In the following code, the original traceback attached to the exception thrown into the generator is ignored:

def gen():
    try:
        yield
    except:
        raise

g = gen()
try:
    1/0
except ZeroDivisionError as v:
    g.throw(v)


But if you replace the last line with:

    g.throw(type(v), v, v.__traceback__)

then the original traceback gets appended.
g.throw() should have fetched the __traceback__ attribute by itself.