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

[Scott, Barry]

On Tuesday, 19 February 2019 11:00:57 GMT Chris Withers wrote:
> Hi All,
> 
> There's this assert:
> 
> https://github.com/twisted/twisted/blob/trunk/src/twisted/internet/defer.py#
> L459
> 
> ...and I'd like to understand why it's there.

We hit this assert when porting from very old twisted to current twisted.
In all cases the problem was with our code that used deferreds in a poor, not
well understood way. After refactoring we are a lot happier with the
resulting code as it easier to maintain now.

Barry