On Thu, Apr 09, 2020 at 08:27:14AM -0300, Soni L. wrote:
> To put it simple, unpacking raises ValueError:
[...]
> But if the iterator raises ValueError, there's no way to tell it apart
> from the unpacking:
>
> >>> def foo():
> ... yield None
> ... raise ValueError
You could start by reading the error message and the traceback, that
will usually make it clear the nature of the value error, and where it
occurred (in the generator, or where the generator was consumed).
For *debugging purposes* this is usually sufficient: the person reading
the exception can tell the difference between an unpacking error:
# outside the iterator
a, b, c = iterator
ValueError: not enough values to unpack (expected 3, got 1)
and some other error:
# inside the iterator
yield int('aaa')
ValueError: invalid literal for int() with base 10: 'aaa'
There may be rare cases where it is difficult to tell. Perhaps the
traceback is missing, or you are debugging a byte-code only library, or
obfuscated code, say. But these are rare cases, and we don't have to
solve those problems in the language.
Where this is not sufficient is for error recovery:
try:
a, b, c = iterator
except ValueError:
recover()
However, this is also true for every exception that Python might raise.
There is no absolutely foolproof solution, but it is usually good enough
to e.g.:
- include as little as possible inside the `try` block;
- carefully audit the contents of the `try` block to ensure it cannot
raise the exception you want to catch;
- wrap the iterator in something that will convert ValueError to
another exception.
At one point some years ago I attempted to write an "Exception Guard"
object that caught an exception and re-raised it as another exception,
so you could do this:
guard = ExceptionGuard(catch=ValueError, throw=RuntimeError)
try:
a, b, c = guard(iterator)
except ValueError:
print('unpacking error')
except RuntimeError:
print('ValueError in iterator caught and coverted')
but it collapsed under the weight of over-engineering and trying to
support versions of Python back to 2.4, and I abandoned it. Perhaps you
will have better luck, and when you have it as a mature, working object,
you can propose it for the standard library.
--
Steven
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/UE4AWB2445ZRZPTIWLT2WMJHLI2OIJYB/
Code of Conduct: http://python.org/psf/codeofconduct/
