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In Python 3.3 and newer, you can return a value from a generator; we use this in Tornado to have functions that call big and slow subprocesses (translate_part below) without blocking the main thread, and also without having more than one of the big&slow functions running concurrently:

@gen.coroutine
def translate(self, text, lock):
    parts = yield [translate_part(part, lock) for part in splitup(text)]
    retval = "".join(parts)
    return retval

To support Python 3.2, we changed it to the uglier, but equivalent:

@gen.coroutine
def translate(self, text, lock):
    parts = yield [translate_part(part, lock) for part in splitup(text)]
    retval = "".join(parts)
    raise StopIteration(retval)

However, this is (understandably) deprecated after Python 3.5. But how do we support both Python 3.5+ and Python 3.2? It's impossible to do 

        if sys.version_info.minor<3:
            raise StopIteration(retval)
        else:
            return retval

since Python 3.2 raises SyntaxError on "return retval", and of course

        try:
            return retval
        except SyntaxError:
            raise StopIteration(retval)

doesn't work. Is there a non-ugly solution to this, or do we have to maintain two versions?

https://www.python.org/dev/peps/pep-0479/ claims fixing things to be backwards-compatible is easy, but all their examples are of StopIteration's/return's without values.

unhammer
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  • Just because `raise StopIteration(retval)` is deprecated and discouraged, doesn't mean it'll stop working anytime soon. Why not just stick with what you have? – Martijn Pieters Jan 14 '16 at 08:34
  • It fails on Travis CI, and the PEP says it's going away. – unhammer Jan 14 '16 at 08:51
  • Is that because you are running against Python nightlies? You'd only get a deprecationwarning there. Are warnings raising an error perhaps? You could just suppress that warning for the one usage, until Tornado is ready to drop 3.2 support. And frankly, very few projects still support 3.2. – Martijn Pieters Jan 14 '16 at 09:23
  • Aha, that might explain why it errored out. I think while we still have 3.2-users we'll keep two files and conditionally import, then eventually remove the 3.2 file. – unhammer Jan 14 '16 at 09:29

0 Answers0