How are generators and coroutines implemented in CPython?

The notion that Python's stack and C stack in a running Python program are intermixed can be misleading.

The Python stack is something completely separated than the actual C stack used by the interpreter. The data structures on Python stack are actually full Python "frame" objects (that can even be introspected and have some attributes changed at run time). This stack is managed by the Python virtual machine, which itself runs in C and thus have a normal C program, machine level, stack.

When using generators and iterators, the interpreter simply stores the respective frame object somewhere else than on the Python program stack, and pushes it back there when execution of the generator resumes. This "somewhere else" is the generator object itself.Calling the method "next" or "send" on the generator object causes this to happen.

The yield instruction takes the current executing context as a closure, and transforms it into an own living object. This object has a __iter__ method which will continue after this yield statement.

So the call stack gets transformed into a heap object.