raise None

Wed Dec 30 21:12:37 EST 2015

On Thu, Dec 31, 2015 at 12:26 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> Traceback (most recent call last):
> File "spam", line 19, in this
> File "spam", line 29, in that
> File "spam", line 39, in other
> ThingyError: ...
>>> I think this is a win for debuggability. (Is that a word?) But it's a bit
> annoying to do it today, since you have to save the return result and
> explicitly compare it to None. If "raise None" was a no-op, it would feel
> more natural to just say raise _validate() and trust that if _validate
> falls out the end and returns None, the raise will be a no-op.

(Yes, it is.)
Gotcha. So here's an alternative possibility. Instead of raising None
doing nothing, what you really want is to have _validate signal an
error with one less level of traceback - that is, you want it to raise
an exception from the calling function.
class remove_traceback_level:
 def __enter__(self): return self
 def __exit__(self, type, value, traceback):
 if type is None: return
 tb = traceback
 while tb.tb_next: tb = tb.tb_next
 tb.tb_next = None
 raise value from traceback
def _validate(a, b):
 with remove_traceback_level():
 if condition(a) or condition(b): raise TypeError
 if other_condition(a) or something_else(b): raise ValueError
 if whatever(a): raise SomethingError
The trouble is that this doesn't actually work, because tb_next is
read-only. But is there something along these lines that would make a
function raise exceptions as if it were in another function?
ChrisA