Author yahya-abou-imran
Recipients yahya-abou-imran
Date 2018-12-11.04:40:28
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Message-id <1544503229.89.0.788709270274.issue35456@psf.upfronthosting.co.za>
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In asyncio.Task help:

 |  set_exception(self, exception, /)
 |      Mark the future done and set an exception.
 |      
 |      If the future is already done when this method is called, raises
 |      InvalidStateError.
 |  
 |  set_result(self, result, /)
 |      Mark the future done and set its result.
 |      
 |      If the future is already done when this method is called, raises
 |      InvalidStateError.

These doctrings are inherited from asyncio.Future.

But in fact it's wrong since:

https://github.com/python/cpython/blob/4824385fec0a1de99b4183f995a3e4923771bf64/Lib/asyncio/tasks.py#L161:

    def set_result(self, result):
        raise RuntimeError('Task does not support set_result operation')

    def set_exception(self, exception):
        raise RuntimeError('Task does not support set_exception operation')

Just adding another docstring is not a good solution - at leas for me - because the problem is in fact deeper:

This prove by itself that a Task is not a Future in fact, or shouldn't be, because this breaks the Liskov substitution principle.

We could have both Future and Task inheriting from some base class like PendingOperation witch would contain all the methods of Future except these two setters.

One problem to deal with might be those calls to super().set_result/exception() in Task._step():

https://github.com/python/cpython/blob/4824385fec0a1de99b4183f995a3e4923771bf64/Lib/asyncio/tasks.py#L254

        except StopIteration as exc:
            if self._must_cancel:
                # Task is cancelled right before coro stops.
                self._must_cancel = False
                super().set_exception(exceptions.CancelledError())
            else:
                super().set_result(exc.value)
        except exceptions.CancelledError:
            super().cancel()  # I.e., Future.cancel(self).
        except Exception as exc:
            super().set_exception(exc)
        except BaseException as exc:
            super().set_exception(exc)
            raise

One way to deal with that would be to let a Task have a Future.
"Prefer composition over inheritance" as they say.

I want to work on PR for this if nobody goes against it...

PS: I really don't like when some people says that Python core developers are known to have poor knowledge in regard to OOP principles. So I really don't like letting something like this in the standard library...
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Date User Action Args
2018-12-11 04:40:29yahya-abou-imransetrecipients: + yahya-abou-imran
2018-12-11 04:40:29yahya-abou-imransetmessageid: <1544503229.89.0.788709270274.issue35456@psf.upfronthosting.co.za>
2018-12-11 04:40:29yahya-abou-imranlinkissue35456 messages
2018-12-11 04:40:28yahya-abou-imrancreate