The documentation for Popen.poll says:

> Check if child process has terminated. Set and return returncode attribute.

This requires the reader to guess that if the child process has not terminated, the call returns immediately, with a return value of ``None``. It would be good to be explicit about this.

Yes we can add "otherwise return None".  However it is pretty clear as is, since "poll" implies an immediate return, and if there's no return code to return, the logical value in Python to get back is None :)

Right, after following the source, I realise that `poll` is indeed returning `self.returncode` in all cases, and if the process hasn't terminated yet then `self.returncode` is still at its initial value of `None`.

I can think of other ways that the result could potentially be indicated, though: when I poll, I'm asking whether the child process is still running or not, so a boolean result wouldn't be unreasonable. Or perhaps an exception (like doing `get(block=False)` on a queue does). I don't there's a good reason not to be explicit here.

> I don't there's

I don't *think* there's ...

Stupid fingers.

Agreed.  Explicit is good.

Try to solve this issue, because it's pretty simple (:

I've added a sentence saying about the explicit return value.

New changeset 006617ff7d6df3fdedcfe53e94ee2c52cc796437 by Gregory P. Smith (Ivan Chernoff) in branch 'master':
bpo-31065: Add doc about Popen.poll returning None. (#3169)
https://github.com/python/cpython/commit/006617ff7d6df3fdedcfe53e94ee2c52cc796437

New changeset 0f1973d06e2116deafb19bbb9443b138187803c7 by Berker Peksag in branch '3.6':
bpo-31065: Add doc about Popen.poll returning None. (GH-3169)
https://github.com/python/cpython/commit/0f1973d06e2116deafb19bbb9443b138187803c7

I've backported 006617ff7d6df3fdedcfe53e94ee2c52cc796437  to 3.6. I think this can be closed now, thank you!