[Twisted-Python] why can't a callback be called with a deferred?

[Chris Withers]

On 21/02/2019 06:55, Glyph wrote:
>
>
>> The methods being hooked don't necessarily return deferreds.

Glyph, this bit ^^^

>> I'd like it to be an explicit choice of the caller, ie:
>>
>> result = yield SomeProtocol.onMessage.called()
>> # okay, we got here, we know onMessage was called,
>> # now we might want to tick a clock, or otherwise simulate
>> # async state manipulation.
>> # now I want to make sure the deferred chain on the onMessage result 
>> has been completed:
>> yield result
>
> I'm not sure I understand your example here. 

Yeah, this is part of carly, that I posted earlier. It stems from the 
need to get the results of method calls when you have no reference to 
the object being calls, or sometimes a result that's a deferred you need 
to wait on, particularly in a test, but have no way of doing so.

If you're feeling brave, have a read of:
https://github.com/cjw296/carly/blob/master/carly/hook.py

> The assertion in question only happens if you call returnValue or do a 
> return with a Deferred directly; this example doesn't do either of 
> those things.

This is the test situation where I hit this issue:
https://github.com/cjw296/carly/blob/master/tests/test_untracked_deferred.py#L28-L35

I'd originally wanted to have that read:

@inlineCallbacks
def test1(self):
...
result = yield pita.asyncMethod.called()
with ShouldRaise(Exception(1)):
yield result

Now, which I'm actually happier with the end result here, I think the 
above it legit, if unusual, and that assert trips it up.

cheers,

Chris