK to allow a bare except, where
> that would catch a limited number of exceptions, in those cases where
> there won't be any risk.

Yeah. I don't like the idea that omitting the exception name(s) would
have a completely different effect in expression or statement form.
Compare:

value = true_value if condition else false_value
# <->
if condition: value = true_value
else: value = false_value

func = lambda x: x+4
# <->
def func(x): return x+4

lst = [x*x for x in range(5)]
# <->
lst = []
for x in range(5): lst.append(x*x)

Each of them has a "corresponding statement form" that's more-or-less
the same in effect (little stuff like name leakage aside), and which
is spelled very similarly. You wouldn't expect, for instance, lambda
functions to specify their args in reverse order compared to def
functions. So if it's syntactically legal to have a bare except in an
expression (still under debate), then that bare except should be
equivalent to "except BaseException" - not "except Exception", not
"except DWIMException", not anything else. Otherwise it'd just cause
confusion when trying to decode a complex expression.

ChrisA