Generator, yes, but used as a coroutine that both emits values and receives them. The introduction of dedicated coroutines in Python 3.5 is something I haven't updated my personal language for very well. Generator-coroutine is the term used in there Python docs, IIRC.

> And exponential growth, you say?

Well... That may not be perfectly accurate and I shouldn't have used that term. It grew large enough, fast enough that the same level of nesting was measured in microseconds for the control and and over an hour for the wrapper so I cancelled the test. Let me put together a reproducible experiment and let others decide the growth rate.

> Where is this implemented?

Unfortunately, our production use of the lib is not something we've open sourced. Similar to the above, I'll make available a reproducible experiment so you can draw your own conclusion.

> Optimizing for CPython looks like adding weird implementation-specific hacks that might start working or be backwards in the next version

This went into the rationale behind making most of the components pluggable. The choice of optimizing for cPython is internal to us and we chose to document it for others. It's not required and not the default.